When do you use Java's @Override annotation and why?

Answer 1

Using the @Override annotation acts as a compile-time safeguard against a common programming mistake. It will throw a compilation error if you have the annotation on a method you're not actually overriding the superclass method.

The most common case where this is useful is when you are changing a method in the base class to have a different parameter list. A method in a subclass that used to override the superclass method will no longer do so due the changed method signature. This can sometimes cause strange and unexpected behavior, especially when dealing with complex inheritance structures. The @Override annotation safeguards against this.

Answer 2

To take advantage from compiler checking you should always use Override annotation. But don’t forget that Java Compiler 1.5 will not allow this annotation when overriding interface methods. You just can use it to override class methods (abstract, or not).

Some IDEs, as Eclipse, even configured with Java 1.6 runtime or higher, they maintain compliance with Java 1.5 and don’t allow the use @override as described above. To avoid that behaviour you must go to: Project Properties ->Java Compiler -> Check “Enable Project Specific Settings” -> Choose “Compiler Compliance Level” = 6.0, or higher.

I like to use this annotation every time I am overriding a method independently, if the base is an interface, or class.

This helps you avoiding some typical errors, as when you are thinking that you are overriding an event handler and then you see nothing happening. Imagine you want to add an event listener to some UI component:

someUIComponent.addMouseListener(new MouseAdapter(){
  public void mouseEntered() {
     ...do something...
  }
});

The above code compiles and run, but if you move the mouse inside someUIComponent the “do something” code will note run, because actually you are not overriding the base method mouseEntered(MouseEvent ev). You just create a new parameter-less method mouseEntered(). Instead of that code, if you have used the @Override annotation you have seen a compile error and you have not been wasting time thinking why your event handler was not running.

Answer 3

Its best to use it for every method intended as an override, and Java 6+, every method intended as an implementation of an interface.

First, it catches misspellings like "hashcode()" instead of "hashCode()" at compile-time. It can be baffling to debug why the result of your method doesn't seem to match your code when the real cause is that your code is never invoked.

Also, if a superclass changes a method signature, overrides of the older signature can be "orphaned", left behind as confusing dead code. The @Override annotation will help you identify these orphans so that they can be modified to match the new signature.