1.10 Calling Class Methods

1. What a class method is

Class methods are associated with the class, not with instances (objects) of the class. Their header includes the keyword static before the method name 1.10.A.1.

Example header: public static double toFahrenheit(double c). The keyword static tells you this is a class method. You can call it without creating any object.

2. Calling a class method from another class

Class methods are typically called using the class name along with the dot operator 1.10.A.2. Here a TempConverter class (written by someone else) has the class method toFahrenheit, and a Weather class uses it:

public class Weather
{
    public static void main(String[] args)
    {
        double hot = TempConverter.toFahrenheit(100.0);
        System.out.println(hot);
        System.out.println(TempConverter.toFahrenheit(30.0));
    }
}

Output:

212.0
86.0

TempConverter.toFahrenheit(100.0) returns 212.0. The return type is double, so the result is stored in a double variable or used in an expression, exactly as in topic 1.9.

In an unrelated class, leaving out the class name does not compile:

// does not compile
public class Weather2
{
    public static void main(String[] args)
    {
        System.out.println(toFahrenheit(100.0));
    }
}

Real view:

Weather2.java:6: error: cannot find symbol
        System.out.println(toFahrenheit(100.0));
                           ^
  symbol:   method toFahrenheit(double)
  location: class Weather2
1 error

3. Calling a class method inside its own class

When the method call occurs in the defining class, the use of the class name is optional 1.10.A.2:

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

    public static void main(String[] args)
    {
        System.out.println(toFahrenheit(100.0));               // class name left out
        System.out.println(TempConverter.toFahrenheit(0.0));   // class name used
    }
}

Output:

212.0
32.0

Both calls work, because main is inside TempConverter.

Background (not tested): “static method”. Many programmers say static method for what the course calls a class method. They mean the same thing. On the exam, expect “class method”.