Abstract classes and methods

Bruce Eckel's Thinking in Java Contents | Prev | Next

and methods

In all the instrument examples, the methods in the base class Instrument were always “dummy” methods. If these methods are ever called, you’ve done something wrong. That’s because the intent of Instrument is to create a common interface for all the classes derived from it.

//: Music4.java
// Abstract classes and methods
import java.util.*;
abstract class Instrument4 {
  int i; // storage allocated for each
  public abstract void play();
  public String what() {
    return "Instrument4";
  public abstract void adjust();
class Wind4 extends Instrument4 {
  public void play() {
  public String what() { return "Wind4"; }
  public void adjust() {}
class Percussion4 extends Instrument4 {
  public void play() {
  public String what() { return "Percussion4"; }
  public void adjust() {}
class Stringed4 extends Instrument4 {
  public void play() {
  public String what() { return "Stringed4"; }
  public void adjust() {}
class Brass4 extends Wind4 {
  public void play() {
  public void adjust() { 
class Woodwind4 extends Wind4 {
  public void play() {
  public String what() { return "Woodwind4"; }
public class Music4 {
  // Doesn't care about type, so new types
  // added to the system still work right:
  static void tune(Instrument4 i) {
    // ...
  static void tuneAll(Instrument4[] e) {
    for(int i = 0; i < e.length; i++)
  public static void main(String[] args) {
    Instrument4[] orchestra = new Instrument4[5];
    int i = 0;
    // Upcasting during addition to the array:
    orchestra[i++] = new Wind4();
    orchestra[i++] = new Percussion4();
    orchestra[i++] = new Stringed4();
    orchestra[i++] = new Brass4();
    orchestra[i++] = new Woodwind4();
} ///:~ 

You can see that there’s really no change except in the base class.

It’s helpful to create abstract classes and methods because they make the abstractness of a class explicit and tell both the user and the compiler how it was intended to be used.


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