Question

In Java, ‘Set’ and ‘List’ are interfaces derived from ‘Collection’ interface.
If we use the code:

import java.util.*;

public class SetExample{

    public stactic void main(String[] args){
      Set set = new HashSet();
      //do something .....
    }

}

Is there a class ‘Set’ in “Collection” API that we are creating an object (‘set’) of? or we are instantiating a interface ‘Set’?

Am really confused…….:O

Answer 1

java.util.Set is an interface, not a class. So

Set set = new HashSet();

creates an object that is a HashSet instance, and assigns a reference to that object to a variable whose type is Set. This works because the HashSet class implements the Set interface. On the other hand:

Set set = new Set();

gives a compilation error because you cannot create an instance of an interface.

An Java interface is essentially a contract between an implementation (a class) and the things that use it. It says what the names and signatures of a conforming object’s methods are, but nothing about the object’s state or how its methods work.

(Just to confuse things a bit … Java also allows you to write something like this:

Set set = new Set() {
    // attributes and methods go here
};

This is does not create an “instance” of the Set interface per se … because that doesn’t make sense. Rather, it declares and instantiates an anonymous class that implements the Set interface.)