Objects Utility Class - Java Programming

When implementing equals(), hashCode(), or handling potentially null values, manual null checks are error-prone and verbose. The Objects utility class provides null-safe methods for these common operations, making defensive programming cleaner and more reliable.

Objects class A utility class in java.util providing null-safe static methods for common object operations.

Null-Safe Equality

Compare objects without risking NullPointerException.

                            Equals.java
                        
Replay: real traced execution (multi-file project)

                            // Objects.equals examples

import java.util.Objects;

public class Equals {
    public static void main(String[] args) {
        System.out.println("Basic equals:");

        String s1 = "hello";
        String s2 = "hello";
        String s3 = "world";
        String s4 = null;

        System.out.println("s1.equals(s2): " + s1.equals(s2));
        System.out.println("Objects.equals(s1, s2): " + Objects.equals(s1, s2));
        System.out.println("Objects.equals(s1, s3): " + Objects.equals(s1, s3));
        System.out.println("\nNull-safe equals:");

        System.out.println("Objects.equals(s1, null): " + Objects.equals(s1, null));
        System.out.println("Objects.equals(null, s1): " + Objects.equals(null, s1));
        System.out.println("Objects.equals(null, null): " + Objects.equals(null, null));

        // This would throw NullPointerException:
        // System.out.println(s4.equals(s1));
        System.out.println("\nDeep equals:");

        int[] arr1 = {1, 2, 3};
        int[] arr2 = {1, 2, 3};
        int[] arr3 = arr1;

        System.out.println("arr1 == arr2: " + (arr1 == arr2));
        System.out.println("arr1.equals(arr2): " + arr1.equals(arr2));
        System.out.println("Objects.equals(arr1, arr2): " + Objects.equals(arr1, arr2));
        System.out.println("Objects.deepEquals(arr1, arr2): " + Objects.deepEquals(arr1, arr2));
        System.out.println("Objects.deepEquals(arr1, arr3): " + Objects.deepEquals(arr1, arr3));
        System.out.println("\nCustom class equals:");

        Person p1 = new Person("Alice", 30);
        Person p2 = new Person("Alice", 30);
        Person p3 = new Person("Bob", 25);
        Person p4 = null;

        System.out.println("p1.equals(p2): " + p1.equals(p2));
        System.out.println("Objects.equals(p1, p2): " + Objects.equals(p1, p2));
        System.out.println("Objects.equals(p1, p3): " + Objects.equals(p1, p3));
        System.out.println("Objects.equals(p1, null): " + Objects.equals(p1, null));
        System.out.println("Objects.equals(null, null): " + Objects.equals(null, null));
        System.out.println("\nComparing arrays:");

        String[] names1 = {"Alice", "Bob"};
        String[] names2 = {"Alice", "Bob"};

        System.out.println("Arrays equal (==): " + (names1 == names2));
        System.out.println("Arrays equal (equals): " + names1.equals(names2));
        System.out.println("Arrays equal (Objects.equals): " + Objects.equals(names1, names2));
        System.out.println("Arrays equal (Objects.deepEquals): " + Objects.deepEquals(names1, names2));
        System.out.println("\n2D arrays:");

        int[][] matrix1 = {{1, 2}, {3, 4}};
        int[][] matrix2 = {{1, 2}, {3, 4}};

        System.out.println("Matrix equal (Objects.equals): " + Objects.equals(matrix1, matrix2));
        System.out.println("Matrix equal (Objects.deepEquals): " + Objects.deepEquals(matrix1, matrix2));
        System.out.println("\nIn collections:");

        java.util.List<String> list1 = java.util.Arrays.asList("a", "b", "c");
        java.util.List<String> list2 = java.util.Arrays.asList("a", "b", "c");
        java.util.List<String> list3 = null;

        System.out.println("Lists equal: " + Objects.equals(list1, list2));
        System.out.println("List vs null: " + Objects.equals(list1, null));
        System.out.println("Null lists: " + Objects.equals(list3, null));
    }

    static class Person {
        String name;
        int age;

        Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) return true;
            if (obj == null || getClass() != obj.getClass()) return false;
            Person other = (Person) obj;
            return age == other.age && Objects.equals(name, other.name);
        }

        @Override
        public int hashCode() {
            return Objects.hash(name, age);
        }
    }
}

                        
1s1 ← hello, s2 ← hello, s3 ← world, s4 ← null⋯5public class Equals {6    public static void main(String[] args) {7        System.out.println("Basic equals:");89        String s1→ hello = "hello";10        String s2→ hello = "hello";11        String s3→ world = "world";12        String s4→ null = null;1314        System.out.println("s1.equals(s2): " + s1.equals(s2hello));15        System.out.println("Objects.equals(s1, s2): " + Objects.equals(s1hello, s2hello));16        System.out.println("Objects.equals(s1, s3): " + Objects.equals(s1hello, s3world));17        System.out.println("\nNull-safe equals:");1819        System.out.println("Objects.equals(s1, null): " + Objects.equals(s1hello, null));20        System.out.println("Objects.equals(null, s1): " + Objects.equals(null, s1hello));21        System.out.println("Objects.equals(null, null): " + Objects.equals(null, null));2223        // This would throw NullPointerException:24        // System.out.println(s4.equals(s1));25        System.out.println("\nDeep equals:");2627        int[] arr1 = {1, 2, 3};28        int[] arr2 = {1, 2, 3};29        int[] arr3 = arr1;3031        System.out.println("arr1 == arr2: " + (arr1 == arr2));32        System.out.println("arr1.equals(arr2): " + arr1.equals(arr2));33        System.out.println("Objects.equals(arr1, arr2): " + Objects.equals(arr1, arr2));34        System.out.println("Objects.deepEquals(arr1, arr2): " + Objects.deepEquals(arr1, arr2));35        System.out.println("Objects.deepEquals(arr1, arr3): " + Objects.deepEquals(arr1, arr3));36        System.out.println("\nCustom class equals:");3738        Person p1 = new Person("Alice", 30);39        Person p2 = new Person("Alice", 30);⋯outputBasic equals:
s1.equals(s2): true
Objects.equals(s1, s2): true
Objects.equals(s1, s3): false

Null-safe equals:
Objects.equals(s1, null): false
Objects.equals(null, s1): false
Objects.equals(null, null): true

Deep equals:
arr1 == arr2: false
arr1.equals(arr2): false
Objects.equals(arr1, arr2): false
Objects.deepEquals(arr1, arr2): true
Objects.deepEquals(arr1, arr3): true

Custom class equals:
2this.name ← Alice, this.age ← 30, p1 ← ⟨Equals$Person A⟩pass 1 of 3⋯38    Person p1→ ⟨Equals$Person A⟩ = new Person("Alice", 30);39    Person p2 = new Person("Alice", 30);40    Person p3 = new Person("Bob", 25);41    Person p4 = null;4243    System.out.println("p1.equals(p2): " + p1.equals(p2));44    System.out.println("Objects.equals(p1, p2): " + Objects.equals(p1, p2));45    System.out.println("Objects.equals(p1, p3): " + Objects.equals(p1, p3));46    System.out.println("Objects.equals(p1, null): " + Objects.equals(p1, null));47    System.out.println("Objects.equals(null, null): " + Objects.equals(null, null));48    System.out.println("\nComparing arrays:");4950    String[] names1 = {"Alice", "Bob"};51    String[] names2 = {"Alice", "Bob"};5253    System.out.println("Arrays equal (==): " + (names1 == names2));54    System.out.println("Arrays equal (equals): " + names1.equals(names2));55    System.out.println("Arrays equal (Objects.equals): " + Objects.equals(names1, names2));56    System.out.println("Arrays equal (Objects.deepEquals): " + Objects.deepEquals(names1, names2));57    System.out.println("\n2D arrays:");5859    int[][] matrix1 = {{1, 2}, {3, 4}};60    int[][] matrix2 = {{1, 2}, {3, 4}};6162    System.out.println("Matrix equal (Objects.equals): " + Objects.equals(matrix1, matrix2));63    System.out.println("Matrix equal (Objects.deepEquals): " + Objects.deepEquals(matrix1, matrix2));64    System.out.println("\nIn collections:");6566    java.util.List<String> list1 = java.util.Arrays.asList("a", "b", "c");67    java.util.List<String> list2 = java.util.Arrays.asList("a", "b", "c");68    java.util.List<String> list3 = null;6970    System.out.println("Lists equal: " + Objects.equals(list1, list2));71    System.out.println("List vs null: " + Objects.equals(list1, null));72    System.out.println("Null lists: " + Objects.equals(list3, null));73}7475static class Person {76    String name;77    int age;7879    Person(String nameAlice, int age30) {80        this.name→ Alice = nameAlice;81        this.age→ 30 = age30;82    }⋯All 3 passes — pass 1 is the card abovepassnameagethis.namethis.agep1p2p3p4list1list2list3
1Alice30Alice30⟨Equals$Person A⟩——————
2Alice30Alice30—⟨Equals$Person A⟩—————
3Bob25Bob25⟨Equals$Person A⟩⟨Equals$Person A⟩⟨Equals$Person B⟩null[a, b, c][a, b, c]null

All 3 passes — pass 1 is the card above
pass	`name`	`age`	`this.name`	`this.age`	`p1`	`p2`	`p3`	`p4`	`list1`	`list2`	`list3`
1	Alice	30	Alice	30	⟨Equals$Person A⟩	—	—	—	—	—	—
2	Alice	30	Alice	30	—	⟨Equals$Person A⟩	—	—	—	—	—
3	Bob	25	Bob	25	⟨Equals$Person A⟩	⟨Equals$Person A⟩	⟨Equals$Person B⟩	null	[a, b, c]	[a, b, c]	null

Null-safe comparison Objects.equals(a, b) handles null values gracefully, returning true if both are null and false if only one is null.

Hash Code Generation

Generate consistent hash codes for multiple fields.

                            Hash.java
                        
Replay: real traced execution (multi-file project)

                            // Objects.hash examples

import java.util.Objects;
import java.util.Arrays;

public class Hash {
    public static void main(String[] args) {
        System.out.println("Basic hash:");

        String s1 = "hello";
        String s2 = "world";

        System.out.println("s1.hashCode(): " + s1.hashCode());
        System.out.println("Objects.hashCode(s1): " + Objects.hashCode(s1));
        System.out.println("Objects.hashCode(null): " + Objects.hashCode(null));
        System.out.println("\nMultiple values:");

        int hash1 = Objects.hash("Alice", 30);
        int hash2 = Objects.hash("Alice", 30);
        int hash3 = Objects.hash("Bob", 30);
        int hash4 = Objects.hash("Alice", 25);

        System.out.println("hash('Alice', 30): " + hash1);
        System.out.println("hash('Alice', 30): " + hash2);
        System.out.println("Same? " + (hash1 == hash2));
        System.out.println("hash('Bob', 30): " + hash3);
        System.out.println("hash('Alice', 25): " + hash4);
        System.out.println("\nCustom class:");

        Person p1 = new Person("Alice", 30);
        Person p2 = new Person("Alice", 30);
        Person p3 = new Person("Bob", 25);

        System.out.println("p1.hashCode(): " + p1.hashCode());
        System.out.println("p2.hashCode(): " + p2.hashCode());
        System.out.println("p3.hashCode(): " + p3.hashCode());
        System.out.println("p1 == p2 hashCode? " + (p1.hashCode() == p2.hashCode()));
        System.out.println("\nIn HashMap:");

        java.util.Map<Person, String> map = new java.util.HashMap<>();

        map.put(new Person("Alice", 30), "Engineer");
        map.put(new Person("Bob", 25), "Designer");

        System.out.println("Get Alice: " + map.get(new Person("Alice", 30)));
        System.out.println("Get Bob: " + map.get(new Person("Bob", 25)));
        System.out.println("\nHash with nulls:");

        int h1 = Objects.hash("Alice", null, 30);
        int h2 = Objects.hash("Alice", null, 30);
        int h3 = Objects.hash(null, null, null);

        System.out.println("hash('Alice', null, 30): " + h1);
        System.out.println("hash('Alice', null, 30): " + h2);
        System.out.println("Same? " + (h1 == h2));
        System.out.println("hash(null, null, null): " + h3);
        System.out.println("\nPoint class:");

        Point pt1 = new Point(10, 20);
        Point pt2 = new Point(10, 20);
        Point pt3 = new Point(20, 10);

        System.out.println("pt1: " + pt1 + " hash=" + pt1.hashCode());
        System.out.println("pt2: " + pt2 + " hash=" + pt2.hashCode());
        System.out.println("pt3: " + pt3 + " hash=" + pt3.hashCode());
        System.out.println("\nHashSet usage:");

        java.util.Set<Point> points = new java.util.HashSet<>();
        points.add(new Point(1, 2));
        points.add(new Point(3, 4));
        points.add(new Point(1, 2));  // Duplicate

        System.out.println("Set size: " + points.size());
        System.out.println("Contains (1,2)? " + points.contains(new Point(1, 2)));
        System.out.println("\nRecord class:");

        Rectangle r1 = new Rectangle(10, 20);
        Rectangle r2 = new Rectangle(10, 20);
        Rectangle r3 = new Rectangle(20, 10);

        System.out.println("r1 hash: " + r1.hashCode());
        System.out.println("r2 hash: " + r2.hashCode());
        System.out.println("r3 hash: " + r3.hashCode());
        System.out.println("r1.equals(r2)? " + r1.equals(r2));
    }

    static class Person {
        String name;
        int age;

        Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) return true;
            if (obj == null || getClass() != obj.getClass()) return false;
            Person other = (Person) obj;
            return age == other.age && Objects.equals(name, other.name);
        }

        @Override
        public int hashCode() {
            return Objects.hash(name, age);
        }
    }

    static class Point {
        int x, y;

        Point(int x, int y) {
            this.x = x;
            this.y = y;
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) return true;
            if (!(obj instanceof Point)) return false;
            Point other = (Point) obj;
            return x == other.x && y == other.y;
        }

        @Override
        public int hashCode() {
            return Objects.hash(x, y);
        }

        @Override
        public String toString() {
            return "(" + x + "," + y + ")";
        }
    }

    static class Rectangle {
        int width, height;

        Rectangle(int width, int height) {
            this.width = width;
            this.height = height;
        }

        @Override
        public boolean equals(Object obj) {
            if (!(obj instanceof Rectangle)) return false;
            Rectangle r = (Rectangle) obj;
            return width == r.width && height == r.height;
        }

        @Override
        public int hashCode() {
            return Objects.hash(width, height);
        }
    }
}

                        
1s1 ← hello, s2 ← world, hash1 ← 1963862399, hash2 ← 1963862399⋯6public class Hash {7    public static void main(String[] args) {8        System.out.println("Basic hash:");910        String s1→ hello = "hello";11        String s2→ world = "world";1213        System.out.println("s1.hashCode(): " + s1.hashCode());14        System.out.println("Objects.hashCode(s1): " + Objects.hashCode(s1hello));15        System.out.println("Objects.hashCode(null): " + Objects.hashCode(null));16        System.out.println("\nMultiple values:");1718        int hash1→ 1963862399 = Objects.hash("Alice", 30);19        int hash2→ 1963862399 = Objects.hash("Alice", 30);20        int hash3→ 2076906 = Objects.hash("Bob", 30);21        int hash4→ 1963862394 = Objects.hash("Alice", 25);2223        System.out.println("hash('Alice', 30): " + hash11963862399);24        System.out.println("hash('Alice', 30): " + hash21963862399);25        System.out.println("Same? " + (hash11963862399 == hash21963862399));26        System.out.println("hash('Bob', 30): " + hash32076906);27        System.out.println("hash('Alice', 25): " + hash41963862394);28        System.out.println("\nCustom class:");2930        Person p1 = new Person("Alice", 30);31        Person p2 = new Person("Alice", 30);⋯outputBasic hash:
s1.hashCode(): 99162322
Objects.hashCode(s1): 99162322
Objects.hashCode(null): 0

Multiple values:
hash('Alice', 30): 1963862399
hash('Alice', 30): 1963862399
Same? true
hash('Bob', 30): 2076906
hash('Alice', 25): 1963862394

Custom class:
2this.name ← Alice, this.age ← 30, p1 ← ⟨Hash$Person A⟩pass 1 of 7⋯30    Person p1→ ⟨Hash$Person A⟩ = new Person("Alice", 30);31    Person p2 = new Person("Alice", 30);32    Person p3 = new Person("Bob", 25);3334    System.out.println("p1.hashCode(): " + p1.hashCode());35    System.out.println("p2.hashCode(): " + p2.hashCode());36    System.out.println("p3.hashCode(): " + p3.hashCode());37    System.out.println("p1 == p2 hashCode? " + (p1.hashCode() == p2.hashCode()));38    System.out.println("\nIn HashMap:");3940    java.util.Map<Person, String> map = new java.util.HashMap<>();4142    map.put(new Person("Alice", 30), "Engineer");43    map.put(new Person("Bob", 25), "Designer");4445    System.out.println("Get Alice: " + map.get(new Person("Alice", 30)));46    System.out.println("Get Bob: " + map.get(new Person("Bob", 25)));47    System.out.println("\nHash with nulls:");4849    int h1 = Objects.hash("Alice", null, 30);50    int h2 = Objects.hash("Alice", null, 30);51    int h3 = Objects.hash(null, null, null);5253    System.out.println("hash('Alice', null, 30): " + h1);54    System.out.println("hash('Alice', null, 30): " + h2);55    System.out.println("Same? " + (h1 == h2));56    System.out.println("hash(null, null, null): " + h3);57    System.out.println("\nPoint class:");5859    Point pt1 = new Point(10, 20);60    Point pt2 = new Point(10, 20);61    Point pt3 = new Point(20, 10);6263    System.out.println("pt1: " + pt1 + " hash=" + pt1.hashCode());64    System.out.println("pt2: " + pt2 + " hash=" + pt2.hashCode());65    System.out.println("pt3: " + pt3 + " hash=" + pt3.hashCode());66    System.out.println("\nHashSet usage:");6768    java.util.Set<Point> points = new java.util.HashSet<>();69    points.add(new Point(1, 2));70    points.add(new Point(3, 4));71    points.add(new Point(1, 2));  // Duplicate7273    System.out.println("Set size: " + points.size());74    System.out.println("Contains (1,2)? " + points.contains(new Point(1, 2)));75    System.out.println("\nRecord class:");7677    Rectangle r1 = new Rectangle(10, 20);78    Rectangle r2 = new Rectangle(10, 20);79    Rectangle r3 = new Rectangle(20, 10);8081    System.out.println("r1 hash: " + r1.hashCode());82    System.out.println("r2 hash: " + r2.hashCode());83    System.out.println("r3 hash: " + r3.hashCode());84    System.out.println("r1.equals(r2)? " + r1.equals(r2));85}8687static class Person {88    String name;89    int age;9091    Person(String nameAlice, int age30) {92        this.name→ Alice = nameAlice;93        this.age→ 30 = age30;94    }⋯All 7 passes — pass 1 is the card abovepassnameagethis.namethis.agep1p2p3maph1h2h3
1Alice30Alice30⟨Hash$Person A⟩——————
2Alice30Alice30—⟨Hash$Person A⟩—————
3Bob25Bob25——⟨Hash$Person B⟩{}———
4Alice30Alice30———————
5Bob25Bob25———————
6Alice30Alice30———————
7Bob25Bob25————75019132575019132529791
3this.x ← 10, this.y ← 20, pt1 ← (10,20)pass 1 of 7⋯59    Point pt1→ (10,20) = new Point(10, 20);60    Point pt2 = new Point(10, 20);61    Point pt3 = new Point(20, 10);6263    System.out.println("pt1: " + pt1 + " hash=" + pt1.hashCode());64    System.out.println("pt2: " + pt2 + " hash=" + pt2.hashCode());65    System.out.println("pt3: " + pt3 + " hash=" + pt3.hashCode());66    System.out.println("\nHashSet usage:");6768    java.util.Set<Point> points = new java.util.HashSet<>();69    points.add(new Point(1, 2));70    points.add(new Point(3, 4));71    points.add(new Point(1, 2));  // Duplicate7273    System.out.println("Set size: " + points.size());74    System.out.println("Contains (1,2)? " + points.contains(new Point(1, 2)));75    System.out.println("\nRecord class:");7677    Rectangle r1 = new Rectangle(10, 20);78    Rectangle r2 = new Rectangle(10, 20);79    Rectangle r3 = new Rectangle(20, 10);8081    System.out.println("r1 hash: " + r1.hashCode());82    System.out.println("r2 hash: " + r2.hashCode());83    System.out.println("r3 hash: " + r3.hashCode());84    System.out.println("r1.equals(r2)? " + r1.equals(r2));85}8687static class Person {88    String name;89    int age;9091    Person(String name, int age) {92        this.name = name;93        this.age = age;94    }9596    @Override97    public boolean equals(Object obj) {98        if (this == obj) return true;99        if (obj == null || getClass() != obj.getClass()) return false;100        Person other = (Person) obj;101        return age == other.age && Objects.equals(name, other.name);102    }103104    @Override105    public int hashCode() {106        return Objects.hash(name, age);107    }108}109110static class Point {111    int x, y;112113    Point(int x10, int y20) {114        this.x→ 10 = x10;115        this.y→ 20 = y20;116    }⋯All 7 passes — pass 1 is the card abovepassxythis.xthis.ypt1pt2pt3points
110201020(10,20)———
210201020—(10,20)——
320102010(10,20)(10,20)(20,10)[]
41212————
53434————
61212————
71212————
4this.width ← 10, this.height ← 20, r1 ← Hash$Rectangle@50bpass 1 of 3⋯77    Rectangle r1→ Hash$Rectangle@50b = new Rectangle(10, 20);78    Rectangle r2 = new Rectangle(10, 20);79    Rectangle r3 = new Rectangle(20, 10);8081    System.out.println("r1 hash: " + r1.hashCode());82    System.out.println("r2 hash: " + r2.hashCode());83    System.out.println("r3 hash: " + r3.hashCode());84    System.out.println("r1.equals(r2)? " + r1.equals(r2));85}8687static class Person {88    String name;89    int age;9091    Person(String name, int age) {92        this.name = name;93        this.age = age;94    }9596    @Override97    public boolean equals(Object obj) {98        if (this == obj) return true;99        if (obj == null || getClass() != obj.getClass()) return false;100        Person other = (Person) obj;101        return age == other.age && Objects.equals(name, other.name);102    }103104    @Override105    public int hashCode() {106        return Objects.hash(name, age);107    }108}109110static class Point {111    int x, y;112113    Point(int x, int y) {114        this.x = x;115        this.y = y;116    }117118    @Override119    public boolean equals(Object obj) {120        if (this == obj) return true;121        if (!(obj instanceof Point)) return false;122        Point other = (Point) obj;123        return x == other.x && y == other.y;124    }125126    @Override127    public int hashCode() {128        return Objects.hash(x, y);129    }130131    @Override132    public String toString() {133        return "(" + x + "," + y + ")";134    }135}136137static class Rectangle {138    int width, height;139140    Rectangle(int width10, int height20) {141        this.width→ 10 = width10;142        this.height→ 20 = height20;143    }⋯All 3 passes — pass 1 is the card abovepasswidthheightthis.widththis.heightr1r2r3
110201020Hash$Rectangle@50b——
210201020—Hash$Rectangle@50b—
320102010—Hash$Rectangle@50bHash$Rectangle@637

All 7 passes — pass 1 is the card above
pass	`name`	`age`	`this.name`	`this.age`	`p1`	`p2`	`p3`	`map`	`h1`	`h2`	`h3`
1	Alice	30	Alice	30	⟨Hash$Person A⟩	—	—	—	—	—	—
2	Alice	30	Alice	30	—	⟨Hash$Person A⟩	—	—	—	—	—
3	Bob	25	Bob	25	—	—	⟨Hash$Person B⟩	{}	—	—	—
4	Alice	30	Alice	30	—	—	—	—	—	—	—
5	Bob	25	Bob	25	—	—	—	—	—	—	—
6	Alice	30	Alice	30	—	—	—	—	—	—	—
7	Bob	25	Bob	25	—	—	—	—	750191325	750191325	29791

All 7 passes — pass 1 is the card above
pass	`x`	`y`	`this.x`	`this.y`	`pt1`	`pt2`	`pt3`	`points`
1	10	20	10	20	(10,20)	—	—	—
2	10	20	10	20	—	(10,20)	—	—
3	20	10	20	10	(10,20)	(10,20)	(20,10)	[]
4	1	2	1	2	—	—	—	—
5	3	4	3	4	—	—	—	—
6	1	2	1	2	—	—	—	—
7	1	2	1	2	—	—	—	—

All 3 passes — pass 1 is the card above
pass	`width`	`height`	`this.width`	`this.height`	`r1`	`r2`	`r3`
1	10	20	10	20	Hash$Rectangle@50b	—	—
2	10	20	10	20	—	Hash$Rectangle@50b	—
3	20	10	20	10	—	Hash$Rectangle@50b	Hash$Rectangle@637

Combined hash Objects.hash() combines multiple fields into a single hash code, ideal for implementing hashCode() in classes with multiple fields.

Null Validation

Enforce non-null requirements with clear error messages.

                            userId
                            userId = null (default)userId = "alice"userId = "guest"
                        

                            Require.java
                        
Replay: real traced execution (multi-file project)

                            // Objects.requireNonNull examples

import java.util.Objects;

public class Require {
    public static void main(String[] args) {
        System.out.println("Basic requireNonNull:");

        String valid = "hello";
        String invalid = null;

        String result = Objects.requireNonNull(valid);
        System.out.println("Valid: " + result);

        try {
            Objects.requireNonNull(invalid);
        } catch (NullPointerException e) {
            System.out.println("Caught NPE: " + e.getMessage());
        }
        System.out.println("\nWith message:");

        try {
            Objects.requireNonNull(null, "Value must not be null");
        } catch (NullPointerException e) {
            System.out.println("Message: " + e.getMessage());
        }
        System.out.println("\nConstructor validation:");

        try {
            Person p1 = new Person("Alice", 30);
            System.out.println("Created: " + p1);

            Person p2 = new Person(null, 25);
        } catch (NullPointerException e) {
            System.out.println("Constructor failed: " + e.getMessage());
        }
        System.out.println("\nMethod parameters:");

        EmailSender sender = new EmailSender();

        try {
            sender.send("user@example.com", "Hello", "Welcome!");
            System.out.println("Email sent successfully");
        } catch (NullPointerException e) {
            System.out.println("Failed: " + e.getMessage());
        }

        try {
            sender.send(null, "Hello", "Welcome!");
        } catch (NullPointerException e) {
            System.out.println("Failed: " + e.getMessage());
        }
        System.out.println("\nSupplier message:");

        String userId = null;

        try {
            Objects.requireNonNull(userId, () ->
                "User ID is required but was null in this request");
            System.out.println("User ID: " + userId);
        } catch (NullPointerException e) {
            System.out.println("Lazy message: " + e.getMessage());
        }
        System.out.println("\nChain validation:");

        Config config = new Config("localhost", 8080, "/api");
        System.out.println("Config: " + config);

        try {
            new Config(null, 8080, "/api");
        } catch (NullPointerException e) {
            System.out.println("Host required: " + e.getMessage());
        }
        System.out.println("\nReturn value validation:");

        UserService service = new UserService();

        User user = service.findUser("alice");
        System.out.println("Found: " + user);

        try {
            service.findUser("unknown");
        } catch (NullPointerException e) {
            System.out.println("Not found: " + e.getMessage());
        }
        System.out.println("\nDefensive copy:");

        String[] items = {"a", "b", "c"};
        Container c1 = new Container(items);
        System.out.println("Container: " + c1);

        try {
            new Container(null);
        } catch (NullPointerException e) {
            System.out.println("Null array: " + e.getMessage());
        }
    }

    static class Person {
        String name;
        int age;

        Person(String name, int age) {
            this.name = Objects.requireNonNull(name, "Name must not be null");
            this.age = age;
        }

        @Override
        public String toString() {
            return name + " (" + age + ")";
        }
    }

    static class EmailSender {
        void send(String to, String subject, String body) {
            Objects.requireNonNull(to, "Recipient must not be null");
            Objects.requireNonNull(subject, "Subject must not be null");
            Objects.requireNonNull(body, "Body must not be null");
            // Send email...
        }
    }

    static class Config {
        String host;
        int port;
        String path;

        Config(String host, int port, String path) {
            this.host = Objects.requireNonNull(host, "Host required");
            this.port = port;
            this.path = Objects.requireNonNull(path, "Path required");
        }

        @Override
        public String toString() {
            return host + ":" + port + path;
        }
    }

    static class UserService {
        User findUser(String id) {
            Objects.requireNonNull(id, "User ID required");

            if ("alice".equals(id)) {
                return new User("alice", "Alice");
            }

            throw new NullPointerException("User not found: " + id);
        }
    }

    static class User {
        String id;
        String name;

        User(String id, String name) {
            this.id = id;
            this.name = name;
        }

        @Override
        public String toString() {
            return name + " (id=" + id + ")";
        }
    }

    static class Container {
        String[] items;

        Container(String[] items) {
            this.items = Objects.requireNonNull(items, "Items array required")
                               .clone();  // Defensive copy
        }

        @Override
        public String toString() {
            return java.util.Arrays.toString(items);
        }
    }
}
// Objects.requireNonNull examples

import java.util.Objects;

public class Require {
    public static void main(String[] args) {
        System.out.println("Basic requireNonNull:");

        String valid = "hello";
        String invalid = null;

        String result = Objects.requireNonNull(valid);
        System.out.println("Valid: " + result);

        try {
            Objects.requireNonNull(invalid);
        } catch (NullPointerException e) {
            System.out.println("Caught NPE: " + e.getMessage());
        }
        System.out.println("\nWith message:");

        try {
            Objects.requireNonNull(null, "Value must not be null");
        } catch (NullPointerException e) {
            System.out.println("Message: " + e.getMessage());
        }
        System.out.println("\nConstructor validation:");

        try {
            Person p1 = new Person("Alice", 30);
            System.out.println("Created: " + p1);

            Person p2 = new Person(null, 25);
        } catch (NullPointerException e) {
            System.out.println("Constructor failed: " + e.getMessage());
        }
        System.out.println("\nMethod parameters:");

        EmailSender sender = new EmailSender();

        try {
            sender.send("user@example.com", "Hello", "Welcome!");
            System.out.println("Email sent successfully");
        } catch (NullPointerException e) {
            System.out.println("Failed: " + e.getMessage());
        }

        try {
            sender.send(null, "Hello", "Welcome!");
        } catch (NullPointerException e) {
            System.out.println("Failed: " + e.getMessage());
        }
        System.out.println("\nSupplier message:");

        String userId = "alice";

        try {
            Objects.requireNonNull(userId, () ->
                "User ID is required but was null in this request");
            System.out.println("User ID: " + userId);
        } catch (NullPointerException e) {
            System.out.println("Lazy message: " + e.getMessage());
        }
        System.out.println("\nChain validation:");

        Config config = new Config("localhost", 8080, "/api");
        System.out.println("Config: " + config);

        try {
            new Config(null, 8080, "/api");
        } catch (NullPointerException e) {
            System.out.println("Host required: " + e.getMessage());
        }
        System.out.println("\nReturn value validation:");

        UserService service = new UserService();

        User user = service.findUser("alice");
        System.out.println("Found: " + user);

        try {
            service.findUser("unknown");
        } catch (NullPointerException e) {
            System.out.println("Not found: " + e.getMessage());
        }
        System.out.println("\nDefensive copy:");

        String[] items = {"a", "b", "c"};
        Container c1 = new Container(items);
        System.out.println("Container: " + c1);

        try {
            new Container(null);
        } catch (NullPointerException e) {
            System.out.println("Null array: " + e.getMessage());
        }
    }

    static class Person {
        String name;
        int age;

        Person(String name, int age) {
            this.name = Objects.requireNonNull(name, "Name must not be null");
            this.age = age;
        }

        @Override
        public String toString() {
            return name + " (" + age + ")";
        }
    }

    static class EmailSender {
        void send(String to, String subject, String body) {
            Objects.requireNonNull(to, "Recipient must not be null");
            Objects.requireNonNull(subject, "Subject must not be null");
            Objects.requireNonNull(body, "Body must not be null");
            // Send email...
        }
    }

    static class Config {
        String host;
        int port;
        String path;

        Config(String host, int port, String path) {
            this.host = Objects.requireNonNull(host, "Host required");
            this.port = port;
            this.path = Objects.requireNonNull(path, "Path required");
        }

        @Override
        public String toString() {
            return host + ":" + port + path;
        }
    }

    static class UserService {
        User findUser(String id) {
            Objects.requireNonNull(id, "User ID required");

            if ("alice".equals(id)) {
                return new User("alice", "Alice");
            }

            throw new NullPointerException("User not found: " + id);
        }
    }

    static class User {
        String id;
        String name;

        User(String id, String name) {
            this.id = id;
            this.name = name;
        }

        @Override
        public String toString() {
            return name + " (id=" + id + ")";
        }
    }

    static class Container {
        String[] items;

        Container(String[] items) {
            this.items = Objects.requireNonNull(items, "Items array required")
                               .clone();  // Defensive copy
        }

        @Override
        public String toString() {
            return java.util.Arrays.toString(items);
        }
    }
}
// Objects.requireNonNull examples

import java.util.Objects;

public class Require {
    public static void main(String[] args) {
        System.out.println("Basic requireNonNull:");

        String valid = "hello";
        String invalid = null;

        String result = Objects.requireNonNull(valid);
        System.out.println("Valid: " + result);

        try {
            Objects.requireNonNull(invalid);
        } catch (NullPointerException e) {
            System.out.println("Caught NPE: " + e.getMessage());
        }
        System.out.println("\nWith message:");

        try {
            Objects.requireNonNull(null, "Value must not be null");
        } catch (NullPointerException e) {
            System.out.println("Message: " + e.getMessage());
        }
        System.out.println("\nConstructor validation:");

        try {
            Person p1 = new Person("Alice", 30);
            System.out.println("Created: " + p1);

            Person p2 = new Person(null, 25);
        } catch (NullPointerException e) {
            System.out.println("Constructor failed: " + e.getMessage());
        }
        System.out.println("\nMethod parameters:");

        EmailSender sender = new EmailSender();

        try {
            sender.send("user@example.com", "Hello", "Welcome!");
            System.out.println("Email sent successfully");
        } catch (NullPointerException e) {
            System.out.println("Failed: " + e.getMessage());
        }

        try {
            sender.send(null, "Hello", "Welcome!");
        } catch (NullPointerException e) {
            System.out.println("Failed: " + e.getMessage());
        }
        System.out.println("\nSupplier message:");

        String userId = "guest";

        try {
            Objects.requireNonNull(userId, () ->
                "User ID is required but was null in this request");
            System.out.println("User ID: " + userId);
        } catch (NullPointerException e) {
            System.out.println("Lazy message: " + e.getMessage());
        }
        System.out.println("\nChain validation:");

        Config config = new Config("localhost", 8080, "/api");
        System.out.println("Config: " + config);

        try {
            new Config(null, 8080, "/api");
        } catch (NullPointerException e) {
            System.out.println("Host required: " + e.getMessage());
        }
        System.out.println("\nReturn value validation:");

        UserService service = new UserService();

        User user = service.findUser("alice");
        System.out.println("Found: " + user);

        try {
            service.findUser("unknown");
        } catch (NullPointerException e) {
            System.out.println("Not found: " + e.getMessage());
        }
        System.out.println("\nDefensive copy:");

        String[] items = {"a", "b", "c"};
        Container c1 = new Container(items);
        System.out.println("Container: " + c1);

        try {
            new Container(null);
        } catch (NullPointerException e) {
            System.out.println("Null array: " + e.getMessage());
        }
    }

    static class Person {
        String name;
        int age;

        Person(String name, int age) {
            this.name = Objects.requireNonNull(name, "Name must not be null");
            this.age = age;
        }

        @Override
        public String toString() {
            return name + " (" + age + ")";
        }
    }

    static class EmailSender {
        void send(String to, String subject, String body) {
            Objects.requireNonNull(to, "Recipient must not be null");
            Objects.requireNonNull(subject, "Subject must not be null");
            Objects.requireNonNull(body, "Body must not be null");
            // Send email...
        }
    }

    static class Config {
        String host;
        int port;
        String path;

        Config(String host, int port, String path) {
            this.host = Objects.requireNonNull(host, "Host required");
            this.port = port;
            this.path = Objects.requireNonNull(path, "Path required");
        }

        @Override
        public String toString() {
            return host + ":" + port + path;
        }
    }

    static class UserService {
        User findUser(String id) {
            Objects.requireNonNull(id, "User ID required");

            if ("alice".equals(id)) {
                return new User("alice", "Alice");
            }

            throw new NullPointerException("User not found: " + id);
        }
    }

    static class User {
        String id;
        String name;

        User(String id, String name) {
            this.id = id;
            this.name = name;
        }

        @Override
        public String toString() {
            return name + " (id=" + id + ")";
        }
    }

    static class Container {
        String[] items;

        Container(String[] items) {
            this.items = Objects.requireNonNull(items, "Items array required")
                               .clone();  // Defensive copy
        }

        @Override
        public String toString() {
            return java.util.Arrays.toString(items);
        }
    }
}

                        
1valid ← hello, invalid ← null, result ← hello⋯5public class Require {6    public static void main(String[] args) {7        System.out.println("Basic requireNonNull:");89        String valid→ hello = "hello";10        String invalid→ null = null;1112        String result→ hello = Objects.requireNonNull(validhello);13        System.out.println("Valid: " + resulthello);⋯outputBasic requireNonNull:
Valid: hello
2try⋯15try {16    Objects.requireNonNull(invalidnull);17} catch (NullPointerException e) {⋯
3catch (NullPointerException e)⋯16    Objects.requireNonNull(invalid);17} catch (NullPointerException ejava.lang.NullPointerException) {18    System.out.println("Caught NPE: " + e.getMessage());19}⋯outputCaught NPE: null
4System.out.println(" With message:");⋯19}20System.out.println("\nWith message:");⋯output
With message:
5catch (NullPointerException e)⋯23    Objects.requireNonNull(null, "Value must not be null");24} catch (NullPointerException ejava.lang.NullPointerException: Value must not be null) {25    System.out.println("Message: " + e.getMessage());26}⋯outputMessage: Value must not be null
6System.out.println(" Constructor validation:");⋯26}27System.out.println("\nConstructor validation:");⋯output
Constructor validation:
7this.name ← Alice, this.age ← 30, p1 ← Alice (30)pass 1 of 2⋯29    try {30        Person p1→ Alice (30) = new Person("Alice", 30);31        System.out.println("Created: " + p1Alice (30));3233        Person p2 = new Person(null, 25);34    } catch (NullPointerException e) {35        System.out.println("Constructor failed: " + e.getMessage());36    }37    System.out.println("\nMethod parameters:");3839    EmailSender sender = new EmailSender();4041    try {42        sender.send("user@example.com", "Hello", "Welcome!");43        System.out.println("Email sent successfully");44    } catch (NullPointerException e) {45        System.out.println("Failed: " + e.getMessage());46    }4748    try {49        sender.send(null, "Hello", "Welcome!");50    } catch (NullPointerException e) {51        System.out.println("Failed: " + e.getMessage());52    }53    System.out.println("\nSupplier message:");5455    String userId = null;  //@userId="alice", "guest"5657    try {58        Objects.requireNonNull(userId, () ->59            "User ID is required but was null in this request");60        System.out.println("User ID: " + userId);61    } catch (NullPointerException e) {62        System.out.println("Lazy message: " + e.getMessage());63    }64    System.out.println("\nChain validation:");6566    Config config = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + config);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String nameAlice, int age30) {104        this.name→ Alice = Objects.requireNonNull(nameAlice, "Name must not be null");105        this.age→ 30 = age30;106    }⋯outputCreated: Alice (30)
8Person(String name, int age)pass 2 of 2⋯103Person(String namenull, int age25) {104    this.name = Objects.requireNonNull(namenull, "Name must not be null");105    this.age = age;⋯
9catch (NullPointerException e)⋯33    Person p2 = new Person(null, 25);34} catch (NullPointerException ejava.lang.NullPointerException: Name must not be null) {35    System.out.println("Constructor failed: " + e.getMessage());36}⋯outputConstructor failed: Name must not be null
10sender ← ⟨Require$EmailSender A⟩⋯36}37System.out.println("\nMethod parameters:");3839EmailSender sender→ ⟨Require$EmailSender A⟩ = new EmailSender();⋯output
Method parameters:
11void send(String to, String subject, String body)pass 1 of 2⋯41    try {42        sender.send("user@example.com", "Hello", "Welcome!");43        System.out.println("Email sent successfully");44    } catch (NullPointerException e) {45        System.out.println("Failed: " + e.getMessage());46    }4748    try {49        sender.send(null, "Hello", "Welcome!");50    } catch (NullPointerException e) {51        System.out.println("Failed: " + e.getMessage());52    }53    System.out.println("\nSupplier message:");5455    String userId = null;  //@userId="alice", "guest"5657    try {58        Objects.requireNonNull(userId, () ->59            "User ID is required but was null in this request");60        System.out.println("User ID: " + userId);61    } catch (NullPointerException e) {62        System.out.println("Lazy message: " + e.getMessage());63    }64    System.out.println("\nChain validation:");6566    Config config = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + config);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subjectHello, String bodyWelcome!) {116        Objects.requireNonNull(touser@example.com, "Recipient must not be null");117        Objects.requireNonNull(subjectHello, "Subject must not be null");118        Objects.requireNonNull(bodyWelcome!, "Body must not be null");119        // Send email...⋯outputEmail sent successfully
12void send(String to, String subject, String body)pass 2 of 2⋯114static class EmailSender {115    void send(String to, String subjectHello, String bodyWelcome!) {116        Objects.requireNonNull(tonull, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");⋯
13catch (NullPointerException e)⋯49    sender.send(null, "Hello", "Welcome!");50} catch (NullPointerException ejava.lang.NullPointerException: Recipient must not be null) {51    System.out.println("Failed: " + e.getMessage());52}⋯outputFailed: Recipient must not be null
14userId ← null⋯52}53System.out.println("\nSupplier message:");5455String userId→ null = null;  //@userId="alice", "guest"⋯output
Supplier message:
15try⋯57try {58    Objects.requireNonNull(userIdnull, () ->59        "User ID is required but was null in this request");60    System.out.println("User ID: " + userId);⋯
16catch (NullPointerException e)⋯60    System.out.println("User ID: " + userId);61} catch (NullPointerException ejava.lang.NullPointerException: User ID is required but was null in this request) {62    System.out.println("Lazy message: " + e.getMessage());63}⋯outputLazy message: User ID is required but was null in this request
17System.out.println(" Chain validation:");⋯63}64System.out.println("\nChain validation:");6566Config config = new Config("localhost", 8080, "/api");67System.out.println("Config: " + config);⋯output
Chain validation:
18this.host ← localhost, this.port ← 8080, this.path ← /api, config ← localhost:8080/apipass 1 of 2⋯66    Config config→ localhost:8080/api = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + configlocalhost:8080/api);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String hostlocalhost, int port8080, String path/api) {129        this.host→ localhost = Objects.requireNonNull(hostlocalhost, "Host required");130        this.port→ 8080 = port8080;131        this.path→ /api = Objects.requireNonNull(path/api, "Path required");132    }⋯outputConfig: localhost:8080/api
19Config(String host, int port, String path)pass 2 of 2⋯128Config(String hostnull, int port8080, String path/api) {129    this.host = Objects.requireNonNull(hostnull, "Host required");130    this.port = port;⋯
20catch (NullPointerException e)⋯70    new Config(null, 8080, "/api");71} catch (NullPointerException ejava.lang.NullPointerException: Host required) {72    System.out.println("Host required: " + e.getMessage());73}⋯outputHost required: Host required
21service ← ⟨Require$UserService B⟩⋯73}74System.out.println("\nReturn value validation:");7576UserService service→ ⟨Require$UserService B⟩ = new UserService();7778User user = service.findUser("alice");79System.out.println("Found: " + user);⋯output
Return value validation:
22User findUser(String id)pass 1 of 2⋯140static class UserService {141    User findUser(String idalice) {142        Objects.requireNonNull(idalice, "User ID required");⋯
23if ("alice".equals(id))⋯144if ("alice".equals(idalice)) {145    return new User("alice", "Alice");146}⋯
24this.id ← alice, this.name ← Alice, user ← Alice (id=alice)⋯78    User user→ Alice (id=alice) = service.findUser("alice");79    System.out.println("Found: " + userAlice (id=alice));8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String host, int port, String path) {129        this.host = Objects.requireNonNull(host, "Host required");130        this.port = port;131        this.path = Objects.requireNonNull(path, "Path required");132    }133134    @Override135    public String toString() {136        return host + ":" + port + path;137    }138}139140static class UserService {141    User findUser(String id) {142        Objects.requireNonNull(id, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }150}151152static class User {153    String id;154    String name;155156    User(String idalice, String nameAlice) {157        this.id→ alice = idalice;158        this.name→ Alice = nameAlice;159    }⋯outputFound: Alice (id=alice)
25User findUser(String id)pass 2 of 2⋯140static class UserService {141    User findUser(String idunknown) {142        Objects.requireNonNull(idunknown, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }⋯
26catch (NullPointerException e)⋯82    service.findUser("unknown");83} catch (NullPointerException ejava.lang.NullPointerException: User not found: unknown) {84    System.out.println("Not found: " + e.getMessage());85}⋯outputNot found: User not found: unknown
27String[] items = {"a", "b", "c"};⋯85}86System.out.println("\nDefensive copy:");8788String[] items = {"a", "b", "c"};89Container c1 = new Container(items);90System.out.println("Container: " + c1);⋯output
Defensive copy:
28c1 ← [a, b, c]pass 1 of 2⋯88    String[] items = {"a", "b", "c"};89    Container c1→ [a, b, c] = new Container(items);90    System.out.println("Container: " + c1[a, b, c]);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String host, int port, String path) {129        this.host = Objects.requireNonNull(host, "Host required");130        this.port = port;131        this.path = Objects.requireNonNull(path, "Path required");132    }133134    @Override135    public String toString() {136        return host + ":" + port + path;137    }138}139140static class UserService {141    User findUser(String id) {142        Objects.requireNonNull(id, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }150}151152static class User {153    String id;154    String name;155156    User(String id, String name) {157        this.id = id;158        this.name = name;159    }160161    @Override162    public String toString() {163        return name + " (id=" + id + ")";164    }165}166167static class Container {168    String[] items;169170    Container(String[] items) {171        this.items = Objects.requireNonNull(items, "Items array required")172                           .clone();  // Defensive copy173    }⋯outputContainer: [a, b, c]
29Container(String[] items)pass 2 of 2⋯170Container(String[] itemsnull) {171    this.items = Objects.requireNonNull(itemsnull, "Items array required")172                       .clone();  // Defensive copy173}⋯
30catch (NullPointerException e)⋯93    new Container(null);94} catch (NullPointerException ejava.lang.NullPointerException: Items array required) {95    System.out.println("Null array: " + e.getMessage());96}⋯outputNull array: Items array required
1valid ← hello, invalid ← null, result ← hello⋯5public class Require {6    public static void main(String[] args) {7        System.out.println("Basic requireNonNull:");89        String valid→ hello = "hello";10        String invalid→ null = null;1112        String result→ hello = Objects.requireNonNull(validhello);13        System.out.println("Valid: " + resulthello);⋯outputBasic requireNonNull:
Valid: hello
2try⋯15try {16    Objects.requireNonNull(invalidnull);17} catch (NullPointerException e) {⋯
3catch (NullPointerException e)⋯16    Objects.requireNonNull(invalid);17} catch (NullPointerException ejava.lang.NullPointerException) {18    System.out.println("Caught NPE: " + e.getMessage());19}⋯outputCaught NPE: null
4System.out.println(" With message:");⋯19}20System.out.println("\nWith message:");⋯output
With message:
5catch (NullPointerException e)⋯23    Objects.requireNonNull(null, "Value must not be null");24} catch (NullPointerException ejava.lang.NullPointerException: Value must not be null) {25    System.out.println("Message: " + e.getMessage());26}⋯outputMessage: Value must not be null
6System.out.println(" Constructor validation:");⋯26}27System.out.println("\nConstructor validation:");⋯output
Constructor validation:
7this.name ← Alice, this.age ← 30, p1 ← Alice (30)pass 1 of 2⋯29    try {30        Person p1→ Alice (30) = new Person("Alice", 30);31        System.out.println("Created: " + p1Alice (30));3233        Person p2 = new Person(null, 25);34    } catch (NullPointerException e) {35        System.out.println("Constructor failed: " + e.getMessage());36    }37    System.out.println("\nMethod parameters:");3839    EmailSender sender = new EmailSender();4041    try {42        sender.send("user@example.com", "Hello", "Welcome!");43        System.out.println("Email sent successfully");44    } catch (NullPointerException e) {45        System.out.println("Failed: " + e.getMessage());46    }4748    try {49        sender.send(null, "Hello", "Welcome!");50    } catch (NullPointerException e) {51        System.out.println("Failed: " + e.getMessage());52    }53    System.out.println("\nSupplier message:");5455    String userId = "alice";5657    try {58        Objects.requireNonNull(userId, () ->59            "User ID is required but was null in this request");60        System.out.println("User ID: " + userId);61    } catch (NullPointerException e) {62        System.out.println("Lazy message: " + e.getMessage());63    }64    System.out.println("\nChain validation:");6566    Config config = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + config);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String nameAlice, int age30) {104        this.name→ Alice = Objects.requireNonNull(nameAlice, "Name must not be null");105        this.age→ 30 = age30;106    }⋯outputCreated: Alice (30)
8Person(String name, int age)pass 2 of 2⋯103Person(String namenull, int age25) {104    this.name = Objects.requireNonNull(namenull, "Name must not be null");105    this.age = age;⋯
9catch (NullPointerException e)⋯33    Person p2 = new Person(null, 25);34} catch (NullPointerException ejava.lang.NullPointerException: Name must not be null) {35    System.out.println("Constructor failed: " + e.getMessage());36}⋯outputConstructor failed: Name must not be null
10sender ← ⟨Require$EmailSender A⟩⋯36}37System.out.println("\nMethod parameters:");3839EmailSender sender→ ⟨Require$EmailSender A⟩ = new EmailSender();⋯output
Method parameters:
11void send(String to, String subject, String body)pass 1 of 2⋯41    try {42        sender.send("user@example.com", "Hello", "Welcome!");43        System.out.println("Email sent successfully");44    } catch (NullPointerException e) {45        System.out.println("Failed: " + e.getMessage());46    }4748    try {49        sender.send(null, "Hello", "Welcome!");50    } catch (NullPointerException e) {51        System.out.println("Failed: " + e.getMessage());52    }53    System.out.println("\nSupplier message:");5455    String userId = "alice";5657    try {58        Objects.requireNonNull(userId, () ->59            "User ID is required but was null in this request");60        System.out.println("User ID: " + userId);61    } catch (NullPointerException e) {62        System.out.println("Lazy message: " + e.getMessage());63    }64    System.out.println("\nChain validation:");6566    Config config = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + config);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subjectHello, String bodyWelcome!) {116        Objects.requireNonNull(touser@example.com, "Recipient must not be null");117        Objects.requireNonNull(subjectHello, "Subject must not be null");118        Objects.requireNonNull(bodyWelcome!, "Body must not be null");119        // Send email...⋯outputEmail sent successfully
12void send(String to, String subject, String body)pass 2 of 2⋯114static class EmailSender {115    void send(String to, String subjectHello, String bodyWelcome!) {116        Objects.requireNonNull(tonull, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");⋯
13catch (NullPointerException e)⋯49    sender.send(null, "Hello", "Welcome!");50} catch (NullPointerException ejava.lang.NullPointerException: Recipient must not be null) {51    System.out.println("Failed: " + e.getMessage());52}⋯outputFailed: Recipient must not be null
14userId ← alice⋯52}53System.out.println("\nSupplier message:");5455String userId→ alice = "alice";⋯output
Supplier message:
15try⋯57try {58    Objects.requireNonNull(userIdalice, () ->59        "User ID is required but was null in this request");60    System.out.println("User ID: " + userIdalice);61} catch (NullPointerException e) {⋯outputUser ID: alice
16System.out.println(" Chain validation:");⋯63}64System.out.println("\nChain validation:");6566Config config = new Config("localhost", 8080, "/api");67System.out.println("Config: " + config);⋯output
Chain validation:
17this.host ← localhost, this.port ← 8080, this.path ← /api, config ← localhost:8080/apipass 1 of 2⋯66    Config config→ localhost:8080/api = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + configlocalhost:8080/api);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String hostlocalhost, int port8080, String path/api) {129        this.host→ localhost = Objects.requireNonNull(hostlocalhost, "Host required");130        this.port→ 8080 = port8080;131        this.path→ /api = Objects.requireNonNull(path/api, "Path required");132    }⋯outputConfig: localhost:8080/api
18Config(String host, int port, String path)pass 2 of 2⋯128Config(String hostnull, int port8080, String path/api) {129    this.host = Objects.requireNonNull(hostnull, "Host required");130    this.port = port;⋯
19catch (NullPointerException e)⋯70    new Config(null, 8080, "/api");71} catch (NullPointerException ejava.lang.NullPointerException: Host required) {72    System.out.println("Host required: " + e.getMessage());73}⋯outputHost required: Host required
20service ← ⟨Require$UserService B⟩⋯73}74System.out.println("\nReturn value validation:");7576UserService service→ ⟨Require$UserService B⟩ = new UserService();7778User user = service.findUser("alice");79System.out.println("Found: " + user);⋯output
Return value validation:
21User findUser(String id)pass 1 of 2⋯140static class UserService {141    User findUser(String idalice) {142        Objects.requireNonNull(idalice, "User ID required");⋯
22if ("alice".equals(id))⋯144if ("alice".equals(idalice)) {145    return new User("alice", "Alice");146}⋯
23this.id ← alice, this.name ← Alice, user ← Alice (id=alice)⋯78    User user→ Alice (id=alice) = service.findUser("alice");79    System.out.println("Found: " + userAlice (id=alice));8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String host, int port, String path) {129        this.host = Objects.requireNonNull(host, "Host required");130        this.port = port;131        this.path = Objects.requireNonNull(path, "Path required");132    }133134    @Override135    public String toString() {136        return host + ":" + port + path;137    }138}139140static class UserService {141    User findUser(String id) {142        Objects.requireNonNull(id, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }150}151152static class User {153    String id;154    String name;155156    User(String idalice, String nameAlice) {157        this.id→ alice = idalice;158        this.name→ Alice = nameAlice;159    }⋯outputFound: Alice (id=alice)
24User findUser(String id)pass 2 of 2⋯140static class UserService {141    User findUser(String idunknown) {142        Objects.requireNonNull(idunknown, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }⋯
25catch (NullPointerException e)⋯82    service.findUser("unknown");83} catch (NullPointerException ejava.lang.NullPointerException: User not found: unknown) {84    System.out.println("Not found: " + e.getMessage());85}⋯outputNot found: User not found: unknown
26String[] items = {"a", "b", "c"};⋯85}86System.out.println("\nDefensive copy:");8788String[] items = {"a", "b", "c"};89Container c1 = new Container(items);90System.out.println("Container: " + c1);⋯output
Defensive copy:
27c1 ← [a, b, c]pass 1 of 2⋯88    String[] items = {"a", "b", "c"};89    Container c1→ [a, b, c] = new Container(items);90    System.out.println("Container: " + c1[a, b, c]);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String host, int port, String path) {129        this.host = Objects.requireNonNull(host, "Host required");130        this.port = port;131        this.path = Objects.requireNonNull(path, "Path required");132    }133134    @Override135    public String toString() {136        return host + ":" + port + path;137    }138}139140static class UserService {141    User findUser(String id) {142        Objects.requireNonNull(id, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }150}151152static class User {153    String id;154    String name;155156    User(String id, String name) {157        this.id = id;158        this.name = name;159    }160161    @Override162    public String toString() {163        return name + " (id=" + id + ")";164    }165}166167static class Container {168    String[] items;169170    Container(String[] items) {171        this.items = Objects.requireNonNull(items, "Items array required")172                           .clone();  // Defensive copy173    }⋯outputContainer: [a, b, c]
28Container(String[] items)pass 2 of 2⋯170Container(String[] itemsnull) {171    this.items = Objects.requireNonNull(itemsnull, "Items array required")172                       .clone();  // Defensive copy173}⋯
29catch (NullPointerException e)⋯93    new Container(null);94} catch (NullPointerException ejava.lang.NullPointerException: Items array required) {95    System.out.println("Null array: " + e.getMessage());96}⋯outputNull array: Items array required
1valid ← hello, invalid ← null, result ← hello⋯5public class Require {6    public static void main(String[] args) {7        System.out.println("Basic requireNonNull:");89        String valid→ hello = "hello";10        String invalid→ null = null;1112        String result→ hello = Objects.requireNonNull(validhello);13        System.out.println("Valid: " + resulthello);⋯outputBasic requireNonNull:
Valid: hello
2try⋯15try {16    Objects.requireNonNull(invalidnull);17} catch (NullPointerException e) {⋯
3catch (NullPointerException e)⋯16    Objects.requireNonNull(invalid);17} catch (NullPointerException ejava.lang.NullPointerException) {18    System.out.println("Caught NPE: " + e.getMessage());19}⋯outputCaught NPE: null
4System.out.println(" With message:");⋯19}20System.out.println("\nWith message:");⋯output
With message:
5catch (NullPointerException e)⋯23    Objects.requireNonNull(null, "Value must not be null");24} catch (NullPointerException ejava.lang.NullPointerException: Value must not be null) {25    System.out.println("Message: " + e.getMessage());26}⋯outputMessage: Value must not be null
6System.out.println(" Constructor validation:");⋯26}27System.out.println("\nConstructor validation:");⋯output
Constructor validation:
7this.name ← Alice, this.age ← 30, p1 ← Alice (30)pass 1 of 2⋯29    try {30        Person p1→ Alice (30) = new Person("Alice", 30);31        System.out.println("Created: " + p1Alice (30));3233        Person p2 = new Person(null, 25);34    } catch (NullPointerException e) {35        System.out.println("Constructor failed: " + e.getMessage());36    }37    System.out.println("\nMethod parameters:");3839    EmailSender sender = new EmailSender();4041    try {42        sender.send("user@example.com", "Hello", "Welcome!");43        System.out.println("Email sent successfully");44    } catch (NullPointerException e) {45        System.out.println("Failed: " + e.getMessage());46    }4748    try {49        sender.send(null, "Hello", "Welcome!");50    } catch (NullPointerException e) {51        System.out.println("Failed: " + e.getMessage());52    }53    System.out.println("\nSupplier message:");5455    String userId = "guest";5657    try {58        Objects.requireNonNull(userId, () ->59            "User ID is required but was null in this request");60        System.out.println("User ID: " + userId);61    } catch (NullPointerException e) {62        System.out.println("Lazy message: " + e.getMessage());63    }64    System.out.println("\nChain validation:");6566    Config config = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + config);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String nameAlice, int age30) {104        this.name→ Alice = Objects.requireNonNull(nameAlice, "Name must not be null");105        this.age→ 30 = age30;106    }⋯outputCreated: Alice (30)
8Person(String name, int age)pass 2 of 2⋯103Person(String namenull, int age25) {104    this.name = Objects.requireNonNull(namenull, "Name must not be null");105    this.age = age;⋯
9catch (NullPointerException e)⋯33    Person p2 = new Person(null, 25);34} catch (NullPointerException ejava.lang.NullPointerException: Name must not be null) {35    System.out.println("Constructor failed: " + e.getMessage());36}⋯outputConstructor failed: Name must not be null
10sender ← ⟨Require$EmailSender A⟩⋯36}37System.out.println("\nMethod parameters:");3839EmailSender sender→ ⟨Require$EmailSender A⟩ = new EmailSender();⋯output
Method parameters:
11void send(String to, String subject, String body)pass 1 of 2⋯41    try {42        sender.send("user@example.com", "Hello", "Welcome!");43        System.out.println("Email sent successfully");44    } catch (NullPointerException e) {45        System.out.println("Failed: " + e.getMessage());46    }4748    try {49        sender.send(null, "Hello", "Welcome!");50    } catch (NullPointerException e) {51        System.out.println("Failed: " + e.getMessage());52    }53    System.out.println("\nSupplier message:");5455    String userId = "guest";5657    try {58        Objects.requireNonNull(userId, () ->59            "User ID is required but was null in this request");60        System.out.println("User ID: " + userId);61    } catch (NullPointerException e) {62        System.out.println("Lazy message: " + e.getMessage());63    }64    System.out.println("\nChain validation:");6566    Config config = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + config);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subjectHello, String bodyWelcome!) {116        Objects.requireNonNull(touser@example.com, "Recipient must not be null");117        Objects.requireNonNull(subjectHello, "Subject must not be null");118        Objects.requireNonNull(bodyWelcome!, "Body must not be null");119        // Send email...⋯outputEmail sent successfully
12void send(String to, String subject, String body)pass 2 of 2⋯114static class EmailSender {115    void send(String to, String subjectHello, String bodyWelcome!) {116        Objects.requireNonNull(tonull, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");⋯
13catch (NullPointerException e)⋯49    sender.send(null, "Hello", "Welcome!");50} catch (NullPointerException ejava.lang.NullPointerException: Recipient must not be null) {51    System.out.println("Failed: " + e.getMessage());52}⋯outputFailed: Recipient must not be null
14userId ← guest⋯52}53System.out.println("\nSupplier message:");5455String userId→ guest = "guest";⋯output
Supplier message:
15try⋯57try {58    Objects.requireNonNull(userIdguest, () ->59        "User ID is required but was null in this request");60    System.out.println("User ID: " + userIdguest);61} catch (NullPointerException e) {⋯outputUser ID: guest
16System.out.println(" Chain validation:");⋯63}64System.out.println("\nChain validation:");6566Config config = new Config("localhost", 8080, "/api");67System.out.println("Config: " + config);⋯output
Chain validation:
17this.host ← localhost, this.port ← 8080, this.path ← /api, config ← localhost:8080/apipass 1 of 2⋯66    Config config→ localhost:8080/api = new Config("localhost", 8080, "/api");67    System.out.println("Config: " + configlocalhost:8080/api);6869    try {70        new Config(null, 8080, "/api");71    } catch (NullPointerException e) {72        System.out.println("Host required: " + e.getMessage());73    }74    System.out.println("\nReturn value validation:");7576    UserService service = new UserService();7778    User user = service.findUser("alice");79    System.out.println("Found: " + user);8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String hostlocalhost, int port8080, String path/api) {129        this.host→ localhost = Objects.requireNonNull(hostlocalhost, "Host required");130        this.port→ 8080 = port8080;131        this.path→ /api = Objects.requireNonNull(path/api, "Path required");132    }⋯outputConfig: localhost:8080/api
18Config(String host, int port, String path)pass 2 of 2⋯128Config(String hostnull, int port8080, String path/api) {129    this.host = Objects.requireNonNull(hostnull, "Host required");130    this.port = port;⋯
19catch (NullPointerException e)⋯70    new Config(null, 8080, "/api");71} catch (NullPointerException ejava.lang.NullPointerException: Host required) {72    System.out.println("Host required: " + e.getMessage());73}⋯outputHost required: Host required
20service ← ⟨Require$UserService B⟩⋯73}74System.out.println("\nReturn value validation:");7576UserService service→ ⟨Require$UserService B⟩ = new UserService();7778User user = service.findUser("alice");79System.out.println("Found: " + user);⋯output
Return value validation:
21User findUser(String id)pass 1 of 2⋯140static class UserService {141    User findUser(String idalice) {142        Objects.requireNonNull(idalice, "User ID required");⋯
22if ("alice".equals(id))⋯144if ("alice".equals(idalice)) {145    return new User("alice", "Alice");146}⋯
23this.id ← alice, this.name ← Alice, user ← Alice (id=alice)⋯78    User user→ Alice (id=alice) = service.findUser("alice");79    System.out.println("Found: " + userAlice (id=alice));8081    try {82        service.findUser("unknown");83    } catch (NullPointerException e) {84        System.out.println("Not found: " + e.getMessage());85    }86    System.out.println("\nDefensive copy:");8788    String[] items = {"a", "b", "c"};89    Container c1 = new Container(items);90    System.out.println("Container: " + c1);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String host, int port, String path) {129        this.host = Objects.requireNonNull(host, "Host required");130        this.port = port;131        this.path = Objects.requireNonNull(path, "Path required");132    }133134    @Override135    public String toString() {136        return host + ":" + port + path;137    }138}139140static class UserService {141    User findUser(String id) {142        Objects.requireNonNull(id, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }150}151152static class User {153    String id;154    String name;155156    User(String idalice, String nameAlice) {157        this.id→ alice = idalice;158        this.name→ Alice = nameAlice;159    }⋯outputFound: Alice (id=alice)
24User findUser(String id)pass 2 of 2⋯140static class UserService {141    User findUser(String idunknown) {142        Objects.requireNonNull(idunknown, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }⋯
25catch (NullPointerException e)⋯82    service.findUser("unknown");83} catch (NullPointerException ejava.lang.NullPointerException: User not found: unknown) {84    System.out.println("Not found: " + e.getMessage());85}⋯outputNot found: User not found: unknown
26String[] items = {"a", "b", "c"};⋯85}86System.out.println("\nDefensive copy:");8788String[] items = {"a", "b", "c"};89Container c1 = new Container(items);90System.out.println("Container: " + c1);⋯output
Defensive copy:
27c1 ← [a, b, c]pass 1 of 2⋯88    String[] items = {"a", "b", "c"};89    Container c1→ [a, b, c] = new Container(items);90    System.out.println("Container: " + c1[a, b, c]);9192    try {93        new Container(null);94    } catch (NullPointerException e) {95        System.out.println("Null array: " + e.getMessage());96    }97}9899static class Person {100    String name;101    int age;102103    Person(String name, int age) {104        this.name = Objects.requireNonNull(name, "Name must not be null");105        this.age = age;106    }107108    @Override109    public String toString() {110        return name + " (" + age + ")";111    }112}113114static class EmailSender {115    void send(String to, String subject, String body) {116        Objects.requireNonNull(to, "Recipient must not be null");117        Objects.requireNonNull(subject, "Subject must not be null");118        Objects.requireNonNull(body, "Body must not be null");119        // Send email...120    }121}122123static class Config {124    String host;125    int port;126    String path;127128    Config(String host, int port, String path) {129        this.host = Objects.requireNonNull(host, "Host required");130        this.port = port;131        this.path = Objects.requireNonNull(path, "Path required");132    }133134    @Override135    public String toString() {136        return host + ":" + port + path;137    }138}139140static class UserService {141    User findUser(String id) {142        Objects.requireNonNull(id, "User ID required");143144        if ("alice".equals(id)) {145            return new User("alice", "Alice");146        }147148        throw new NullPointerException("User not found: " + id);149    }150}151152static class User {153    String id;154    String name;155156    User(String id, String name) {157        this.id = id;158        this.name = name;159    }160161    @Override162    public String toString() {163        return name + " (id=" + id + ")";164    }165}166167static class Container {168    String[] items;169170    Container(String[] items) {171        this.items = Objects.requireNonNull(items, "Items array required")172                           .clone();  // Defensive copy173    }⋯outputContainer: [a, b, c]
28Container(String[] items)pass 2 of 2⋯170Container(String[] itemsnull) {171    this.items = Objects.requireNonNull(itemsnull, "Items array required")172                       .clone();  // Defensive copy173}⋯
29catch (NullPointerException e)⋯93    new Container(null);94} catch (NullPointerException ejava.lang.NullPointerException: Items array required) {95    System.out.println("Null array: " + e.getMessage());96}⋯outputNull array: Items array required

Fail-fast validation Objects.requireNonNull() throws NullPointerException immediately with a descriptive message, catching bugs early.

String Conversion

Convert objects to strings safely.

                            Tostring.java
                        
Replay: real traced execution (multi-file project)

                            // Objects.toString examples

import java.util.Objects;

public class Tostring {
    public static void main(String[] args) {
        System.out.println("Basic toString:");

        String s1 = "hello";
        String s2 = null;

        System.out.println("s1.toString(): " + s1.toString());
        System.out.println("Objects.toString(s1): " + Objects.toString(s1));
        System.out.println("Objects.toString(null): " + Objects.toString(s2));

        // This would throw NPE:
        // System.out.println(s2.toString());
        System.out.println("\nWith default:");

        String name = null;
        String result = Objects.toString(name, "Unknown");
        System.out.println("Name: " + result);

        Integer age = null;
        String ageStr = Objects.toString(age, "N/A");
        System.out.println("Age: " + ageStr);
        System.out.println("\nArray toString:");

        int[] numbers = {1, 2, 3, 4, 5};
        int[] empty = null;

        System.out.println("Array: " + Objects.toString(numbers));
        System.out.println("Null array: " + Objects.toString(empty, "[]"));

        // Better for arrays:
        System.out.println("Arrays.toString: " + java.util.Arrays.toString(numbers));
        System.out.println("\nCustom objects:");

        Person p1 = new Person("Alice", 30);
        Person p2 = null;

        System.out.println("Person: " + Objects.toString(p1));
        System.out.println("Null person: " + Objects.toString(p2, "No person"));
        System.out.println("\nCollections:");

        java.util.List<String> list = java.util.Arrays.asList("a", "b", "c");
        java.util.List<String> nullList = null;

        System.out.println("List: " + Objects.toString(list));
        System.out.println("Null list: " + Objects.toString(nullList, "[]"));
        System.out.println("\nFormatting output:");

        String username = "alice";
        String email = null;
        String phone = "555-1234";

        System.out.println("Username: " + Objects.toString(username, "N/A"));
        System.out.println("Email: " + Objects.toString(email, "N/A"));
        System.out.println("Phone: " + Objects.toString(phone, "N/A"));
        System.out.println("\nIn logging:");

        Config config = new Config();
        config.host = "localhost";
        config.port = 8080;
        config.database = null;

        System.out.println("Config:");
        System.out.println("  Host: " + Objects.toString(config.host, "default"));
        System.out.println("  Port: " + config.port);
        System.out.println("  DB: " + Objects.toString(config.database, "not configured"));
        System.out.println("\nOptional values:");

        Integer count = getCount();
        String status = getStatus();

        System.out.println("Count: " + Objects.toString(count, "0"));
        System.out.println("Status: " + Objects.toString(status, "unknown"));
        System.out.println("\nDebug output:");

        Object obj1 = "hello";
        Object obj2 = 123;
        Object obj3 = null;
        Object obj4 = new Person("Bob", 25);

        printDebug("obj1", obj1);
        printDebug("obj2", obj2);
        printDebug("obj3", obj3);
        printDebug("obj4", obj4);
    }

    static Integer getCount() {
        return null;  // Simulate optional value
    }

    static String getStatus() {
        return null;  // Simulate optional value
    }

    static void printDebug(String name, Object value) {
        System.out.println(name + " = " + Objects.toString(value, "<null>"));
    }

    static class Person {
        String name;
        int age;

        Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        @Override
        public String toString() {
            return "Person{name='" + name + "', age=" + age + "}";
        }
    }

    static class Config {
        String host;
        int port;
        String database;
    }
}

                        
1s1 ← hello, s2 ← null, name ← null, result ← Unknown, age ← null⋯5public class Tostring {6    public static void main(String[] args) {7        System.out.println("Basic toString:");89        String s1→ hello = "hello";10        String s2→ null = null;1112        System.out.println("s1.toString(): " + s1.toString());13        System.out.println("Objects.toString(s1): " + Objects.toString(s1hello));14        System.out.println("Objects.toString(null): " + Objects.toString(s2null));1516        // This would throw NPE:17        // System.out.println(s2.toString());18        System.out.println("\nWith default:");1920        String name→ null = null;21        String result→ Unknown = Objects.toString(namenull, "Unknown");22        System.out.println("Name: " + resultUnknown);2324        Integer age→ null = null;25        String ageStr→ N/A = Objects.toString(agenull, "N/A");26        System.out.println("Age: " + ageStrN/A);27        System.out.println("\nArray toString:");2829        int[] numbers = {1, 2, 3, 4, 5};30        int[] empty→ null = null;3132        System.out.println("Array: " + Objects.toString(numbers));33        System.out.println("Null array: " + Objects.toString(emptynull, "[]"));3435        // Better for arrays:36        System.out.println("Arrays.toString: " + java.util.Arrays.toString(numbers));37        System.out.println("\nCustom objects:");3839        Person p1 = new Person("Alice", 30);40        Person p2 = null;⋯outputBasic toString:
s1.toString(): hello
Objects.toString(s1): hello
Objects.toString(null): null

With default:
Name: Unknown
Age: N/A

Array toString:
Array: ⟨int[] A⟩
Null array: []
Arrays.toString: [1, 2, 3, 4, 5]

Custom objects:
2this.name ← Alice, this.age ← 30, p1 ← Person{name='Alice', age=30}pass 1 of 2⋯39    Person p1→ Person{name='Alice', age=30} = new Person("Alice", 30);40    Person p2→ null = null;4142    System.out.println("Person: " + Objects.toString(p1Person{name='Alice', age=30}));43    System.out.println("Null person: " + Objects.toString(p2null, "No person"));44    System.out.println("\nCollections:");4546    java.util.List<String> list→ [a, b, c] = java.util.Arrays.asList("a", "b", "c");47    java.util.List<String> nullList→ null = null;4849    System.out.println("List: " + Objects.toString(list[a, b, c]));50    System.out.println("Null list: " + Objects.toString(nullListnull, "[]"));51    System.out.println("\nFormatting output:");5253    String username→ alice = "alice";54    String email→ null = null;55    String phone→ 555-1234 = "555-1234";5657    System.out.println("Username: " + Objects.toString(usernamealice, "N/A"));58    System.out.println("Email: " + Objects.toString(emailnull, "N/A"));59    System.out.println("Phone: " + Objects.toString(phone555-1234, "N/A"));60    System.out.println("\nIn logging:");6162    Config config→ ⟨Tostring$Config B⟩ = new Config();63    config.host→ localhost = "localhost";64    config.port→ 8080 = 8080;65    config.database→ null = null;6667    System.out.println("Config:");68    System.out.println("  Host: " + Objects.toString(config.hostlocalhost, "default"));69    System.out.println("  Port: " + config.port8080);70    System.out.println("  DB: " + Objects.toString(config.databasenull, "not configured"));71    System.out.println("\nOptional values:");7273    Integer count = getCount();74    String status = getStatus();7576    System.out.println("Count: " + Objects.toString(count, "0"));77    System.out.println("Status: " + Objects.toString(status, "unknown"));78    System.out.println("\nDebug output:");7980    Object obj1 = "hello";81    Object obj2 = 123;82    Object obj3 = null;83    Object obj4 = new Person("Bob", 25);8485    printDebug("obj1", obj1);86    printDebug("obj2", obj2);87    printDebug("obj3", obj3);88    printDebug("obj4", obj4);89}9091static Integer getCount() {92    return null;  // Simulate optional value93}9495static String getStatus() {96    return null;  // Simulate optional value97}9899static void printDebug(String name, Object value) {100    System.out.println(name + " = " + Objects.toString(value, "<null>"));101}102103static class Person {104    String name;105    int age;106107    Person(String nameAlice, int age30) {108        this.name→ Alice = nameAlice;109        this.age→ 30 = age30;110    }⋯outputPerson: Person{name='Alice', age=30}
Null person: No person

Collections:
List: [a, b, c]
Null list: []

Formatting output:
Username: alice
Email: N/A
Phone: 555-1234

In logging:
Config:
  Host: localhost
  Port: 8080
  DB: not configured

Optional values:
3count ← null⋯73Integer count→ null = getCount();74String status = getStatus();⋯
4status ← null, obj1 ← hello, obj2 ← 123, obj3 ← null⋯73Integer count = getCount();74String status→ null = getStatus();7576System.out.println("Count: " + Objects.toString(countnull, "0"));77System.out.println("Status: " + Objects.toString(statusnull, "unknown"));78System.out.println("\nDebug output:");7980Object obj1→ hello = "hello";81Object obj2→ 123 = 123;82Object obj3→ null = null;83Object obj4 = new Person("Bob", 25);⋯outputCount: 0
Status: unknown

Debug output:
5this.name ← Bob, this.age ← 25, obj4 ← Person{name='Bob', age=25}pass 2 of 2⋯82    Object obj3 = null;83    Object obj4→ Person{name='Bob', age=25} = new Person("Bob", 25);8485    printDebug("obj1", obj1hello);86    printDebug("obj2", obj2);87    printDebug("obj3", obj3);88    printDebug("obj4", obj4);89}9091static Integer getCount() {92    return null;  // Simulate optional value93}9495static String getStatus() {96    return null;  // Simulate optional value97}9899static void printDebug(String name, Object value) {100    System.out.println(name + " = " + Objects.toString(value, "<null>"));101}102103static class Person {104    String name;105    int age;106107    Person(String nameBob, int age25) {108        this.name→ Bob = nameBob;109        this.age→ 25 = age25;110    }⋯
6static void printDebug(String name, Object value)pass 1 of 4⋯85    printDebug("obj1", obj1hello);86    printDebug("obj2", obj2123);87    printDebug("obj3", obj3);88    printDebug("obj4", obj4);89}9091static Integer getCount() {92    return null;  // Simulate optional value93}9495static String getStatus() {96    return null;  // Simulate optional value97}9899static void printDebug(String nameobj1, Object valuehello) {100    System.out.println(nameobj1 + " = " + Objects.toString(valuehello, "<null>"));101}⋯outputobj1 = helloAll 4 passes — pass 1 is the card abovepassnamevalueobj1obj2obj3obj4
1obj1hellohello123——
2obj2123—123null—
3obj3null——nullPerson{name='Bob', age=25}
4obj4Person{name='Bob', age=25}———Person{name='Bob', age=25}

All 4 passes — pass 1 is the card above
pass	`name`	`value`	`obj1`	`obj2`	`obj3`	`obj4`
1	obj1	hello	hello	123	—	—
2	obj2	123	—	123	null	—
3	obj3	null	—	—	null	Person{name='Bob', age=25}
4	obj4	Person{name='Bob', age=25}	—	—	—	Person{name='Bob', age=25}

Comparison Operations

Compare objects with null handling.

                            Compare.java
                        
Replay: real traced execution (multi-file project)

                            // Objects.compare examples

import java.util.Objects;
import java.util.Comparator;

public class Compare {
    public static void main(String[] args) {
        System.out.println("Basic compare:");

        String s1 = "apple";
        String s2 = "banana";
        String s3 = "apple";

        int result = Objects.compare(s1, s2, String::compareTo);
        System.out.println("compare('apple', 'banana'): " + result);

        result = Objects.compare(s1, s3, String::compareTo);
        System.out.println("compare('apple', 'apple'): " + result);
        System.out.println("\nWith nulls:");

        String n1 = null;
        String n2 = "hello";

        // Nulls-first comparator
        Comparator<String> nullsFirst = Comparator.nullsFirst(String::compareTo);

        result = Objects.compare(n1, n2, nullsFirst);
        System.out.println("compare(null, 'hello', nullsFirst): " + result);

        result = Objects.compare(n2, n1, nullsFirst);
        System.out.println("compare('hello', null, nullsFirst): " + result);

        result = Objects.compare(n1, n1, nullsFirst);
        System.out.println("compare(null, null, nullsFirst): " + result);
        System.out.println("\nNulls-last:");

        Comparator<String> nullsLast = Comparator.nullsLast(String::compareTo);

        result = Objects.compare(null, "hello", nullsLast);
        System.out.println("compare(null, 'hello', nullsLast): " + result);

        result = Objects.compare("hello", null, nullsLast);
        System.out.println("compare('hello', null, nullsLast): " + result);
        System.out.println("\nNumbers:");

        Integer i1 = 10;
        Integer i2 = 20;
        Integer i3 = null;

        result = Objects.compare(i1, i2, Integer::compareTo);
        System.out.println("compare(10, 20): " + result);

        result = Objects.compare(i1, i3, Comparator.nullsLast(Integer::compareTo));
        System.out.println("compare(10, null): " + result);
        System.out.println("\nCustom objects:");

        Person p1 = new Person("Alice", 30);
        Person p2 = new Person("Bob", 25);
        Person p3 = new Person("Charlie", 30);

        // Compare by age
        result = Objects.compare(p1, p2, Comparator.comparing(p -> p.age));
        System.out.println("Alice vs Bob (by age): " + result);

        result = Objects.compare(p1, p3, Comparator.comparing(p -> p.age));
        System.out.println("Alice vs Charlie (by age): " + result);

        // Compare by name
        result = Objects.compare(p1, p2, Comparator.comparing(p -> p.name));
        System.out.println("Alice vs Bob (by name): " + result);
        System.out.println("\nSorting:");

        java.util.List<Person> people = new java.util.ArrayList<>();
        people.add(new Person("Charlie", 30));
        people.add(new Person("Alice", 25));
        people.add(new Person("Bob", 35));

        System.out.println("Before sort:");
        people.forEach(System.out::println);

        people.sort((a, b) -> Objects.compare(a, b, Comparator.comparing(p -> p.name)));

        System.out.println("\nAfter sort by name:");
        people.forEach(System.out::println);
        System.out.println("\nMultiple criteria:");

        java.util.List<Person> students = new java.util.ArrayList<>();
        students.add(new Person("Alice", 30));
        students.add(new Person("Bob", 25));
        students.add(new Person("Charlie", 30));
        students.add(new Person("Alice", 25));

        Comparator<Person> byAgeThenName =
            Comparator.comparing((Person p) -> p.age)
                      .thenComparing(p -> p.name);

        students.sort((a, b) -> Objects.compare(a, b, byAgeThenName));

        System.out.println("Sorted by age then name:");
        students.forEach(System.out::println);
        System.out.println("\nMin/Max:");

        Person youngest = people.stream()
            .min((a, b) -> Objects.compare(a, b, Comparator.comparing(p -> p.age)))
            .orElse(null);

        Person oldest = people.stream()
            .max((a, b) -> Objects.compare(a, b, Comparator.comparing(p -> p.age)))
            .orElse(null);

        System.out.println("Youngest: " + youngest);
        System.out.println("Oldest: " + oldest);
    }

    static class Person {
        String name;
        int age;

        Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        @Override
        public String toString() {
            return name + " (" + age + ")";
        }
    }
}

                        
1s1 ← apple, s2 ← banana, s3 ← apple, result ← -1, n1 ← null, n2 ← hello⋯6public class Compare {7    public static void main(String[] args) {8        System.out.println("Basic compare:");910        String s1→ apple = "apple";11        String s2→ banana = "banana";12        String s3→ apple = "apple";1314        int result→ -1 = Objects.compare(s1apple, s2banana, String::compareTo);15        System.out.println("compare('apple', 'banana'): " + result-1);1617        result→ 0 = Objects.compare(s1apple, s3apple, String::compareTo);18        System.out.println("compare('apple', 'apple'): " + result0);19        System.out.println("\nWith nulls:");2021        String n1→ null = null;22        String n2→ hello = "hello";2324        // Nulls-first comparator25        Comparator<String> nullsFirst→ ⟨Comparators$NullComparator A⟩ = Comparator.nullsFirst(String::compareTo);2627        result→ -1 = Objects.compare(n1null, n2hello, nullsFirst⟨Comparators$NullComparator A⟩);28        System.out.println("compare(null, 'hello', nullsFirst): " + result-1);2930        result→ 1 = Objects.compare(n2hello, n1null, nullsFirst⟨Comparators$NullComparator A⟩);31        System.out.println("compare('hello', null, nullsFirst): " + result1);3233        result→ 0 = Objects.compare(n1null, n1, nullsFirst⟨Comparators$NullComparator A⟩);34        System.out.println("compare(null, null, nullsFirst): " + result0);35        System.out.println("\nNulls-last:");3637        Comparator<String> nullsLast→ ⟨Comparators$NullComparator B⟩ = Comparator.nullsLast(String::compareTo);3839        result→ 1 = Objects.compare(null, "hello", nullsLast⟨Comparators$NullComparator B⟩);40        System.out.println("compare(null, 'hello', nullsLast): " + result1);4142        result→ -1 = Objects.compare("hello", null, nullsLast⟨Comparators$NullComparator B⟩);43        System.out.println("compare('hello', null, nullsLast): " + result-1);44        System.out.println("\nNumbers:");4546        Integer i1→ 10 = 10;47        Integer i2→ 20 = 20;48        Integer i3→ null = null;4950        result→ -1 = Objects.compare(i110, i220, Integer::compareTo);51        System.out.println("compare(10, 20): " + result-1);5253        result→ -1 = Objects.compare(i110, i3null, Comparator.nullsLast(Integer::compareTo));54        System.out.println("compare(10, null): " + result-1);55        System.out.println("\nCustom objects:");5657        Person p1 = new Person("Alice", 30);58        Person p2 = new Person("Bob", 25);⋯outputBasic compare:
compare('apple', 'banana'): -1
compare('apple', 'apple'): 0

With nulls:
compare(null, 'hello', nullsFirst): -1
compare('hello', null, nullsFirst): 1
compare(null, null, nullsFirst): 0

Nulls-last:
compare(null, 'hello', nullsLast): 1
compare('hello', null, nullsLast): -1

Numbers:
compare(10, 20): -1
compare(10, null): -1

Custom objects:
2this.name ← Alice, this.age ← 30, p1 ← Alice (30)pass 1 of 10⋯57    Person p1→ Alice (30) = new Person("Alice", 30);58    Person p2 = new Person("Bob", 25);59    Person p3 = new Person("Charlie", 30);6061    // Compare by age62    result = Objects.compare(p1, p2, Comparator.comparing(p -> p.age));63    System.out.println("Alice vs Bob (by age): " + result);6465    result = Objects.compare(p1, p3, Comparator.comparing(p -> p.age));66    System.out.println("Alice vs Charlie (by age): " + result);6768    // Compare by name69    result = Objects.compare(p1, p2, Comparator.comparing(p -> p.name));70    System.out.println("Alice vs Bob (by name): " + result);71    System.out.println("\nSorting:");7273    java.util.List<Person> people = new java.util.ArrayList<>();74    people.add(new Person("Charlie", 30));75    people.add(new Person("Alice", 25));76    people.add(new Person("Bob", 35));7778    System.out.println("Before sort:");79    people.forEach(System.out::println);8081    people.sort((a, b) -> Objects.compare(a, b, Comparator.comparing(p -> p.name)));8283    System.out.println("\nAfter sort by name:");84    people.forEach(System.out::println);85    System.out.println("\nMultiple criteria:");8687    java.util.List<Person> students = new java.util.ArrayList<>();88    students.add(new Person("Alice", 30));89    students.add(new Person("Bob", 25));90    students.add(new Person("Charlie", 30));91    students.add(new Person("Alice", 25));9293    Comparator<Person> byAgeThenName =94        Comparator.comparing((Person p) -> p.age)95                  .thenComparing(p -> p.name);9697    students.sort((a, b) -> Objects.compare(a, b, byAgeThenName));9899    System.out.println("Sorted by age then name:");100    students.forEach(System.out::println);101    System.out.println("\nMin/Max:");102103    Person youngest = people.stream()104        .min((a, b) -> Objects.compare(a, b, Comparator.comparing(p -> p.age)))105        .orElse(null);106107    Person oldest = people.stream()108        .max((a, b) -> Objects.compare(a, b, Comparator.comparing(p -> p.age)))109        .orElse(null);110111    System.out.println("Youngest: " + youngest);112    System.out.println("Oldest: " + oldest);113}114115static class Person {116    String name;117    int age;118119    Person(String nameAlice, int age30) {120        this.name→ Alice = nameAlice;121        this.age→ 30 = age30;122    }⋯All 10 passes — pass 1 is the card abovepassnameagethis.namethis.agep1p2p3resultpeoplestudentsbyAgeThenNameyoungestoldest
1Alice30Alice30Alice (30)————————
2Bob25Bob25—Bob (25)———————
3Charlie30Charlie30Alice (30)Bob (25)Charlie (30)1[]————
4Charlie30Charlie30—————————
5Alice25Alice25—————————
6Bob35Bob35—————[]———
7Alice30Alice30—————————
8Bob25Bob25—————————
9Charlie30Charlie30—————————
10Alice25Alice25——————⟨Comparator lambda C⟩Alice (25)Bob (35)

All 10 passes — pass 1 is the card above
pass	`name`	`age`	`this.name`	`this.age`	`p1`	`p2`	`p3`	`result`	`people`	`students`	`byAgeThenName`	`youngest`	`oldest`
1	Alice	30	Alice	30	Alice (30)	—	—	—	—	—	—	—	—
2	Bob	25	Bob	25	—	Bob (25)	—	—	—	—	—	—	—
3	Charlie	30	Charlie	30	Alice (30)	Bob (25)	Charlie (30)	1	[]	—	—	—	—
4	Charlie	30	Charlie	30	—	—	—	—	—	—	—	—	—
5	Alice	25	Alice	25	—	—	—	—	—	—	—	—	—
6	Bob	35	Bob	35	—	—	—	—	—	[]	—	—	—
7	Alice	30	Alice	30	—	—	—	—	—	—	—	—	—
8	Bob	25	Bob	25	—	—	—	—	—	—	—	—	—
9	Charlie	30	Charlie	30	—	—	—	—	—	—	—	—	—
10	Alice	25	Alice	25	—	—	—	—	—	—	⟨Comparator lambda C⟩	Alice (25)	Bob (35)

@seealso collections_util

Exercise: Practical.java

Implement equals and hashCode for a Product class using Objects methods