An Introduction to Annotations

Sun Microsystem added the features like annotation to make the development easier and more efficient in jdk 5. The main objective to develop the annotations is to make the development easier. Annotations behaves like the meta. The literal meaning of meta data is data about data. Java also signifies this meaning. Annotations are like meta data, means you are free to add your code and can also apply them to variables, parameters, fields type declarations, methods and constructors. Metadata is also used to create the documentation to perform rudimentary compile time checking and even for tracking down the dependencies in code. XDoclet contains all these features and is widely used. Annotations provide a means of indicating about methods, classes, dependencies, incompleteness and also about the references on other methods and classes respectively. Quoting from Sun’s official site, “It (annotation-based development) lets us avoid writing boilerplate code under many circumstances by enabling tools to generate it from annotations in the source code. This leads to a declarative programming style where the programmer says what should be done and tools emit the code to do it.”

Annotation is the way of associating the program elements with the meta tags so that the compiler can extract program behavior to support the annotated elements to generate interdependent code when necessary.

Fundamentals of annotations

While going through the annotations you should consider two things. The first one is the “annotation” itself and second one is the “annotations types”. An annotation is the meta tag, used to give some life to the code you are using. While annotation type is used to define annotations so that you can use them while creating your own custom annotations. An annotation type definition appends an “at” @ sign at the start of the interface keyword with the annotation name. On the other hand, an annotation includes the “at” @ sign followed by the annotation type. You can also add the

data within the parenthesis after the annotation name. Lets illustrate the concept more clearly by using some examples.

Annotation Types:

Three types of annotations types are there in java.

I. Marker:

Like the marker interface, marker annotations does not contain any elements except the name itself. The example given below clarifies the concept of marker interface.

Example:

II. Single-value:

This type of elements provide only single value. It means that these can be represented with the data and value pair or we can use the shortcut syntax (just by using the value only within the parenthesis).

Example: