Hidden Features of Java

Answer 1

You can define an anonymous subclass and directly call a method on it even if it implements no interfaces.

new Object() {
  void foo(String s) {
    System.out.println(s);
  }
}.foo("Hello");

Answer 2

Not really a feature, but an amusing trick I discovered recently in some Web page:

class Example
{
  public static void main(String[] args)
  {
    System.out.println("Hello World!");
    http://Phi.Lho.free.fr

    System.exit(0);
  }
}

is a valid Java program (although it generates a warning). If you don't see why, see Gregory's answer! ;-) Well, syntax highlighting here also gives a hint!

Answer 3

When working in Swing I like the hidden Ctrl - Shift - F1 feature.

It dumps the component tree of the current window.
(Assuming you have not bound that keystroke to something else.)

Answer 4

Language-level assert keyword.

Answer 5

Not really part of the Java language, but the javap disassembler which comes with Sun's JDK is not widely known or used.

Answer 6

Haven't seen anyone mention instanceof being implemented in such a way that checking for null is not necessary.

Instead of:

if( null != aObject && aObject instanceof String )
{
    ...
}

just use:

if( aObject instanceof String )
{
    ...
}