Answers to Questions and Exercises: Classes (The Java™ Tutorials Classes and Objects) (original) (raw)

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: 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.