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


Question

What are the best practices for using Java's @Override annotation and why?

It seems like it would be overkill to mark every single overridden method with the @Override annotation. Are there certain programming situations that call for using the @Override and others that should never use the @Override?

2011/11/09
1
498
11/9/2011 12:12:28 AM

Accepted Answer

Use it every time you override a method for two benefits. Do it so that you can take advantage of the compiler checking to make sure you actually are overriding a method when you think you are. This way, if you make a common mistake of misspelling a method name or not correctly matching the parameters, you will be warned that you method does not actually override as you think it does. Secondly, it makes your code easier to understand because it is more obvious when methods are overwritten.

Additionally, in Java 1.6 you can use it to mark when a method implements an interface for the same benefits. I think it would be better to have a separate annotation (like @Implements), but it's better than nothing.

2008/09/18
515
9/18/2008 4:53:19 PM

I think it is most useful as a compile-time reminder that the intention of the method is to override a parent method. As an example:

protected boolean displaySensitiveInformation() {
  return false;
}

You will often see something like the above method that overrides a method in the base class. This is an important implementation detail of this class -- we don't want sensitive information to be displayed.

Suppose this method is changed in the parent class to

protected boolean displaySensitiveInformation(Context context) {
  return true;
}

This change will not cause any compile time errors or warnings - but it completely changes the intended behavior of the subclass.

To answer your question: you should use the @Override annotation if the lack of a method with the same signature in a superclass is indicative of a bug.

2008/09/18

There are many good answers here, so let me offer another way to look at it...

There is no overkill when you are coding. It doesn't cost you anything to type @override, but the savings can be immense if you misspelled a method name or got the signature slightly wrong.

Think about it this way: In the time you navigated here and typed this post, you pretty much used more time than you will spend typing @override for the rest of your life; but one error it prevents can save you hours.

Java does all it can to make sure you didn't make any mistakes at edit/compile time, this is a virtually free way to solve an entire class of mistakes that aren't preventable in any other way outside of comprehensive testing.

Could you come up with a better mechanism in Java to ensure that when the user intended to override a method, he actually did?

Another neat effect is that if you don't provide the annotation it will warn you at compile time that you accidentally overrode a parent method--something that could be significant if you didn't intend to do it.

2008/09/18

I always use the tag. It is a simple compile-time flag to catch little mistakes that I might make.

It will catch things like tostring() instead of toString()

The little things help in large projects.

2008/09/18

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.

2008/09/18

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.

2010/11/03