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Thinking in Java
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Forgetting the object type

Music.java might seem strange to you. Why should anyone intentionally forget the type of an object? This is what happens when you upcast, and it seems like it could be much more straightforward if tune( ) simply takes a Wind reference as its argument. This brings up an essential point: If you did that, you’d need to write a new tune( ) for every type of Instrument in your system. Suppose we follow this reasoning and add Stringed and Brass instruments:

//: c07:music:Music2.java
// Overloading instead of upcasting.
package c07.music;
import com.bruceeckel.simpletest.*;

class Stringed extends Instrument {
  public void play(Note n) {
    System.out.println("Stringed.play() " + n);
  }
}

class Brass extends Instrument {
  public void play(Note n) {
    System.out.println("Brass.play() " + n);
  }
}

public class Music2 {
  private static Test monitor = new Test();
  public static void tune(Wind i) {
    i.play(Note.MIDDLE_C);
  }
  public static void tune(Stringed i) {
    i.play(Note.MIDDLE_C);
  }
  public static void tune(Brass i) {
    i.play(Note.MIDDLE_C);
  }
  public static void main(String[] args) {
    Wind flute = new Wind();
    Stringed violin = new Stringed();
    Brass frenchHorn = new Brass();
    tune(flute); // No upcasting
    tune(violin);
    tune(frenchHorn);
    monitor.expect(new String[] {
      "Wind.play() Middle C",
      "Stringed.play() Middle C",
      "Brass.play() Middle C"
    });
  }
} ///:~


This works, but there’s a major drawback: you must write type-specific methods for each new Instrument class you add. This means more programming in the first place, but it also means that if you want to add a new method like tune( ) or a new type of Instrument, you’ve got a lot of work to do. Add the fact that the compiler won’t give you any error messages if you forget to overload one of your methods and the whole process of working with types becomes unmanageable.

Wouldn’t it be much nicer if you could just write a single method that takes the base class as its argument, and not any of the specific derived classes? That is, wouldn’t it be nice if you could forget that there are derived classes, and write your code to talk only to the base class?

That’s exactly what polymorphism allows you to do. However, most programmers who come from a procedural programming background have a bit of trouble with the way polymorphism works.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire