Iinherited fields value are not changed

I'm not sure if these question is still appropriate to be asked as there could be an answer already. But i still dont understand the concept of inheritance when it comes to attributes in parent and child class relationship. Please note the example below.

class Animal{
    public int lifeSpan = 50;

    public String imAn(){
        return "I'm an Animal";
    }
}

class Elephant extends Animal{
   public int lifeSpan = 100;

   public String imAn(){
        return "I'm an Elephant";
    }
}

public class Test{
  public static void main(String args[]){
    Animal animal = new Elephant();
    System.out.println(animal.imAn()+" and i live around "+animal.lifeSpan+" years");  
  }
}

Answer would be : I'm an Elephant and i live around 50 years.

I do understand the virtual method invocation concept here, but what makes it not load the lifeSpan attribute of child class. Does this means the class attributes are not polymorphic?

Jon Skeet
people
quotationmark

Does this means the class attributes are not polymorphic?

No, fields aren't polymorphic. You've actually got two fields in your Elephant class - one declared in Animal and one declared in Elephant, which hides the one in Animal. That's the problem. You should get rid of the declaration of lifeSpan in Elephant, and instead initialize the one remaining field in a constructor.

You should also get into the habit of making fields private - and final, where possible. Assuming you really want to have a method to describe the name (rather than a field, which would be more usual) I would write your code as:

class Animal {
    private final int lifeSpan;

    public Animal() {
        // Default to a life-span of 50
        this(50);
    }

    public Animal(int lifeSpan) {
        this.lifeSpan = lifeSpan;
    }

    public String getName(){
        return "Animal";
    }

    public int getLifeSpan() {
        return lifeSpan;
    }
}

class Elephant extends Animal {
    public Elephant() {
        // Give every elephant a life-span of 100.
        super(100);
    }

    @Override public String getName() {
        return "Elephant";
    }
}

public class Test {
    public static void main(String args[]){
        Animal animal = new Elephant();
        System.out.printf("I am an %s and I live around %d years%n",
            animal.getName(), animal.getLifeSpan());
    }
}

people

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