1.10 Calling Class Methods

1. Exam Points

• Class methods are declared using the static keyword.
• Call class methods(static) using the class name.
• Call instance methods(non-static) using an instance name.
• An instance method(non-static) can call a static method from the same class directly.
• If you call a method of a class by the class name, the method must be static.

2. Knowledge Points

(1) Class Methods (Static Methods)

• Class methods are static methods.
• A method is declared as a class method by using the keyword static in the header before the method name.
• A method is an instance method if it is not declared as static.
• Class methods are typically called using the class name along with the dot operator.
• Example:
  

  public class Circle {
  
      static double pi; // a class variable (类变量-static)
      double radius; // an instance variable (实例变量-not static)
      
      public static void printPi(){ // a class method (类方法)
          System.out.println(pi);
      }
      
      public void printRadius(){ // an instance method (实例方法)
          System.out.println(radius);
      }
  }
  
  // Another class other than the Circle class
  public class Test{
    public static void main(String[] args) {
        // printPi() is a class method,
        // so we call it using the class name Circle,
        // no need to create an instance/object of the Circle class.
        Circle.printPi();	
        
        Circle c1 = new Circle();
        c1.radius = 2.0;
        Circle c2 = new Circle();
        c2.radius = 3.0;
        
        // printRadius() is an instance method,
        // so we call it using an instance/object name.
        c1.printRadius();
        c1.printRadius();
    }
  }
  

3. Exercises

• Practice
• Solution