Question

The following program when run will product stackoverflow as output.

I want to know what is happening in the line where a TestA is being instantiated.

interface TestA { String toString(); }

class Test 
{
  public static void main(String[] args) 
  {
     // whats going on in this line ???
     System.out.println(new TestA() {public String toString() { return "stackoverflow"; } });
  }
}

Answer 1

You are instantiating an anonymous class, that implements the TestA interface.

This technique is very useful for e.g. event listeners in GUI programming, since it saves you from creating a bunch of named single-use classes.