Foundations

Hello, Numbers!

You're helping a friend split a restaurant bill. The total is $47.50 and there are 4 people. You need to calculate each person's share: 47.50 / 4 = 11.875, round to $11.88.

Print a single number

The simplest thing we can do - tell the computer to show us a number.

Number.java
Replay: real traced execution (multi-file project)
public class Number {
    public static void main(String[] args) {
        System.out.println(42);
        System.out.println(100);
        System.out.println(-7);
    }
}

  1. public static void main(String[] args)

    1public class Number {2    public static void main(String[] args) {3        System.out.println(42);4        System.out.println(100);5        System.out.println(-7);6    }
    output42
100
-7

The computer echoes back exactly what we ask for.

println In Java, `System.out.println(...)` displays the value to the screen.

Basic arithmetic

Now let's do some math. We can add, subtract, multiply, and divide.

Arithmetic.java
Replay: real traced execution (multi-file project)
public class Arithmetic {
    public static void main(String[] args) {
        System.out.println(10 + 5);
        System.out.println(10 - 5);
        System.out.println(10 * 5);
        System.out.println(10 / 5);
        System.out.println(10 % 3);
    }
}

  1. public static void main(String[] args)

    1public class Arithmetic {2    public static void main(String[] args) {3        System.out.println(10 + 5);4        System.out.println(10 - 5);5        System.out.println(10 * 5);6        System.out.println(10 / 5);7        System.out.println(10 % 3);8    }
    output15
5
50
2
1
operators Arithmetic operators: `+` add, `-` subtract, `*` multiply, `/` divide, `%` remainder.

Grouping with parentheses

What if we want to control the order of operations? Use parentheses.

Grouping.java
Replay: real traced execution (multi-file project)
public class Grouping {
    public static void main(String[] args) {
        System.out.println(2 + 3 * 4);
        System.out.println((2 + 3) * 4);
        System.out.println(100 / (5 + 5));
    }
}

  1. public static void main(String[] args)

    1public class Grouping {2    public static void main(String[] args) {3        System.out.println(2 + 3 * 4);4        System.out.println((2 + 3) * 4);5        System.out.println(100 / (5 + 5));6    }
    output14
20
10

Without parentheses: 2 + 3 * 4 gives 14 (multiply first). With parentheses: (2 + 3) * 4 gives 20 (add first).

precedence Multiplication and division happen before addition and subtraction, unless you use parentheses.

A real calculation

Let's calculate the area of a rectangle with sides 7 and 5.

Area.java
Replay: real traced execution (multi-file project)
public class Area {
    public static void main(String[] args) {
        System.out.println(7 * 5);
    }
}

  1. public static void main(String[] args)

    1public class Area {2    public static void main(String[] args) {3        System.out.println(7 * 5);4    }
    output35

Complex expression

Combine everything: multiple operations, nested parentheses.

Complex.java
Replay: real traced execution (multi-file project)
public class Complex {
    public static void main(String[] args) {
        System.out.println((10 + 20) * (30 - 15) / 5);
        System.out.println(((2 + 3) * (4 + 5)) - 10);
        System.out.println(100 / 10 / 2);
    }
}

  1. public static void main(String[] args)

    1public class Complex {2    public static void main(String[] args) {3        System.out.println((10 + 20) * (30 - 15) / 5);4        System.out.println(((2 + 3) * (4 + 5)) - 10);5        System.out.println(100 / 10 / 2);6    }
    output90
35
5

The computer follows the same math rules you learned in school.