1.14 Calling Instance Methods

1. Exam Points

• How to declare class methods (static) and instance methods.
• How to call class methods (static) and instance methods.
• Pay attention to the return type and arguments passed into a method.
• Example:
  • declare: public boolean check(int a, String b, double c) {}
  • call: boolean result =check(10, ”anna”, 5.3);
• Pay attention: call methods from the same class or a different class.
  • an instance method can call another method in the same class directly.
  • an method needs to call another instance method in a different class by using an object name.

2. Knowledge Points

(1) Class Methods and Instance Methods

• A class method is declared using the static keyword, and is called using the class name.
  • Example:

  public class A{    
      public static void test1(){}
  }
  
  public class B{
      public static void main(String[] args){
          A.test1();
      }
  }
  

• An instance method is declared without using the static keyword, and is called using an object/instance name.
  • Example:

  public class A{    
      public void test2(){}
  }
  
  public class B{
      public static void main(String[] args){
          A a = new A();
          a.test2();
      }
  }
  

(2) Calling Instance Methods

• Instance methods are called on objects of the class, are non-static methods.
• A method call on a null reference will result in a NullPointerException.
• Example:

  Dog d = null;
  d.bark(); // d is null, leading to NullPointerException
  

3. Exercises

• Practice
• Solution