CS141: Topics for Nov 4, 6, & 8, 2013

Homework and Reading

The reading and homework for this week can be found here: Lab 11

Chp 9 Vocabulary

Inheritance superclass subclass extends override Polymorphism dynamic method look-up Abstract (vs concrete) class

final methods protected access inheritance hierarchy overriding equals instanceof operator interface comparable interface Function objects

Topics: Chp 9 - Inheritance and Interfaces

• What is inheritance and why use it?
• Subclasses vs superclasses - See Hierarchy
• A class can have only one direct superclass. A class can have multiple subclasses.
• Don't overuse subclasses.
• Subclasses:
  • How are they defined (using extends)
  • Overriding methods
  • What variables and methods in the super class can be accessed by the subclass?
    • Public vs Protected: Protected modifier allows access by all subclasses and all classes in the same package (folder). Your text suggests leaving variables as private.
    • Private variables in the superclass are not accessible from the subclass.
    • Using super() in the first line of constructor
    • Using super.method_name() anywhere in a subclass method.
• Polymorphism:

      Shape[] shapes = new Shape[3];
      shapes[0] = new Circle(2);
      shapes[1] = new Rectangle(2,3);
      shapes[2] = new Square(5);
      for (int i=0; i < shapes.length; i++) {
  	     System.out.println("The area for " + shapes[i] + " is " + shapes[i].calcArea());
       }
  

  • Which toString() method is called? Which calcArea() is called? The one in superclass (Shape) or in the subclass (Circle, Rectangle, Square)?
• Abstract class:
  • Declare as

    public abstract class Shape { 
    	    public abstract double calcArea(); 
    	    ... 
    

    Must define method in the subclass. Can't create a Shape object; can only create a subclass.
• Using the keyword final so that one can't override a method in the subclass.
• The universal super class: Object
  All Java classes are subclasses of the Object class and inherit the methods: toString(), equals(), hashCode(). These methods should generally be overriden.
• instanceof operator can be used to check what type of object you have. For example, if you need to cast an object. Should be used sparingly. Don't use as a way to implement different behavior, i.e. to bypass polymorphism.
• Interface - using the implements keyword.
  • An interface has no variables, only methods which are not implemented.
  • An interface is used to force a class to define a set of methods so as to force consistency across a set a classes.
  • Examples: The Comparable interface
  • A class can have only one superclass but can implement multiple interfaces.
• Function Objects.

In Class Practice Activities

• Object Diagram Practice: Assume there is a Card class defined. Draw the object diagram after each line is executed:

  Line  0:   public static void main(String[] args) {
  Line  1:      Card[] myCards;
  Line  2:      myCards = new Card[4];
  Line  3:      for (int i = 0; i < myCards.length ; i++) {
  Line  4:         myCards[i] = new Card(i);  
  Line  5:         System.out.print(myCards[i] + " "); 
  Line  6:      }
  Line  7:      Card c = myCards[3];
  Line  8:      myCards[0] = myCards[1];
  Line  9:      myCards[2] = null;
  Line 10:      myCards[3] = null;
  Line 11:      myCards[3] = c;
  Line 12:      System.out.println("\n");
  Line 13:      for (int i = 0; i < myCards.length ; i++) {
  Line 14:         System.out.print(myCards[i] + " "); 
  Line 15:      }
  Line 16:   }

  Assuming that the output from Line 5 gives the A♠, 2♠ 3♠ 4♠, what is the output at Line 14?
  Solution may be found here: Lines 1-7 and Lines 8-16

• We will go over how to get your database to read from a file. See WISE for instructions.

• Try the Chp 9 programming exercises: P9.8 and P9.9 on page 459

• You may begin the new programming assignment on inheritance: assignment (pdf). This will be due sometime after the exam. Here is some code that you will also need (see instructions in assignment): the card class and main testing code.

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