很难让我的孩子 class 准确地覆盖我的父母 class

Question

I'm not new to coding, but new to polymorphism/inheritance/etc. I'm confused about static/dynamic binding, and specifically what happens when I put different subclasses in an array together. I'm working with some very simple animal classes, and just trying to learn the basics.

I tried defining them as ligers too, but then when I tried to print the array of animals' sizes, they both had -1 as their size. The way I have it now gives p0 the right size, but not p1.

   public abstract class Animal {
    public int size = -1;   
   }

   public class Tiger extends Animal {

   }

   public class Liger extends Animal {
     public int size = 121;
   }

   public static void main(final String[] args) {

    Animal[] animal = new Animal[10];
    Animal p0 = new Liger();
    p0.size = 11;
    animals[0] = p0;

    Animal p1 = new Liger();
    animals[1] = p1;
  }

When I define the ligers as animals, p1 gets -1 as its size instead of the 121 I want it to have. I'm sure the issue is with me calling it an Animal instead of a liger, but I'm not sure what the correct syntax is to fix it. I want them to be able to be in an array with tigers.

Answer 1

Please, what is fs in your line

Animal p1 = fs.new Liger();

By the way, take a look at this code and tell me if it does what you are looking for.

public static void main(final String[] args) {

    Animal[] animals = new Animal[10];

    Animal p0 = new Liger();
    p0.size = 11;
    animals[0] = p0;

    Animal p1 = new Liger();
    animals[1] = p1;

    System.out.println("p0.size "+ p0.size); // call as a superclass instance
    System.out.println("real p0.size "+ ((p0 instanceof Liger) ? ((Liger)p0).size : ((Tiger)p0).size) ); // cast to call the effective instance of the subclass
    System.out.println("p1.size "+ p1.size); // call as a superclass instance
    System.out.println("real p1.size "+ ((p1 instanceof Liger) ? ((Liger)p1).size : ((Tiger)p1).size) ); // cast to call the effective instance of the subclass


}

Answer 2

public abstract class Animal {
    private int size;

    public Animal() {
        size = -1;
    }

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

    public void setSize( int size ) {
        this.size = size;
    }

    public int getSize() {
        return size;
    }

    public static void main(final String[] args) {

      Animal[] animals  = new Animal[2];
      animals[0] = new Liger(20);
      animals[1] = new Liger();

      for(int i = 0; i < animals.length; ++i) {
          System.out.println("Size : " + animals[i].getSize());
      }   
    }
}

public class Tiger extends Animal {
      public Tiger() {
         super();    
      }

      public Tiger(int size) {
         super(size);
      }
}

public class Liger extends Animal {
      public Liger() {
         super();    
      }

      public Liger(int size) {
         super(size);
      }  
}

This is just a basic guideline, just for reference. The point here is behaviour can be overridden in subsequent child classes. Java Tutorial