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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

      Becausex is defined as apublic static int in the classIdentifyMyParts, every reference tox will have the value that was last assigned becausex is a static variable (and therefore a class variable) shared across all instances of the class. That is, there is only onex: when the value ofx changes in any instance it affects the value ofx for all instances ofIdentifyMyParts.

      This is covered in the Class Variables section ofUnderstanding 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 inEnum Types.

   Answer:Card.java.

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

   Answer: SeeDeck.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: SeeDisplayDeck.java.