Java Methods: Defining, Calling, Parameters and Return Values

A method is a named block of code that performs one task. You define it once and call it whenever you need it. Methods make programs shorter, easier to test and easier to read, because each piece of work has a name. (Other languages call them functions; in Java every function lives inside a class, so it is called a method.)

You have already written one method in every program: main.

Defining and calling a method#

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

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

Welcome to Laviyx Learn!
Welcome to Laviyx Learn!

A method definition has:

• modifiers: here static, which means the method belongs to the class and can be called from main without creating an object (you will learn about non-static methods in Classes and Objects);
• a return type: void means it returns nothing;
• a name in camelCase, usually a verb;
• a parameter list in parentheses;
• a body in braces.

Parameters#

Parameters receive the values (arguments) passed in a call:

public class Main {
    static void greet(String name, int visits) {
        System.out.println("Hello, " + name + "! Visit number " + visits + ".");
    }

    public static void main(String[] args) {
        greet("Asha", 3);
        greet("Ravi", 1);
    }
}

Hello, Asha! Visit number 3.
Hello, Ravi! Visit number 1.

Arguments match parameters by position, and each must have a compatible type. greet(3, "Asha") does not compile.

Returning a value#

A method with a return type sends a result back with return:

public class Main {
    static double rectangleArea(double width, double height) {
        return width * height;
    }

    public static void main(String[] args) {
        double area = rectangleArea(4.5, 2);
        System.out.println("Area: " + area);
        System.out.println("Twice: " + 2 * rectangleArea(3, 3));
    }
}

Area: 9.0
Twice: 18.0

The call rectangleArea(4.5, 2) is an expression whose value is the returned result, so you can store it, print it or calculate with it.

return also ends the method immediately, which is handy for special cases:

public class Main {
    static int safeDivide(int a, int b) {
        if (b == 0) {
            return 0;     // early return for the special case
        }
        return a / b;
    }

    public static void main(String[] args) {
        System.out.println(safeDivide(10, 3) + " " + safeDivide(5, 0));
    }
}

3 0

The compiler checks that every path through a non-void method returns a value: "missing return statement" means one path does not.

Methods that answer a question#

Methods returning boolean are usually named like a question:

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

    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            System.out.println(i + (isEven(i) ? " even" : " odd"));
        }
    }
}

1 odd
2 even
3 odd
4 even

Order does not matter#

Unlike C and C++, Java methods can be defined in any order inside a class: main can call a method defined below it, with no declarations needed.

The method signature#

A method's name plus its parameter types form its signature, for example rectangleArea(double, double). Two methods in the same class may share a name if their signatures differ; that is overloading, covered in Method Overloading.

Writing good methods#

• One task per method. If describing it needs "and", consider splitting it.
• Name it with a verb (printReport, calculateTax) or a question for boolean results (isValid, hasStock).
• Keep it short enough to read in one screen.
• Prefer returning results over printing them, so callers decide what to do with the value.

Common mistakes#

Practice#

public class Temperatures {
    static double celsiusToFahrenheit(double c) {
        return c * 9 / 5 + 32;
    }

    static void describe(double c) {
        System.out.println(c + " C = " + celsiusToFahrenheit(c) + " F");
    }

    public static void main(String[] args) {
        describe(0);
        describe(37);
        describe(100);
    }
}

0.0 C = 32.0 F
37.0 C = 98.6 F
100.0 C = 212.0 F

public class PrimeList {
    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) {
        StringBuilder primes = new StringBuilder();
        for (int i = 1; i <= 30; i++) {
            if (isPrime(i)) primes.append(i).append(' ');
        }
        System.out.println(primes.toString().trim());
    }
}

2 3 5 7 11 13 17 19 23 29

Key takeaway#