abstraction

[manohar reddy]

how to instatiate abstraction

[Stan James]

An abstraction in Java is probably an interface or abstract class. You can't instantiate these. Both of these will fail:

x = new Runnable();
y = new AbstractList();

You can make a concrete class that implements the interface or extends the abstract class. The compiler will require the concrete class to provide all the abstract methods. Here's a concrete class you could instantiate:

public class MyRunnable implements Runnable
{
   public void run() 
   {
      whatever...
   }
}

And there's another tricky way to create a class right in line instead of defining it in its own file:

Runnable r = new Runnable() {
   public void run()
   {
      do something
   }
};

That looks a lot like we instantiated Runnable, an interface, but in fact we defined a tiny class with a run() method. It has no name so it's called anonymous, but it's a perfectly good little class as far as the compiler's concerned.

Does that all make sense?