An array for different objects in java?

Question

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.

• Ousmane D.: As a good practice, your class should be called Animal not Animals.

Answer 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");
 }
}

Answer 2

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.

Answer 3

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()
}