Java Annotations

Annotations in Java are a form of metadata that provide data about a program but are not part of the program itself. Introduced in Java 5, annotations allow developers to write cleaner, more expressive code. They can be used to give the compiler instructions, to be processed during runtime, or to be used by various tools and frameworks.

Types of Annotations in Java

Java annotations are broadly categorized into three types:

Built-in Annotations: Provided by the Java language specification. Examples include @Override, @Deprecated, @SuppressWarnings.
Meta-annotations: Used to create custom annotations. They annotate other annotations, such as @Target, @Retention, @Inherited.
Custom Annotations: Defined by developers to provide specific functionalities in their applications.

Examples of Using Annotations

Built-in Annotations

@Override: Indicates that a method overrides a method from a superclass.

class Animal {
    void eat() {
        System.out.println("Animal is eating");
    }
}

class Dog extends Animal {
    @Override
    void eat() {
        System.out.println("Dog is eating");
    }
}

class Animal {

void eat() {

System.out.println("Animal is eating");

}

class Dog extends Animal {

@Override

void eat() {

System.out.println("Dog is eating");

}

@Deprecated: Marks a program element as deprecated, meaning it should no longer be used.

public class Utility {
    @Deprecated
    public void oldMethod() {
        System.out.println("Old method, do not use.");
    }
}

public class Utility {

@Deprecated

public void oldMethod() {

System.out.println("Old method, do not use.");

}

@SuppressWarnings: Instructs the compiler to suppress specific warnings.

@SuppressWarnings("unchecked")
public void myMethod() {
    List list = new ArrayList();
    list.add("test"); // This causes an unchecked warning
}

@SuppressWarnings("unchecked")

public void myMethod() {

List list = new ArrayList();

list.add("test"); // This causes an unchecked warning

}

Meta-annotations

@Retention: Specifies how long annotations with the annotated type are to be retained.

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation { }

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)

public @interface MyAnnotation { }

@Target: Specifies the kinds of elements an annotation type can be applied to.

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

@Target(ElementType.METHOD)
public @interface MyMethodAnnotation { }

import java.lang.annotation.ElementType;

import java.lang.annotation.Target;

@Target(ElementType.METHOD)

public @interface MyMethodAnnotation { }

Custom Annotations

Custom annotations can be defined to include information that can be processed at runtime or compile time.

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Test {
    // This can be used to annotate test methods
}

import java.lang.annotation.ElementType;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)

@Target(ElementType.METHOD)

public @interface Test {

// This can be used to annotate test methods

}

Usage:

public class ExampleTest {
    
    @Test
    public void testMethod() {
        // Test code here
    }
}

public class ExampleTest {

@Test

public void testMethod() {

// Test code here

}

Conclusion

Annotations in Java provide a powerful way to add metadata to your Java code, making it more readable, maintainable, and versatile. They are extensively used in modern Java applications for various purposes, including documentation, code analysis, and runtime processing by frameworks and libraries. Understanding how to use built-in annotations, meta-annotations, and how to define custom annotations is essential for effective Java development.