Java Methods

A method is a named block of code that does one job. You have been calling methods all along: println, length, Math.max. Now you will write your own. Methods let you give a name to a piece of work, reuse it as often as you like, and split a long program into small parts that are easy to understand and test.

Defining and calling a method

A method is written inside the class, next to main. For now every method starts with static; the Class Methods topic explains what that word means.

public class Main {
    static void greet() {
        System.out.println("Hello!");
        System.out.println("Welcome to Java.");
    }

    public static void main(String[] args) {
        greet();
        greet();
    }
}

Hello!
Welcome to Java.
Hello!
Welcome to Java.

• static void greet() defines a method called greet. void means it gives nothing back.
• The empty brackets mean it needs no input.
• greet(); calls the method: Java runs its block, then comes back to the line after the call.

The order of methods in the class does not matter; main is always where the program starts.

Parameters: giving a method input

Inside the brackets you list parameters, variables that receive values from the call:

public class Main {
    static void greet(String name, int times) {
        for (int i = 0; i < times; i++) {
            System.out.println("Hello, " + name + "!");
        }
    }

    public static void main(String[] args) {
        greet("Anil", 2);
        greet("Sara", 1);
    }
}

Hello, Anil!
Hello, Anil!
Hello, Sara!

The values in the call ("Anil", 2) are called arguments. They are matched to the parameters by position. The next topic, Method Parameters, goes deeper.

Returning a value

Instead of void, a method can declare a return type and give a value back with return. The call can then be used like a value:

public class Main {
    static int square(int x) {
        return x * x;
    }

    static double circleArea(double radius) {
        return Math.PI * radius * radius;
    }

    public static void main(String[] args) {
        int s = square(9);
        System.out.println(s);
        System.out.println(square(3) + square(4));
        System.out.printf("%.2f%n", circleArea(2));
    }
}

81
25
12.57

return ends the method immediately. A method with a return type must return a value on every path, or it does not compile.

Methods that decide

Methods often return a boolean, which makes conditions read like sentences:

public class Main {
    static boolean isPrime(int n) {
        if (n < 2) {
            return false;
        }
        for (int d = 2; d * d <= n; d++) {
            if (n % d == 0) {
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        for (int n = 1; n <= 30; n++) {
            if (isPrime(n)) {
                System.out.print(n + " ");
            }
        }
        System.out.println();
    }
}

2 3 5 7 11 13 17 19 23 29 

The main method now reads almost like English: for every n, if n is prime, print it. That is the real value of methods: they hide the details behind a good name.

Practice exercises

1. Write a method printLine(int length) that prints a line of that many dashes.
2. Write a method max3(int a, int b, int c) that returns the largest of three numbers.
3. Write a method isEven(int n) and use it to print the even numbers from 1 to 20.