interface question

[Timothy Sam]

Hi guys! I know that an interface is like a 100% abstract class... And an abstract class cannot be instantiated... But are there exceptions to this rule? Is there any way an interface can be instantiated?

[MathuKollappan Kollappan]

This is the one way of initializing

interface Interface
{
public void showSomeThing();
}

class InterfaceImpl implements Interface
{
public void showSomeThing()
{
System.out.println("Interface Demo");
}
public static void main(String[] args)
{
Interface obj = new InterfaceImpl();
obj.showSomeThing();
}
}

[memati bas]

Interfaces can't have instance variables (non-static fields) or instance initializers.

[marc weber]

You can instantiate a concrete class that extends an abstract class and/or implements an interface, and then upcast the reference to the type of the abstract class or interface. But you cannot directly instantiate an abstract class or interface.

[Timothy Sam]

Understood... Thanks!

[memati bas]

Ohh,
Good explanation Marc...

[Stan James]

Say SomeInterface has one requiredMethod(). There is syntax like:

SomeInterface x = new SomeInterface 
   {
      public void requireMethod()
      {
         method implementation
      }
   }
x.requiredMethod();

That looks for all the world like you're instantiating an interface, but you're really defining a new class that implements the interface and instantiating the new class. The new class doesn't have the "public class name" clause so it looks pretty odd. This "anonymous class" trick is often seen with listeners for Swing events, defining them on the fly rather than putting them in their own class files.

[Tony Morris]

Correction

SomeInterface x = new SomeInterface() 
{
   public void requireMethod()
   {
      // method implementation
   }
}
x.requiredMethod();

[Ken Blair]

Originally posted by Tony Morris:
Correction

SomeInterface x = new SomeInterface() 
{
   public void requireMethod()
   {
      // method implementation
   }
}
x.requiredMethod();

That's still not going to compile on account of the fact that neither of you have implemented the requiredMethod(). Pun intended.

*grin*

[Jon Egan]

further harrassment....

and even then it won't compile, because you need a semicolon after the closing brace on the next-to-last line, to complete the assignment statement started on the first line....

-- Jon

[Ken Blair]

Oh dear how did I miss that.

[Tony Morris]

interface SomeInterface
{
    void requiredMethod();
}
 
final class X
{
    {
        SomeInterface x = new SomeInterface() 
        {
           public void requiredMethod()
           {
              // method implementation
           }
        };
 
x.requiredMethod();
    }    
}

[Stan James]

Yeah, I carefully said the syntax is "like" what I showed. Didn't take the time to try to compile it. Thanks for the fixes, all.