Why assigning a subclass instance to a variable of type of superclass ?

Question

Hi all,
What is the significance of assigning an instance of a subclass to a variable whose type is a superclass's type.
For eg. List list=new ArrayList();
If I do so is it possible to execute the methods of the subclass ?
regards
Anto Paul.

Answer 1

In addition, this is what polymorphism is all about:
abstract class Animal {
  String name;
  Animal(String name) {
    this.name = name;
  public abstract void voice();
class Dog extends Animal {
  Dog(String name) { super(name); }
  public void voice() {
    System.out.println(name+" barks");
class Cat extends Animal {
  Cat(String name) { super(name); }
  public void voice() {
    System.out.println(name+" miaows");
public class Polymorphism {
  public static void main(String args[]) {
    Animal[] animals = {
      new Dog("Fido"),
      new Cat("Felix")
    for (int i = 0; i < animals.length; i++) {
      animals.voice();
Here, we are looping through an array of Animals. In fact, these are concrete subclasses of the abstract Animal class. In this simple example, you can see from the code that the animals array contains a dog and a cat. But we could even extend this to read in an arbitrary Animal object that had been serialized into a file. At compile time, the exact class of the serialized animal would not be known. But polymorphism occurs at runtime to ensure the correct voice() method is called.
For the List, or Map example, conisder this:
public SomeClass {
  public HashMap map1 = new HashMap();  // this ties you to hashmap
  public Map map2 = new HashMap();      // this allows you to change Map implementation
  public void process(HashMap map) {}   // this ties you to hashmap
  public void process(Map map) {}       // this allows you to change Map implementation
}Suppose you use a HashMap for map2 to start with. But at some point in the future you would like to ensure your map is sorted. By specifying map2 to be a Map, you can change the implementation without having to modify each method call by simplying changing the initiliastion to be:
Map map2 = new TreeMap();Hope some of this helps :) Cheers, Neil