Java Methods

A method is a named block of code which you can run many times. Methods make programs short, clear and easy to test. In other languages they are called functions.

Declare and call a method

public class Main {
    // method declaration
    static void greet() {
        System.out.println("Hello from a method!");
    }

    public static void main(String[] args) {
        greet();      // call it
        greet();      // call it again
    }
}

Parts of a method

static int add(int a, int b) {
    return a + b;
}

Part	Meaning
static	Belongs to the class (we use it in main for now)
int	The return type. Use void when nothing is returned
add	The name of the method (camelCase)
(int a, int b)	The parameters: the inputs
return a + b;	Sends the result back to the caller

Parameters and return values

public class Calc {
    static int add(int a, int b) {
        return a + b;
    }

    static double average(int[] numbers) {
        int sum = 0;
        for (int n : numbers) {
            sum += n;
        }
        return (double) sum / numbers.length;
    }

    static boolean isEven(int n) {
        return n % 2 == 0;
    }

    public static void main(String[] args) {
        int total = add(5, 7);
        System.out.println(total);
        System.out.println(average(new int[]{70, 80, 95}));
        System.out.println(isEven(10) + " " + isEven(7));
    }
}

Pass by value

Java always passes a copy of the value. For a primitive, the original variable can not be changed by the method. For an object or an array the copy is a copy of the reference, so the method can change the object itself.

import java.util.Arrays;

public class Pass {
    static void changeNumber(int x) {
        x = 100;                  // changes only the copy
    }

    static void changeArray(int[] a) {
        a[0] = 100;               // changes the real array
    }

    public static void main(String[] args) {
        int n = 5;
        changeNumber(n);
        System.out.println(n);

        int[] arr = {1, 2, 3};
        changeArray(arr);
        System.out.println(Arrays.toString(arr));
    }
}

Return early

public class Early {
    static String sign(int n) {
        if (n > 0) return "positive";
        if (n < 0) return "negative";
        return "zero";
    }

    public static void main(String[] args) {
        System.out.println(sign(5) + " " + sign(-3) + " " + sign(0));
    }
}

Rules

• A method with a return type must return a value on every path.
• A method can return only one value (use an array, a list or a record for more).
• Give methods verb names: calculateTotal(), isValid().
• A method should do one job.

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