Unit 1.14: Instance Methods

Instance Methods

• Definition: Instance methods define the behaviors of an object. Unlike static methods, they must be called on a specific instance (object) of a class.

• Syntax: Use the dot operator (.) to call a method on an object reference.
  • objectVariable.methodName(arguments);

NullPointerException

As introduced in Unit 1.13 (The null Reference), assigning null to a variable is valid, but using it carelessly leads to errors.

• Recap: A NullPointerException is a runtime error that stops your program.
• The Cause: This exception happens specifically when you try to use the dot operator (.) on a reference that points to nothing. You are asking "nobody" to do "something," which is impossible.

Examples

Calling Methods

This program demonstrates how to call an instance method (length()) on a String object.

Task: Invoking an instance method on a String object.

public class MethodExample {
    public static void main(String[] args) {
        String s = "AP Computer Science";
        
        // .length() is an instance method called on the object 's'
        int len = s.length(); 
        
        System.out.println("The string is: " + s);
        System.out.println("The length of the string is: " + len);
    }
}

Causing an Exception (NullPointerException)

This program demonstrates what happens when you try to call a method on a variable that is null. It will compile successfully, but it will crash when run.

Task: Triggering a NullPointerException by calling a method on a null reference.

public class NullExample {
    public static void main(String[] args) {
        // emptyRef is declared but points to no object
        String emptyRef = null;
        
        System.out.println("Attempting to call .length() on a null reference...");
        
        // This line causes a NullPointerException because there 
        // is no object to perform the action on.
        int x = emptyRef.length(); 
        
        // This line will NEVER be reached because the program crashes above.
        System.out.println("The length is: " + x);
    }
}