Data Types

Type Conversion

Changing Data Types

A web form returns the user's age as text: "25". To check if they're old enough to vote (age >= 18), you need to convert "25" to the number 25. Type conversion bridges the gap between data formats.

Int to double (precise division)

When dividing integers, you might want a decimal result.

example
IntToDouble.java
Replay: real traced execution (multi-file project)
public class IntToDouble {
    public static void main(String[] args) {
        int a = 7;
        int b = 3;

        // Integer division - truncates!
        int intResult = a / b;
        System.out.println("Integer division: " + a + "/" + b + " = " + intResult);

        // Convert to double for precise division
        double doubleResult = (double) a / b;
        System.out.println("Double division: " + a + "/" + b + " = " + doubleResult);

        // Practical: calculate percentage
        int score = 85;
        int total = 100;
        double percentage = (double) score / total * 100;
        System.out.println("Percentage: " + percentage + "%");
    }
}

public class IntToDouble {
    public static void main(String[] args) {
        int a = 5;
        int b = 3;

        // Integer division - truncates!
        int intResult = a / b;
        System.out.println("Integer division: " + a + "/" + b + " = " + intResult);

        // Convert to double for precise division
        double doubleResult = (double) a / b;
        System.out.println("Double division: " + a + "/" + b + " = " + doubleResult);

        // Practical: calculate percentage
        int score = 85;
        int total = 100;
        double percentage = (double) score / total * 100;
        System.out.println("Percentage: " + percentage + "%");
    }
}

public class IntToDouble {
    public static void main(String[] args) {
        int a = 10;
        int b = 3;

        // Integer division - truncates!
        int intResult = a / b;
        System.out.println("Integer division: " + a + "/" + b + " = " + intResult);

        // Convert to double for precise division
        double doubleResult = (double) a / b;
        System.out.println("Double division: " + a + "/" + b + " = " + doubleResult);

        // Practical: calculate percentage
        int score = 85;
        int total = 100;
        double percentage = (double) score / total * 100;
        System.out.println("Percentage: " + percentage + "%");
    }
}

public class IntToDouble {
    public static void main(String[] args) {
        int a = 7;
        int b = 2;

        // Integer division - truncates!
        int intResult = a / b;
        System.out.println("Integer division: " + a + "/" + b + " = " + intResult);

        // Convert to double for precise division
        double doubleResult = (double) a / b;
        System.out.println("Double division: " + a + "/" + b + " = " + doubleResult);

        // Practical: calculate percentage
        int score = 85;
        int total = 100;
        double percentage = (double) score / total * 100;
        System.out.println("Percentage: " + percentage + "%");
    }
}

public class IntToDouble {
    public static void main(String[] args) {
        int a = 7;
        int b = 4;

        // Integer division - truncates!
        int intResult = a / b;
        System.out.println("Integer division: " + a + "/" + b + " = " + intResult);

        // Convert to double for precise division
        double doubleResult = (double) a / b;
        System.out.println("Double division: " + a + "/" + b + " = " + doubleResult);

        // Practical: calculate percentage
        int score = 85;
        int total = 100;
        double percentage = (double) score / total * 100;
        System.out.println("Percentage: " + percentage + "%");
    }
}

  1. a ← 7, b ← 3, intResult ← 2, doubleResult ← 2.3333333333333335

    1public class IntToDouble {2    public static void main(String[] args) {3        int a→ 7 = 7;   //@a=5, 104        int b→ 3 = 3;   //@b=2, 45        6        // Integer division - truncates!7        int intResult→ 2 = a7 / b3;8        System.out.println("Integer division: " + a7 + "/" + b3 + " = " + intResult2);9        10        // Convert to double for precise division11        double doubleResult→ 2.3333333333333335 = (double) a / b3;12        System.out.println("Double division: " + a7 + "/" + b3 + " = " + doubleResult2.3333333333333335);13        14        // Practical: calculate percentage15        int score→ 85 = 85;16        int total→ 100 = 100;17        double percentage→ 85.0 = (double) score / total100 * 100;18        System.out.println("Percentage: " + percentage85.0 + "%");19    }
    outputInteger division: 7/3 = 2
Double division: 7/3 = 2.3333333333333335
Percentage: 85.0%

  1. a ← 5, b ← 3, intResult ← 1, doubleResult ← 1.6666666666666667

    1public class IntToDouble {2    public static void main(String[] args) {3        int a→ 5 = 5;4        int b→ 3 = 3;5        6        // Integer division - truncates!7        int intResult→ 1 = a5 / b3;8        System.out.println("Integer division: " + a5 + "/" + b3 + " = " + intResult1);9        10        // Convert to double for precise division11        double doubleResult→ 1.6666666666666667 = (double) a / b3;12        System.out.println("Double division: " + a5 + "/" + b3 + " = " + doubleResult1.6666666666666667);13        14        // Practical: calculate percentage15        int score→ 85 = 85;16        int total→ 100 = 100;17        double percentage→ 85.0 = (double) score / total100 * 100;18        System.out.println("Percentage: " + percentage85.0 + "%");19    }
    outputInteger division: 5/3 = 1
Double division: 5/3 = 1.6666666666666667
Percentage: 85.0%

  1. a ← 10, b ← 3, intResult ← 3, doubleResult ← 3.3333333333333335

    1public class IntToDouble {2    public static void main(String[] args) {3        int a→ 10 = 10;4        int b→ 3 = 3;5        6        // Integer division - truncates!7        int intResult→ 3 = a10 / b3;8        System.out.println("Integer division: " + a10 + "/" + b3 + " = " + intResult3);9        10        // Convert to double for precise division11        double doubleResult→ 3.3333333333333335 = (double) a / b3;12        System.out.println("Double division: " + a10 + "/" + b3 + " = " + doubleResult3.3333333333333335);13        14        // Practical: calculate percentage15        int score→ 85 = 85;16        int total→ 100 = 100;17        double percentage→ 85.0 = (double) score / total100 * 100;18        System.out.println("Percentage: " + percentage85.0 + "%");19    }
    outputInteger division: 10/3 = 3
Double division: 10/3 = 3.3333333333333335
Percentage: 85.0%

  1. a ← 7, b ← 2, intResult ← 3, doubleResult ← 3.5, score ← 85, total ← 100

    1public class IntToDouble {2    public static void main(String[] args) {3        int a→ 7 = 7;4        int b→ 2 = 2;5        6        // Integer division - truncates!7        int intResult→ 3 = a7 / b2;8        System.out.println("Integer division: " + a7 + "/" + b2 + " = " + intResult3);9        10        // Convert to double for precise division11        double doubleResult→ 3.5 = (double) a / b2;12        System.out.println("Double division: " + a7 + "/" + b2 + " = " + doubleResult3.5);13        14        // Practical: calculate percentage15        int score→ 85 = 85;16        int total→ 100 = 100;17        double percentage→ 85.0 = (double) score / total100 * 100;18        System.out.println("Percentage: " + percentage85.0 + "%");19    }
    outputInteger division: 7/2 = 3
Double division: 7/2 = 3.5
Percentage: 85.0%

  1. a ← 7, b ← 4, intResult ← 1, doubleResult ← 1.75, score ← 85, total ← 100

    1public class IntToDouble {2    public static void main(String[] args) {3        int a→ 7 = 7;4        int b→ 4 = 4;5        6        // Integer division - truncates!7        int intResult→ 1 = a7 / b4;8        System.out.println("Integer division: " + a7 + "/" + b4 + " = " + intResult1);9        10        // Convert to double for precise division11        double doubleResult→ 1.75 = (double) a / b4;12        System.out.println("Double division: " + a7 + "/" + b4 + " = " + doubleResult1.75);13        14        // Practical: calculate percentage15        int score→ 85 = 85;16        int total→ 100 = 100;17        double percentage→ 85.0 = (double) score / total100 * 100;18        System.out.println("Percentage: " + percentage85.0 + "%");19    }
    outputInteger division: 7/4 = 1
Double division: 7/4 = 1.75
Percentage: 85.0%

Integer division truncates. Convert to double for precise results.

cast Explicit conversion: `(int)3.7` → `3`, `(double)5` → `5.0`

Double to int (truncation)

Sometimes you need a whole number from a decimal.

price
DoubleToInt.java
Replay: real traced execution (multi-file project)
public class DoubleToInt {
    public static void main(String[] args) {
        double price = 19.99;

        // Cast truncates (drops decimals)
        int truncated = (int) price;
        System.out.println("Price: " + price);
        System.out.println("Truncated: " + truncated);

        // Math.round() rounds to nearest
        long rounded = Math.round(price);
        System.out.println("Rounded: " + rounded);

        // Floor and ceiling
        int floor = (int) Math.floor(price);
        int ceiling = (int) Math.ceil(price);
        System.out.println("Floor: " + floor);
        System.out.println("Ceiling: " + ceiling);

        // Practical: convert dollars to cents (avoid floating point)
        int cents = (int) Math.round(price * 100);
        System.out.println("Cents: " + cents);
    }
}

public class DoubleToInt {
    public static void main(String[] args) {
        double price = 25.50;

        // Cast truncates (drops decimals)
        int truncated = (int) price;
        System.out.println("Price: " + price);
        System.out.println("Truncated: " + truncated);

        // Math.round() rounds to nearest
        long rounded = Math.round(price);
        System.out.println("Rounded: " + rounded);

        // Floor and ceiling
        int floor = (int) Math.floor(price);
        int ceiling = (int) Math.ceil(price);
        System.out.println("Floor: " + floor);
        System.out.println("Ceiling: " + ceiling);

        // Practical: convert dollars to cents (avoid floating point)
        int cents = (int) Math.round(price * 100);
        System.out.println("Cents: " + cents);
    }
}

public class DoubleToInt {
    public static void main(String[] args) {
        double price = 9.95;

        // Cast truncates (drops decimals)
        int truncated = (int) price;
        System.out.println("Price: " + price);
        System.out.println("Truncated: " + truncated);

        // Math.round() rounds to nearest
        long rounded = Math.round(price);
        System.out.println("Rounded: " + rounded);

        // Floor and ceiling
        int floor = (int) Math.floor(price);
        int ceiling = (int) Math.ceil(price);
        System.out.println("Floor: " + floor);
        System.out.println("Ceiling: " + ceiling);

        // Practical: convert dollars to cents (avoid floating point)
        int cents = (int) Math.round(price * 100);
        System.out.println("Cents: " + cents);
    }
}

  1. price ← 19.99, truncated ← 19, rounded ← 20, floor ← 19, ceiling ← 20

    1public class DoubleToInt {2    public static void main(String[] args) {3        double price→ 19.99 = 19.99;   //@price=25.50, 9.954        5        // Cast truncates (drops decimals)6        int truncated→ 19 = (int) price;7        System.out.println("Price: " + price19.99);8        System.out.println("Truncated: " + truncated19);9        10        // Math.round() rounds to nearest11        long rounded→ 20 = Math.round(price19.99);12        System.out.println("Rounded: " + rounded20);13        14        // Floor and ceiling15        int floor→ 19 = (int) Math.floor(price19.99);16        int ceiling→ 20 = (int) Math.ceil(price19.99);17        System.out.println("Floor: " + floor19);18        System.out.println("Ceiling: " + ceiling20);19        20        // Practical: convert dollars to cents (avoid floating point)21        int cents→ 1999 = (int) Math.round(price19.99 * 100);22        System.out.println("Cents: " + cents1999);23    }
    outputPrice: 19.99
Truncated: 19
Rounded: 20
Floor: 19
Ceiling: 20
Cents: 1999

  1. price ← 25.5, truncated ← 25, rounded ← 26, floor ← 25, ceiling ← 26

    1public class DoubleToInt {2    public static void main(String[] args) {3        double price→ 25.5 = 25.50;4        5        // Cast truncates (drops decimals)6        int truncated→ 25 = (int) price;7        System.out.println("Price: " + price25.5);8        System.out.println("Truncated: " + truncated25);9        10        // Math.round() rounds to nearest11        long rounded→ 26 = Math.round(price25.5);12        System.out.println("Rounded: " + rounded26);13        14        // Floor and ceiling15        int floor→ 25 = (int) Math.floor(price25.5);16        int ceiling→ 26 = (int) Math.ceil(price25.5);17        System.out.println("Floor: " + floor25);18        System.out.println("Ceiling: " + ceiling26);19        20        // Practical: convert dollars to cents (avoid floating point)21        int cents→ 2550 = (int) Math.round(price25.5 * 100);22        System.out.println("Cents: " + cents2550);23    }
    outputPrice: 25.5
Truncated: 25
Rounded: 26
Floor: 25
Ceiling: 26
Cents: 2550

  1. price ← 9.95, truncated ← 9, rounded ← 10, floor ← 9, ceiling ← 10

    1public class DoubleToInt {2    public static void main(String[] args) {3        double price→ 9.95 = 9.95;4        5        // Cast truncates (drops decimals)6        int truncated→ 9 = (int) price;7        System.out.println("Price: " + price9.95);8        System.out.println("Truncated: " + truncated9);9        10        // Math.round() rounds to nearest11        long rounded→ 10 = Math.round(price9.95);12        System.out.println("Rounded: " + rounded10);13        14        // Floor and ceiling15        int floor→ 9 = (int) Math.floor(price9.95);16        int ceiling→ 10 = (int) Math.ceil(price9.95);17        System.out.println("Floor: " + floor9);18        System.out.println("Ceiling: " + ceiling10);19        20        // Practical: convert dollars to cents (avoid floating point)21        int cents→ 995 = (int) Math.round(price9.95 * 100);22        System.out.println("Cents: " + cents995);23    }
    outputPrice: 9.95
Truncated: 9
Rounded: 10
Floor: 9
Ceiling: 10
Cents: 995

Casting to int truncates (drops decimals). Use Math.round() to round.

String to number (parse input)

User input is text. Convert to numbers for calculation.

example
Parse.java
Replay: real traced execution (multi-file project)
public class Parse {
    public static void main(String[] args) {
        // Simulated user input
        String ageInput = "25";
        String priceInput = "19.99";

        // Parse string to number
        int age = Integer.parseInt(ageInput);
        double price = Double.parseDouble(priceInput);

        System.out.println("Age string: " + ageInput);
        System.out.println("Age number: " + age);
        System.out.println("Age + 1 = " + (age + 1));

        System.out.println("Price string: " + priceInput);
        System.out.println("Price number: " + price);
        System.out.println("With tax: " + (price * 1.1));

        // Also works for other types
        boolean active = Boolean.parseBoolean("true");
        System.out.println("Active: " + active);
    }
}

public class Parse {
    public static void main(String[] args) {
        // Simulated user input
        String ageInput = "30";
        String priceInput = "19.99";

        // Parse string to number
        int age = Integer.parseInt(ageInput);
        double price = Double.parseDouble(priceInput);

        System.out.println("Age string: " + ageInput);
        System.out.println("Age number: " + age);
        System.out.println("Age + 1 = " + (age + 1));

        System.out.println("Price string: " + priceInput);
        System.out.println("Price number: " + price);
        System.out.println("With tax: " + (price * 1.1));

        // Also works for other types
        boolean active = Boolean.parseBoolean("true");
        System.out.println("Active: " + active);
    }
}

public class Parse {
    public static void main(String[] args) {
        // Simulated user input
        String ageInput = "17";
        String priceInput = "19.99";

        // Parse string to number
        int age = Integer.parseInt(ageInput);
        double price = Double.parseDouble(priceInput);

        System.out.println("Age string: " + ageInput);
        System.out.println("Age number: " + age);
        System.out.println("Age + 1 = " + (age + 1));

        System.out.println("Price string: " + priceInput);
        System.out.println("Price number: " + price);
        System.out.println("With tax: " + (price * 1.1));

        // Also works for other types
        boolean active = Boolean.parseBoolean("true");
        System.out.println("Active: " + active);
    }
}

public class Parse {
    public static void main(String[] args) {
        // Simulated user input
        String ageInput = "25";
        String priceInput = "9.95";

        // Parse string to number
        int age = Integer.parseInt(ageInput);
        double price = Double.parseDouble(priceInput);

        System.out.println("Age string: " + ageInput);
        System.out.println("Age number: " + age);
        System.out.println("Age + 1 = " + (age + 1));

        System.out.println("Price string: " + priceInput);
        System.out.println("Price number: " + price);
        System.out.println("With tax: " + (price * 1.1));

        // Also works for other types
        boolean active = Boolean.parseBoolean("true");
        System.out.println("Active: " + active);
    }
}

public class Parse {
    public static void main(String[] args) {
        // Simulated user input
        String ageInput = "25";
        String priceInput = "100.00";

        // Parse string to number
        int age = Integer.parseInt(ageInput);
        double price = Double.parseDouble(priceInput);

        System.out.println("Age string: " + ageInput);
        System.out.println("Age number: " + age);
        System.out.println("Age + 1 = " + (age + 1));

        System.out.println("Price string: " + priceInput);
        System.out.println("Price number: " + price);
        System.out.println("With tax: " + (price * 1.1));

        // Also works for other types
        boolean active = Boolean.parseBoolean("true");
        System.out.println("Active: " + active);
    }
}

  1. ageInput ← 25, priceInput ← 19.99, age ← 25, price ← 19.99, active ← true

    1public class Parse {2    public static void main(String[] args) {3        // Simulated user input4        String ageInput→ 25 = "25";       //@ageInput="30", "17"5        String priceInput→ 19.99 = "19.99";  //@priceInput="9.95", "100.00"6        7        // Parse string to number8        int age→ 25 = Integer.parseInt(ageInput25);9        double price→ 19.99 = Double.parseDouble(priceInput19.99);10        11        System.out.println("Age string: " + ageInput25);12        System.out.println("Age number: " + age25);13        System.out.println("Age + 1 = " + (age25 + 1));14        15        System.out.println("Price string: " + priceInput19.99);16        System.out.println("Price number: " + price19.99);17        System.out.println("With tax: " + (price19.99 * 1.1));18        19        // Also works for other types20        boolean active→ true = Boolean.parseBoolean("true");21        System.out.println("Active: " + activetrue);22    }
    outputAge string: 25
Age number: 25
Age + 1 = 26
Price string: 19.99
Price number: 19.99
With tax: 21.989
Active: true

  1. ageInput ← 30, priceInput ← 19.99, age ← 30, price ← 19.99, active ← true

    1public class Parse {2    public static void main(String[] args) {3        // Simulated user input4        String ageInput→ 30 = "30";5        String priceInput→ 19.99 = "19.99";6        7        // Parse string to number8        int age→ 30 = Integer.parseInt(ageInput30);9        double price→ 19.99 = Double.parseDouble(priceInput19.99);10        11        System.out.println("Age string: " + ageInput30);12        System.out.println("Age number: " + age30);13        System.out.println("Age + 1 = " + (age30 + 1));14        15        System.out.println("Price string: " + priceInput19.99);16        System.out.println("Price number: " + price19.99);17        System.out.println("With tax: " + (price19.99 * 1.1));18        19        // Also works for other types20        boolean active→ true = Boolean.parseBoolean("true");21        System.out.println("Active: " + activetrue);22    }
    outputAge string: 30
Age number: 30
Age + 1 = 31
Price string: 19.99
Price number: 19.99
With tax: 21.989
Active: true

  1. ageInput ← 17, priceInput ← 19.99, age ← 17, price ← 19.99, active ← true

    1public class Parse {2    public static void main(String[] args) {3        // Simulated user input4        String ageInput→ 17 = "17";5        String priceInput→ 19.99 = "19.99";6        7        // Parse string to number8        int age→ 17 = Integer.parseInt(ageInput17);9        double price→ 19.99 = Double.parseDouble(priceInput19.99);10        11        System.out.println("Age string: " + ageInput17);12        System.out.println("Age number: " + age17);13        System.out.println("Age + 1 = " + (age17 + 1));14        15        System.out.println("Price string: " + priceInput19.99);16        System.out.println("Price number: " + price19.99);17        System.out.println("With tax: " + (price19.99 * 1.1));18        19        // Also works for other types20        boolean active→ true = Boolean.parseBoolean("true");21        System.out.println("Active: " + activetrue);22    }
    outputAge string: 17
Age number: 17
Age + 1 = 18
Price string: 19.99
Price number: 19.99
With tax: 21.989
Active: true

  1. ageInput ← 25, priceInput ← 9.95, age ← 25, price ← 9.95, active ← true

    1public class Parse {2    public static void main(String[] args) {3        // Simulated user input4        String ageInput→ 25 = "25";5        String priceInput→ 9.95 = "9.95";6        7        // Parse string to number8        int age→ 25 = Integer.parseInt(ageInput25);9        double price→ 9.95 = Double.parseDouble(priceInput9.95);10        11        System.out.println("Age string: " + ageInput25);12        System.out.println("Age number: " + age25);13        System.out.println("Age + 1 = " + (age25 + 1));14        15        System.out.println("Price string: " + priceInput9.95);16        System.out.println("Price number: " + price9.95);17        System.out.println("With tax: " + (price9.95 * 1.1));18        19        // Also works for other types20        boolean active→ true = Boolean.parseBoolean("true");21        System.out.println("Active: " + activetrue);22    }
    outputAge string: 25
Age number: 25
Age + 1 = 26
Price string: 9.95
Price number: 9.95
With tax: 10.945
Active: true

  1. ageInput ← 25, priceInput ← 100.00, age ← 25, price ← 100.0, active ← true

    1public class Parse {2    public static void main(String[] args) {3        // Simulated user input4        String ageInput→ 25 = "25";5        String priceInput→ 100.00 = "100.00";6        7        // Parse string to number8        int age→ 25 = Integer.parseInt(ageInput25);9        double price→ 100.0 = Double.parseDouble(priceInput100.00);10        11        System.out.println("Age string: " + ageInput25);12        System.out.println("Age number: " + age25);13        System.out.println("Age + 1 = " + (age25 + 1));14        15        System.out.println("Price string: " + priceInput100.00);16        System.out.println("Price number: " + price100.0);17        System.out.println("With tax: " + (price100.0 * 1.1));18        19        // Also works for other types20        boolean active→ true = Boolean.parseBoolean("true");21        System.out.println("Active: " + activetrue);22    }
    outputAge string: 25
Age number: 25
Age + 1 = 26
Price string: 100.00
Price number: 100.0
With tax: 110.00000000000001
Active: true
parseInt Convert string to int: `Integer.parseInt("42")` → `42`
parseDouble Convert string to double: `Double.parseDouble("3.14")` → `3.14`

Number to string (format output)

Convert numbers to strings for display or concatenation.

ToString.java
Replay: real traced execution (multi-file project)
public class ToString {
    public static void main(String[] args) {
        int count = 42;
        double price = 19.99;
        boolean active = true;

        // Explicit conversion
        String countStr = String.valueOf(count);
        String priceStr = String.valueOf(price);
        String activeStr = String.valueOf(active);

        System.out.println("Count as string: '" + countStr + "'");
        System.out.println("Price as string: '" + priceStr + "'");
        System.out.println("Active as string: '" + activeStr + "'");

        // Integer wrapper method
        String intStr = Integer.toString(count);
        System.out.println("Using Integer.toString: '" + intStr + "'");

        // Concatenation converts automatically
        String message = "You have " + count + " items";
        System.out.println(message);

        // Format with specific decimal places
        String formatted = String.format("Price: $%.2f", price);
        System.out.println(formatted);
    }
}

  1. count ← 42, price ← 19.99, active ← true, countStr ← 42, priceStr ← 19.99

    1public class ToString {2    public static void main(String[] args) {3        int count→ 42 = 42;4        double price→ 19.99 = 19.99;5        boolean active→ true = true;6        7        // Explicit conversion8        String countStr→ 42 = String.valueOf(count42);9        String priceStr→ 19.99 = String.valueOf(price19.99);10        String activeStr→ true = String.valueOf(activetrue);11        12        System.out.println("Count as string: '" + countStr42 + "'");13        System.out.println("Price as string: '" + priceStr19.99 + "'");14        System.out.println("Active as string: '" + activeStrtrue + "'");15        16        // Integer wrapper method17        String intStr→ 42 = Integer.toString(count42);18        System.out.println("Using Integer.toString: '" + intStr42 + "'");19        20        // Concatenation converts automatically21        String message→ You have 42 items = "You have " + count42 + " items";22        System.out.println(messageYou have 42 items);23        24        // Format with specific decimal places25        String formatted→ Price: $19.99 = String.format("Price: $%.2f", price19.99);26        System.out.println(formattedPrice: $19.99);27    }
    outputCount as string: '42'
Price as string: '19.99'
Active as string: 'true'
Using Integer.toString: '42'
You have 42 items
Price: $19.99

Numbers automatically convert when concatenated with strings using +.

Implicit vs explicit conversion

Java automatically converts in some cases, but not others.

Implicit.java
Replay: real traced execution (multi-file project)
public class Implicit {
    public static void main(String[] args) {
        // Widening: automatic (no data loss possible)
        int i = 100;
        long l = i;      // int → long (automatic)
        float f = i;     // int → float (automatic)
        double d = i;    // int → double (automatic)

        System.out.println("int to long: " + l);
        System.out.println("int to float: " + f);
        System.out.println("int to double: " + d);

        // Narrowing: requires cast (may lose data)
        double bigDouble = 123.456;
        // int x = bigDouble;     // ERROR: won't compile
        int x = (int) bigDouble;
        System.out.println("double to int (cast): " + x);

        // Mixed expressions widen automatically
        int quantity = 3;
        double priceEach = 19.99;
        double total = quantity * priceEach;  // int widened to double
        System.out.println("Total: " + total);

    }
}

  1. i ← 100, l ← 100, f ← 100.0, d ← 100.0, bigDouble ← 123.456, x ← 123

    1public class Implicit {2    public static void main(String[] args) {3        // Widening: automatic (no data loss possible)4        int i→ 100 = 100;5        long l→ 100 = i;      // int → long (automatic)6        float f→ 100.0 = i;     // int → float (automatic)7        double d→ 100.0 = i;    // int → double (automatic)8        9        System.out.println("int to long: " + l100);10        System.out.println("int to float: " + f100.0);11        System.out.println("int to double: " + d100.0);12        13        // Narrowing: requires cast (may lose data)14        double bigDouble→ 123.456 = 123.456;15        // int x = bigDouble;     // ERROR: won't compile16        int x→ 123 = (int) bigDouble;  //#?explicit_cast17        System.out.println("double to int (cast): " + x123);18        19        // Mixed expressions widen automatically20        int quantity→ 3 = 3;21        double priceEach→ 19.99 = 19.99;22        double total→ 59.97 = quantity3 * priceEach19.99;  // int widened to double23        System.out.println("Total: " + total59.97);
    outputint to long: 100
int to float: 100.0
int to double: 100.0
double to int (cast): 123
Total: 59.97
widening Automatic conversion to larger type: `int` → `double`
narrowing Requires explicit cast, may lose data: `double` → `int`

Exercise: ParseErrors.java

Handle invalid input: what happens with NumberFormatException?