Interface instance?

[Serena Zhou]

Can a interface have a instance? It doesn't compile when I tried in websphere. But I have seen mock questions that has instance of interface and it didn't report compilation error. Why?

[Dirk Schreckmann]

Serena,
Welcome to JavaRanch!
We ain't got many rules 'round these parts, but we do got one. Please change your display name to comply with The JavaRanch Naming Policy.
Thanks Pardner! Hope to see you 'round the Ranch!

[Abdulla Mamuwala]

Hi
An interface is 100% abstract class. We know that an abstract class cannot be instantiated, therefore the same applies to an interface.

[Corey McGlone]

Originally posted by Abdulla Mamuwala:
Hi
An interface is [b]100% abstract class. We know that an abstract class cannot be instantiated, therefore the same applies to an interface.[/B]
An interface is not the same as an abstract class - they are quite different.
However, it is correct that you can not instantiate an interface.
If you look at some mock exam questions, though, you might be lead to believe that you can have instances of interfaces. Check out this example:

doItButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            ....
        }
    }
);

ActionListener is an interface and it appears that we're instantiating it. However, it's important to realize that we're not actually making a new instance of an interface, we're creating an anonymous object that implements ActionListener and that's certainly legal.
I hope that helps. If you have some specific examples that you're still confused about, post them here and I'll try to help.
Corey

[C. Magmanum]

Originally posted by Serena:
Can a interface have a instance? It doesn't compile when I tried in websphere. But I have seen mock questions that has instance of interface and it didn't report compilation error. Why?

maybe u saw something like this

Set someSet = new HashSet();

...in this case, u are actually creating an object of HashSet, which is an implementation of Set. (I am new to java but I think I am right)

[Serena Zhou]

Sorry I didn't include code I am puzzled about. Here is one of the questions:
Given the following classes, which of the following will compile without error?

  
interface IFace{}
class CFace implements IFace{}
class Base{}
public class ObRef extends Base
{
   public static void main(String argv[])
   {
      ObRef ob = new ObRef();
      Base b = new Base();
      Object o1 = new Object();
      IFace o2 = new CFace();
    }
}

A. o1 = o2;
B. ob = b;
C. b = ob;
D. o1 =b;
Ans: A D
Another one I am having problem:
What will be the result of an attempt to compile and run the following code?

  
interface Inter{}
class Base implements Inter{}
class Derived extends Base{}
public class Test{
public static void main(String[] args){
  Base b = new Base();
  Derived d = new Derived();
  Inter i = (Base)b; //1
  i = (Base)d; //2
  Derived bd = (Derived)b; //3
  b = (Base)i; //4
  }
}

A. compile error at line 1
B. compile error at line 2
C. compile error at line 3
D. compile error at line 4
E. ClassCastException at runtime
F. Compiles and runs without errors
Ans: E
Thanks

[Corey McGlone]

Serena,
Hmmm. I'm not entirely sure what it is you're confused about. You can have a variable with an Interface type, such as Comparable, but you can't have an instance of an interface - that variable would have to reference some object that implements that interface. In your examples, that's exactly what you have.
That aspect of it is no different from having a variable with the type of an abstract class. We know that we can't instantiate the abstract class so the only use of the variable would be to reference a subclass of that abstract class.
Perhaps if you can point out the line(s) that are giving you trouble, I can be of more help.
Corey

[Serena Zhou]

Ok, I understand it from your following explanation:
You can have a variable with an Interface type, such as Comparable, but you can't have an instance of an interface - that variable would have to reference some object that implements that interface.
Thank you very much!

[Kiran Kumar]

Isn't 'C' also one of answers besides 'A' and 'D' for the first code snippet you have posted?
Correct me if I am wrong.
Regards,
Kiran

[kalpana appala]

I am also thinking that one of the answers for the first snippet is 'C'.
Can anyone explain whether 'C' is correct or not???

Thanks
kalpana

[kalpana appala]

I am also thinking that one of the answers for the first snippet is 'C'.
Can anyone explain whether 'C' is correct or not???

Thanks
kalpana

[Serena Zhou]

Yeah, You are right. C is the right answer, too.
Sorry about that.
You all have sharp eyes.