Programming to an interface rather than an implementation.

1.Animal.java

package com.chp1.simuduck.v3;

public interface Animal {
    void makeSound();
}

2.Dog.java

package com.chp1.simuduck.v3;

public class Dog implements Animal{

    @Override
    public void makeSound() {
        System.out.println("hav hav hav hav");
    }
}

3.Cat.java

package com.chp1.simuduck.v3;

public class Cat implements Animal{

    @Override
    public void makeSound() {
        System.out.println("miyav miyav miyav.");
    }
}

4.Test.java

    @Test 
    public void test_animal(){
        Animal animal = new Dog();
        animal.makeSound();
        animal = new Cat();
        animal.makeSound();
    }

5.Out.txt

hav hav hav hav
miyav miyav miyav.