1

So, for example, let's say I have this class named for example "Animals", a subclass called "Dogs" and other one called "Cats" and a class for running the program.

class Animals{
    int size;
    String name;
}
class Dogs extends Animals{
    void bark(){
        System.out.println("Woff");
    }
}

class Cats extends Animals{
    void meow(){
        System.out.println("Meow");
    }
}

And in the class Test I want to create an array that holds both of them. How should I declare them so I can also access to their methods? I was thinking, for example, in something like this: 

class Test{
    public static void main(String[] args){
        Animals [] array = new Animals[2];
        array[0] = new Dogs();
        array[1] = new Cats();
        array[0].bark();
    }
}

But it doesn't seem to work that way since Java recognizes the method as a Animals' method.

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  • 2
    As a good practice, your class should be called Animal not Animals. Commented Sep 28, 2017 at 21:54

3 Answers 3

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1

You need to use instanceof to check wether the animal is a cat or a dog, then safe cast and call the good method : 

public static void main(String[] args){
    Animals [] array = new Animals[2];
    array[0] = new Dogs();
    array[1] = new Cats();
    array[0].bark();
}

public static void makeSound(Animal[] animals){
    for(Animal a : animals){
        if(a instanceof Dogs){
            ((Dogs)a).bark();
        }else if(a instanceof Cats){
            ((Cats)a).meow();
        }
}

This will print :

Woff
Meow

Better practice : 

class Animals{
    int size;
    String name;
    abstract void makeSound();
}
class Dogs extends Animals{
    void makeSound(){
        System.out.println("Woff");
    }
}

class Cats extends Animals{
    void makeSound(){
        System.out.println("Meow");
    }
}
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Comments

1

Why doesn't this work

As the array is of animals it won't let you call the subclasses methods, as the compiler can't be sure any animal would have a bark or meow method.

How to fix

You can do one of the following:

  • Add an abstract makeSound method to animal, and use for both, so the compiler knows the method exists.
  • Have 2 arrays, 1 of dogs and one of cats, so the compiler knows the method exists.
  • Check the types of each item using instanceof and call the methods after casting to the right type, so the type is checked at runtime.
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0

You can ask for the instance of the object at the array position and after this it's safe to call the method by casting the object to the dogs class:

if(array[0] instanceof Dogs) {
    ((Dogs)array[0]).bark()
}
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