CS203
Spring 2013
Apr 5

• Experiment with types and subtypes

1. Launch Netbeans, and create a new project named lab9 under your cs203 course directory
   • make the main class Examples rather than the default
2. Add the tester.jar file to your project’s Libraries
3. Add an A class and a B class to your project, and copy/paste the respective code below:

   /**
    * cs203
    * Spring 2013
    * Lab 9
    * 
    * file: B.java
    */
    
   public class B {
    
       public void bmethod1() {
           System.out.println("class B: bmethod1() called...");
       }
    
       public void bmethod2() {
           System.out.println("class B: bmethod2() called, calling which class's bmethod1?...");
           bmethod1();
       }
   }
    
   /**
    * cs203
    * Spring 2013
    * Lab 9
    * 
    * file: A.java
    */
    
   public class A extends B {
    
       @Override
       public void bmethod1() {
           System.out.println("class A: bmethod1() called...");
       }
   }

4. Add code to the main() method in your Examples class to experiment with classes A and B. Your code should generate the following output when run:

   run:
   Initializing myB, an object of type B, to an instance of type B...
   class B: bmethod1() called...
   class B: bmethod2() called, which class's bmethod1() will I call???
   class B: bmethod1() called...
    
   Initializing myB, and object of type B, to an instance of type A...
   class A: bmethod1() called...
   class B: bmethod2() called, which class's bmethod1() will I call???
   class A: bmethod1() called...
   BUILD SUCCESSFUL (total time: 0 seconds)

5. Notice how this is a way to exploit the behavior of B type objects. Malicious coders could potentially do bad things in class A’s bmethod1.
6. One way to protect the behavior of B objects is to make class B final:

   public final class B {

   Make class B final and observe what happens when you try to compile your code. Discuss with Marc or one of our coaches.

• When you’ve had enough fun, submit your work using the submit203 script:
  submit203 lab9 lab9

• Logout and have a good weekend!