Java 1.5 Tiger A Developers Notebook [Electronic resources]

6.2 Annotating an Overriding Method

The Override annotation type is a marker interface, and has no members to initialize. It is used to indicate that a method is overriding a method from its superclass. In particular, it's supposed to help ensure you actually do override a methodby avoiding misspellings or confusion over the correct name and parameter list of the method you're trying to override. In these cases of error, a compiler can catch the problem and report it.

6.2.1 How do I do that?

Because Override is a marker interface, there are no values that you need to supply it. Just preface it with the annotation syntax marker, the at sign (@), and type "Override". This should be on a line by itself, just before the method declaration you want to indicate as an overriding method, as seen in Example 6-1.

NOTE

Largely by coincidence, the @ sign, pronounced "at", is a mnemonic for Annotation Type.

Example 6-1. Using the Override annotation type

package com.oreilly.tiger.ch06;
public class OverrideTester {
public OverrideTester( ) { }
@Override
public String toString( ) {
return super.toString( ) + " [OverrideTester Implementation]";
}
@Override
public int hashCode( ) {
return toString( ).hashCode( );
}
}

This isn't very sexy or glamorous, but it compiles without any problem. Where things start to become useful is when you don't do what you intended do. Change the hashCode( ) method to look like this:

     @Override
public int hasCode( ) {
return toString( ).hashCode( );
}

NOTE

This method exists, but is commented out of, the Override- Tester's source listing in the book's sample code.

Here, hashCode( ) is misspelled, and the method is no longer overriding anything, making the annotation speak up. Compile this class, and you'll get the following error:

       [javac] src\ch06\OverrideTester.java:1:
method does not override a method from its superclass
[javac]   @Override
[javac]    ^
[javac] 1 error

Suddenly, that little annotation becomes quite a boon, catching mistakes at compile time. It's also such an easy thing to integrate into your programming practices that I'd recommend you use it often.

6.2.2 What about...

...the methods that should be overridden? Override marks the overriding method, not the overridden methodthis is an important distinction, and you should learn it well. Java has ways to indicate that a method should be overridden alreadynamely, by declaring that method as abstract.