Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition

6.6. Abstract Classes

In the Animal class, you introduced a version of the sound() method that did nothing because you wanted to call the sound() method in the subclass objects dynamically. The method sound() has no meaning in the context of the generic class Animal, so implementing it does not make much sense. This situation often arises in object-oriented programming. You will often find yourself creating a superclass from which you will derive a number of subclasses, just to take advantage of polymorphism.

To cater for this, Java has abstract classes. An abstract class is a class in which one or more methods are declared, but not defined. The bodies of these methods are omitted, because, as in the case of the method sound() in the Animal class, implementing the methods does not make sense. Since they have no definition and cannot be executed, they are called abstract methods. The declaration for an abstract method ends with a semicolon and you specify the method with the keyword abstract to identify it as such. To declare that a class is abstract you just use the keyword abstract in front of the class keyword in the first line of the class definition.

You could have defined the class Animal as an abstract class by amending it as follows:

public abstract class Animal {
  public abstract void sound();   // Abstract method

  public Animal(String aType) {
    type = new String(aType);
  }

  public String toString() {
    return "This is a " + type;
  }

  private String type;
}

The previous program will ...