Answers to Questions and Exercises: Classes

Questions

1. Consider the following class:

   public class IdentifyMyParts {
       public static int x = 7;
       public int y = 3;
   } 
   

   1. Question: What are the class variables?

      Answer: x

   2. Question: What are the instance variables?

      Answer: y

   3. Question: What is the output from the following code:

      IdentifyMyParts a = new IdentifyMyParts(); 
      IdentifyMyParts b = new IdentifyMyParts(); 
      a.y = 5; 
      b.y = 6; 
      a.x = 1; 
      b.x = 2; 
      System.out.println("a.y = " + a.y); 
      System.out.println("b.y = " + b.y); 
      System.out.println("a.x = " + a.x); 
      System.out.println("b.x = " + b.x); 
      System.out.println("IdentifyMyParts.x = " + IdentifyMyParts.x);
      

      Answer: Here is the output:

       a.y = 5 
       b.y = 6 
       a.x = 2 
       b.x = 2
       IdentifyMyParts.x = 2
      

      Because x is defined as a public static int in the class IdentifyMyParts, every reference to x will have the value that was last assigned because x is a static variable (and therefore a class variable) shared across all instances of the class. That is, there is only one x: when the value of x changes in any instance it affects the value of x for all instances of IdentifyMyParts.

      This is covered in the Class Variables section of Understanding Instance and Class Members.

Exercises

1. Exercise: Write a class whose instances represent a single playing card from a deck of cards. Playing cards have two distinguishing properties: rank and suit. Be sure to keep your solution as you will be asked to rewrite it in Enum Types.

   Answer: Card.java.

2. Exercise: Write a class whose instances represents a full deck of cards. You should also keep this solution.

   Answer: See Deck.java.

3. Exercise: Write a small program to test your deck and card classes. The program can be as simple as creating a deck of cards and displaying its cards.

   Answer: See DisplayDeck.java.