Introduction Of Annotation In Java

Introduction

In this article, we discuss Annotation in Java.

What is Annotation in Java?

Annotation is a tag that represents the metadata (data about data). It is attached to a class, interface, method or field to indicate some additional information that can be used by the Java compiler and JVM.

Annotations, a form of metadata, provide data about a program that is not part of the program itself. Annotations have no direct effect on the operation of the code they annotate.

Some uses of Annotations

Runtime processing
At runtime, some annotations are available to be examined.
Provide information for the compiler.
It can be used by the compiler to detect errors or suppress warnings.
Deployment-time and Compile-time processing.
Software tools can process annotation information to generate code, XML files, and so forth.

Built-In-Annotations

Several built-in annotations are available in Java. Some annotations are applied to Java code and some to other annotations.

Built-In-Annotation that are applied in Java code

@Deprecated
@Override
@SuppressWarnings

Built-In Annotations that are applied in other annotations

@Documented
@Target
@Retention
@Inherited

Understanding Built-In Annotations that are applied in Java code

Let's understand Built-In annotation first.

@override

This annotation provides assurance about the overriding, in other words, that the subclass overrides the parent correctly.
If it does not override correctly then it produces a compile-time error.
Sometimes, due to some silly mistakes like spelling mistakes etcetera, an error is generated. So to avoid the error use @Override annotation.

Example

class Parent {
void doSomework() {
System.out.println("Do some work");
}
}
class Children extends Parent {
@Override
void doSomework() {
System.out.println("No work should be done");
}
}
class Check {
public static void main(String args[]) {
Parent prnt = new Children();
prnt.doSomework();
}
}

Output

@SuppressWarnings

This annotation is used to suppress warnings issued by the compiler.

Example

import java.util.*;
class Test {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
ArrayList arylst = new ArrayList();
arylst.add("Jaun");
arylst.add("Paul");
arylst.add("San");
for (Object o: arylst)
System.out.println(o);
}
}

Output

@Deprecated

@Deprecated marks methods as deprecated, so the compiler understands it and produces a warning and it shows a message to us that it will be removed in the future. So it is not a good approach to use such type of methods.

Example

class Main {
void main() {
System.out.println("hello main");
}
@Deprecated
void child() {
System.out.println("hello Child");
}
}
class Test1 {
public static void main(String args[]) {
Main mn = new Main();
mn.child();
}
}

Output

At compile-Time

It shows an error at compile-time as shown below:

At Run-Time

But at run-time it runs well, as in the following:

Introduction Of Annotation In Java

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Introduction

What is Annotation in Java?

Some uses of Annotations

Built-In-Annotations

Built-In-Annotation that are applied in Java code

Built-In Annotations that are applied in other annotations

@override

@SuppressWarnings

@Deprecated