In the first two parts of this series of articles, we had a look at generics, enhanced for loop and auto boxing/ unboxing. In this final article in this series, we will look at metadata and annotations. What's new in J2SE 5.0? Part 3: MetaData / Annotations and Enumerated Types

Three part series: What's new in J2SE 5.0?:

• Part 1: Generics
• Part 2: Enhanced For Loop and Auto-boxing / Unboxing
• Part 3: MetaData / Annotations and Enumerated Types

What's new in J2SE 5.0?- Part 3

MetaData / Annotations

Today, annotations and metadata are the buzzwords in the world of Java. J2SE 5.0 has brought this feature for us. In simple words metadata means data about data.

Javadoc comments are widely used to document the code. Then, why are annotations required?

Annotations go well beyond what Javadoc is capable of and achieve compile-time checking and analysis of Java code. This means we can perform many checks on compile-time itself, significantly reducing the debugging time spent trying to fix runtime errors.. For example, we override methods form the super class. And by using @Override annotation the information about method overriding is provided.

Annotations can be categorized as follows:

• Marker annotations: These annotations do not use any additional data. So no variables are used and just name is used.
• Single-value annotations: In this case, data supplied is just one argument or a single data member.
• Full annotations: More than one data elements are used by these types of annotations.

Listing 1 shows how the annotations are declared.

Listing 1

public class MyAnnotate {

@Override

public String toSring() {

return super.toString() + " Annotation Override";

}

}

In Listing 1 the annotation @Override has been used... Override is an in-built annotation provided with J2SE 5.0. We will use this annotation as an example.

• Now let?s see how annotations help us in providing data/information about our code and how at compile-time the code is checked with the help of annotations.

Listing 2

Compiling 1 source file to C: JavaApplication1buildclasses
C:JavaApplication1srcjavaapplication1MyAnnotate.java:3: method does not override a method from its superclass
@Override
1 error
BUILD FAILED (total time: 1 minute 11 seconds)

When we define the override annotation and compile, we get an error as shown in Listing 2. This error clearly states that the method we are trying to override doesn?t exist in the super class. Thus this annotation notifies that the method, which is annotated, is actually overriding the method from superclass. This annotation is used on methods and not on classes, interfaces or package declarations.

In J2SE 5.0 one more built-in annotation Deprecated have been provided. You can also define your own custom annotations.