What's wrong with this example of Java property inheritance?

Question

Inheritance.java

public class InheritanceExample {
  static public void main(String[] args){
    Cat c = new Cat();
    System.out.println(c.speak());

            


        

Answer 1

You're shadowing the field inherited from Animal. You have a few options, but the prettiest way of doing it is passing the sound in the constructor:

public class Animal {
  private final String sound;
  protected Animal(String sound){
    if (sound == null)
      throw new NullPointerException("sound");
    this.sound = sound;
  }
  public String speak(){
    return sound;
  }
}

public class Cat extends Animal {
  public Cat(){ super("meow"); }
}

public class Dog extends Animal {
  public Dog(){ super("woof"); }
}

This way, you can make sure that an Animal always has a valid sound, right from construction.