Superclass/Subclass methods

Question

I have a situation

public class Animal
{
   String noise;

  public String makeNoise()
  {
      return noise;
  }
}

Then there will be a subclass with the concrete definition of the noise.

public class Dog extends Animal{
   String noise = "woof";
}

also

public class Cat extends Animal{
   String noise = "meow";
}

What I want to do is

 Animal cat = new Cat();
 cat.makeNoise();  // This will be 'meow'

and

Animal dog = new Dog();
dog.makeNoise();  // This will be 'woof'

Basically, I don't want to repeat the makeNoise() method when I create an animal. However, this will not work. (Noise is an empty string)

I could use a static object like

static String NoiseDog = "woof"
static String NoiseCat = "meow"

but then again I have to write the makeNoise() method for each animal. Is there a better way to architect this?

Set the noise member of the super class in the constructor on each of the sub classes. You only need to implement makeNoise() in the super. — bhspencer, Jul 07 '16 at 19:29

score 2 · Answer 1 · answered Jul 07 '16 at 19:37

If you want to force all sub-classes of Animal to have a noise defined, you can enforce that in the constructor:

public abstract class Animal {
    private final String noise;

    public Animal(final String noise) {
        this.noise = noise;
    }

    public String makeNoise() {
        return noise;
    }
}

Then Dog:

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

And Cat:

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

And to test it out:

public class Test {
    public static void main(String[] args) {
        final Animal dog = new Dog();
        System.out.println(dog.makeNoise());
        final Animal cat = new Cat();
        System.out.println(cat.makeNoise());
    }
}

Output:

woof
meow

Thanks for this. In my real world example, there are 5 different parameters which need to be set differently for each class. So not really easy to pass 5 values in the constructor. — user2689782, Jul 07 '16 at 20:00
@user2689782 If you have 5 parameters to pass to `Animal`, you could wrap those parameters into another class `AnimalCharacteristics` and pass that to `Animal`. — Dan W, Jul 07 '16 at 20:04

score 1 · Answer 2 · answered Jul 07 '16 at 19:33

1

public class Cat extends Animal{
   String noise = "meow";
}

This creates an instance variable named "noise" that hides the superclass variable. Instead, you need this to set the superclass value:

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

answered Jul 07 '16 at 19:33

FredK

4,094
1
9
11

I did not understand this? – user2689782 Jul 07 '16 at 20:02
@user2689782 this sets the String noise in the superclass to "meow", giving you the result you want. – Orin Jul 07 '16 at 20:16

score 0 · Answer 3 · answered Jul 07 '16 at 19:31

0

Make the Animal class abstract. This way, there can be no such thing as an Animal object which calls makeNoise.

Then, set the noise String to a value within the constructor of each subclass as appropriate to that animal's sound.

answered Jul 07 '16 at 19:31

Zircon

4,677
15
32

Same problem. I have a large number of parameters which are specific to each subclass. – user2689782 Jul 07 '16 at 20:01
If they're specific to the subclass, then you don't have to pass them to the constructor as parameters. Just set the values in the constructor body. – Zircon Jul 07 '16 at 20:06
Set the values in the constructor of the superclass? Or subclass? – user2689782 Jul 07 '16 at 20:54
Each subclass. So in the `Dog` constructor you'd have `noise = "Woof"` and so on. – Zircon Jul 07 '16 at 20:55

Maxim Bravo · Answer 4 · 2020-01-02T22:39:56.957

0

Alternatively, you could implement an interface like so:

Animal:

public interface Animal {
    public String makeNoise();
}

Dog:

public class Dog implements Animal {
    public String makeNoise() {
        return "woof";
    }
}

Cat:

public class Cat implements Animal {
    public String makeNoise() {
        return "meow";
    }
}

Test:

public class Test {
    public static void main(String[] args) {
        Animal dog = new Dog();
        System.out.println(dog.makeNoise());
        Animal cat = new Cat();
        System.out.println(cat.makeNoise());
    }
}

Output:

woof
meow

edited Jan 02 '20 at 22:39

answered Jul 07 '16 at 20:24

Maxim Bravo

75
2
11

In this case, I have to define the makeNoise() method in each class. I am trying to get away from that. – user2689782 Jul 07 '16 at 20:53

Superclass/Subclass methods

4 Answers4