Java Code House

Today in this article I will show you how to create your own annotation. Here we will start with a very basic annotation and then slowly show you how to include arrays and also how to use default values. We will also show you how to yo use annotations. We have divided our tutorial on creating and using annotations in two parts. The first part meaning this one is a very basic and good one for beginners to start with.
In our first annotation we will just start with an empty annotation specifying only @target and @Retention.

import java.lang.annotation.*;

@Target(value={ElementType.FIELD})
@Retention(value=RetentionPolicy.RUNTIME)
public @interface Author {
}

In the next example you will see that when there is only one value for an annotation then we can omit the name as well as braces{} indicating arrays. The above sample is re-written as

import java.lang.annotation.*;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Author {
}

In our next example you will see how to mention the target type when it is more than one say FIELD and METHOD.

import java.lang.annotation.*;

@Target({ElementType.FIELD, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Author {
}

In our next example we will deal with annotation which has annotation type elements. It looks almost like methods. We will also show you how to use this annotation in a class.

import java.lang.annotation.*;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Author {
 String name();
}

class MyClass {
 @Author(name="Nirupam")
 int i = 10;
}

In our next and last example of today, we will show you how to mention default values of annotation type elements. It is done using keyword default . If a default value is mentioned, then you can omit adding your own value. But it does not mean that you cannot add your own value, of course you can do it.

import java.lang.annotation.*;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Author {
 String name() default "Nirupam";
}

class MyClass {
 @Author
 int i = 10;
}

• @Deprecated : This annotation is used to indicate that the element marked with it should not be used any longer. The compiler will generate warning if any program uses class, field or method marked with this annotation. It should also be documented with Javadoc annotation @deprecated. Many people make mistakes between the use of @ sign in annotation and javadoc comment. They are completely different and most importantly the annotation has upper-case while the Javadoc comment has a lower-case.

class DeprecationDemo{
   /**
   * @deprecated
   * defines how to use deprecated annotation
   */
   @Deprecated
   public void deprecatedMethod(){}
}

• @Override : This annotation is used to indicate that an element marked with it is meant to override an element of super-class. This is not necessary to override an element but helps to prevent runtime errors if Java fails to override during run-time. To know more why should you use this read our article here
• @SupressWarnings : This annotation is used to supress some warnings that would be generated by the compiler if not used. In our example we will inherit a serializable class JPanel and not write the variable serialVersionUID which will generally result in warnings. But using this annotation will supress it.

@SupressWarnings("serial")
class SupressWarningsDemo extends javax.swing.JPanel{
   
}

It can also be used to supress other warnings like deprecated using "deprecation" if using any deprecated elements. Also unchecked warnings using "unchecked" while interfacing with legacy codes before the inclusion of generics.
• @SafeVarargs : This annotation when applied to a method confirms that there will be no unsafe operation with the variable argument parameter. When it is used unchecked warnings associated with varargs are supressed.
• @FunctionalInterface : This  annotation is used to indicate that an interface is functional interface. this one is related to lambda expression and method references. This annotation is included in Java 8. It can only used with interfaces and if used with classes or enums it will create an error.

• Information used by compiler : They can be used which carries special meaning to the compiler and helps in detecting any compile time errors.
• Deployment-time processing : They can be used by softwares, servers to generate codes, files and also to deploy codes as in case of servlets.
• Runtime processing : They can be used to carry information during runtime.