Creating objects for class and referenced to interface

[mkarthik90]

Hi ,
I want to know the advantages of creating objects for class and referencing to interface.

For example

interface test{
public void display();
}
class Testing implements test{
public void display(){
System.out.println("Inside Testing ");
}
}

class MainClass{
public static void main(String a[]){
Test testingObject = new Testing();
testingObject.display();
}
}

Here why is it a good practice to hold the testingObject in test interface?

[DarrylBurke]

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db

[gimbal2]

Consider this: there is an ArrayList and a LinkedList. Now what if you wanted to make a method which does not care what kind of list it is, but just wants "any" list?

The answer:

public void doSomethingWithAList(List<SomeObject> theList){

}

To this method you can pass a LinkedList or an ArrayList (or a Vector if you really want to) - because they all implement the List interface. But if you'd have done this:

public void doSomethingWithAList(ArrayList<SomeObject> theList){

}

Then you would only be able to pass an ArrayList.

[Tolls]

I have a method in a class:

public ArrayList<String> getMyList() {
   return myList;
}

Now, should I discover that it would actually be better to use a LinkedList for myList (for whatever reason) not only do I have to change the method signature, but I also now have to fix all the other places that use this method. If I'd used an interface instead I would have been able to switch out the concrete class represented by myList without any of the hassle.

It also allows you to stub these things out when testing.

In essence, you want to be coding against the contract of a class (the interface), not the actual implementation of that contract.
The using class shouldn't care how the implementing class does its job, so long as it does its job.