Java Annotations

What are Annotations?

Annotations are basically syntactic meta-data that can be applied to packages, classes, methods, variables & parameters. 

Purpose

Annotations serve several purposes such as...

• Provide extra information to the compiler
• @Override
• @SuppressWarnings
• @Deprecated
• Reduce boiler plate code
• See project LomBok
• AOP as in Spring AOP
• Replacement for marker interfaces
• Used by frameworks to glue user defined classes together into an application instead of specifying the same in an external XML configuration file
• Spring
• Provide information to be used at run-time 

Examples
Annotations can have elements which have values
@Author(name="kirk",  date="1/1/2014")

If annotations have a single element named value, it can be omitted as shown below:
@SuppressWarnings("unchecked")

If annotations have no elements, then the braces can be omitted as well:
@Override

References
Java documentation: http://docs.oracle.com/javase/tutorial/java/annotations/
Wikipedia: http://en.wikipedia.org/wiki/Java_annotation