CS 424 Java

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CS 424 Java

• Class 14
• Tuesday, February 22, 2000

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Thought for the Day

• The illiterate of the 21st century will not be
  those who cannot read and write, but those who
  cannot learn, unlearn, and relearn.
• Alvin Toffler

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Todays Agenda

• Chapter 9 Object-Oriented Programming
• Sections 9.1-9.9
• Hope to return exam Thursday
• Next project assignment Thursday

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Reading Assignment

• Today Finish chapter 9.
• Thursday Chapter 10 Strings Characters

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Seminar Today

• Electronic Commerce The End of the Beginning?
• Nenad Jukic, PhD from UA
• Now at Loyola University
• 5PM today, EE 119

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ACM Meeting

• Bret McKee, Exxon-Mobil
• What you (dont) need to know in the real
  world!
• Thursday, 5PM, EE 119

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Reading Quiz

• Assume BaseClass is extended by Class1 which
  is extended by Class2
• Assume objects bC, c1 and c2 of those
  classes. Which of the following can you do?
  What happens if you cant?
• c1 bC
• bC c1
• c2 bC
• c1 c2

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Quiz Answers

• Background you need
• BaseClass is the superclass to Class1, which
  is the superclass to Class2
• bC, c1 and c2 are references, not objects!
• You can make a superclass reference point to a
  subclass object, but you cant make a subclass
  reference point to a superclass object!

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Quiz Answers (2)

• Assuming that each reference points to an object
  of its own type
• c1 bC Cant gets a ClassCastException
• bC c1 Can
• c2 bC Cant same exception
• c1 c2 Can

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Chapter 9 Object Oriented Programming

• Superclass and subclass
• Polymorphism
• Abstract methods, abstract classes
• Overriding methods
• Dynamic binding
• Interfaces

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Polymorphism

• Makes programs more extensible.
• If several classes inherit from one superclass,
  they can all use the same method.
• Example Window methods
• Open, close, move, repaint,
• All sub-Window objects could inherit, probably
  override them.

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Polymorphism (2)

• Same command ?
• different objects ?
• many forms (polymorph)
• myWindow.open()
• If myWindow is a text window, or a graphics
  window, or What would be different?

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Abstract classes

• Similar to Cs abstract base class
• Javas abstract methods similar to Cs pure
  virtual functions
• Meant to be overridden in subclasses
• You cant create an object of an abstract class,
• But you can create a reference to an abstract
  class!
• Try it! Whats the difference?

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Overriding methods

• In the subclass, write a new version of the
  method.
• You can call the super.method() if you wish.

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Dynamic binding

• Which method gets called for an object?
• If the reference to the object is a superclass
  reference, it could be the superclass method, or
  the subclass method.
• If the hierarchy is deep, several options
• Binding is at execution time.

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Example Code

• Figure 9.9

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Interfaces

• Can use an interface instead of a superclass.
• Subclass must implement all the methods in the
  interface.
• Old example chapter 6
• Implemented ActionListener
• Wrote actionPerformed
• Use interface when there are no instance
  variables and no default method implementations.

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Example Code

• Figure 9.11

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Exercise

• Page 452, 9.25