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Abstract Classes and Interfaces
Design Issues


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Review
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  • Abstract Class:
    • A class without any instances
  • Interface:
    • A set of services provided by a class
Review (cont.)
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  • Similarities:
    • Both can contain abstract methods
  • Differences:
    • An interface only contains abstract methods
    • An abstract class can contain concrete methods
Design Issues
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  • When to use an abstract class:
    • You have several classes that provide the same functionality in the same way but you do not want to move that functionality into a concrete parent because it doesn't make sense to have instances of the parent
  • When to use an interface:
    • You have classes that provide the same functionality in different ways and/or are not related hierachically
    • You need to reveal the capabilities of an object without revealing its class
Some Examples in Java
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  • Abstract Classes:
    • AbstractButton javax.swing.AbstractButton : A generic button-like object in a GUI
    • AbstractCollection java.util.AbstractCollection : A skeletal implementation of a class that contains a "set" of other objects
  • Interfaces:
    • Comparable java.lanag.Comparable : Objects that can be ordered
    • ActionListener java.awt.event.ActionListener : Objects that can be performed of GUI actions (e.g., button clicks)
A Design Exercise
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  • Motivation:
    • The only way to get better is to practice
  • The System to be Designed:
    • The Madison Music Machine 3000: A software implementation of a "karaoke machine" that can play (instrumental) music while simultaneously displaying the lyrics to be sung.
A Design Exercise (cont.)
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An Initial Design

images/madisonmusicmachine-v1.gif
A Design Exercise (cont.)
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An Improved Design

images/madisonmusicmachine-v2.gif
A Design Exercise (cont.)
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More Improvements

images/madisonmusicmachine-v3.gif
A Design Exercise (cont.)
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Eliminating the MadisonMusicMachine Class

images/madisonmusicmachine-v4.gif
A Design Exercise (cont.)
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Synchronizing the Music and Lyrics

images/madisonmusicmachine-v5.gif
A Design Exercise (cont.)
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Improved Synchronization

images/madisonmusicmachine-v6.gif
A Design Exercise (cont.)
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Adding a Presenter Interface

images/madisonmusicmachine-v7.gif
A Design Exercise (cont.)
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Combining the Lyrics and Music Classes

images/madisonmusicmachine-v8.gif
A Design Exercise (cont.)
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Adding a PlayList Class

images/madisonmusicmachine-v9.gif
A Simple Implementation
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AbstractLineElement

javaexamples/oopbasics/musicmachine/AbstractLineElement.java
 

Lyric

javaexamples/oopbasics/musicmachine/Lyric.java
 

Note

javaexamples/oopbasics/musicmachine/Note.java
 
A Simple Implementation (cont.)
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Presenter

javaexamples/oopbasics/musicmachine/Presenter.java
 

Console

javaexamples/oopbasics/musicmachine/Console.java
 

InstrumentSynthesizer

javaexamples/oopbasics/musicmachine/InstrumentSynthesizer.java
 
A Simple Implementation (cont.)
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MetronomeListener

javaexamples/oopbasics/musicmachine/MetronomeListener.java
 
A Simple Implementation (cont.)
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Part

javaexamples/oopbasics/musicmachine/Part.java
 
A Simple Implementation (cont.)
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Song

javaexamples/oopbasics/musicmachine/Song.java
 
Concluding Thoughts
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  • The Design Process:
    • Could have been improved by differentiating between conceptual models (for analysis) and class models (for engineering design)
  • The Design Itself:
    • Could have been improved using design patterns, most notably the Model-View-Controller Pattern
There's Always More to Learn
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