Lesson 1.9 · Unit 1 · CED topic 1.10

Calling class methods

Some methods belong to a class as a whole rather than to any one object. You have used several already without creating anything: Math.sqrt(49) does not need a Math object, because Math is a toolbox of methods that stand on their own. This lesson explains what makes those methods different, and how to call and write them.

What a class method is

Class methods are associated with the class, not with instances of the class. A class method has the keyword static in its header, before the return type.

public static int square(int n)

Because a class method belongs to the class, you do not create an object to call it. The usual call uses the class name with the dot operator.

public class Converter {
    public static double toFahrenheit(double celsius) {
        return celsius * 9.0 / 5.0 + 32.0;
    }

    public static int square(int n) {
        return n * n;
    }
}

double boiling = Converter.toFahrenheit(100);
int area = Converter.square(7);
System.out.println(boiling);
System.out.println(area);

212.0
49

When the call appears in the class that defines the method, the class name is optional: inside Converter you may write square(7) instead of Converter.square(7). From any other class you must name the class.

Calling a method and using its value

Everything from Lesson 1.9 still applies. The arguments must match the parameter types in number and order. A void class method is a statement by itself; a non-void class method returns a value that you store or use in an expression, and a method call can be an argument to another method call.

System.out.println(Converter.square(Converter.square(3)));

81

The inner call runs first and returns 9; that value is the argument for the outer call, which returns 81.

Why call a class method without an object

A class method is the right tool when the work depends only on its arguments. Converting a temperature needs no stored state: give it a number, get a number. Methods that depend on the data stored in a particular object are instance methods (Lesson 1.14), and they are called on an object instead.

int a = Converter.square(5);
double f = Converter.toFahrenheit(0);
System.out.println(a + " " + f);

25 32.0

public class Main {
    public static int cube(int n) {
        return n * n * n;
    }

    public static int sumOfCubes(int a, int b) {
        return cube(a) + cube(b);
    }

    public static void main(String[] args) {
        System.out.println(sumOfCubes(2, 3));
        System.out.println(Main.cube(4));
    }
}

35
64

public class Main {
    public int getFive() {
        return 5;
    }

    public static void main(String[] args) {
        int x = getFive();
    }
}

Main.java:7: error: non-static method getFive() cannot be referenced from a static context
        int x = getFive();
                ^
1 error