The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
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Consider the following class:
public class IdentifyMyParts { public static int x = 7; public int y = 3; }
Question: What are the class variables?
Answer: x
Question: What are the instance variables?
Answer: y
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.
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
Exercise: Write a class whose instances represents a full deck of cards. You should also keep this solution.
Answer: See
.Deck.java
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