Annotations

Intermediate ~15 min read

Annotations provide metadata (data about data) for our code. They do not change the action of a compiled program directly but are used by compilers and tools.

Built-in Annotations

Java has several built-in annotations:

@Override: Ensures a method is overriding a parent method.
@Deprecated: Marks a method/class as obsolete.
@SuppressWarnings: Tells the compiler to ignore specific warnings.

@Override
public String toString() {
    return "My Object";
}

@Deprecated
public void oldMethod() { }

Creating Custom Annotations

You can create your own annotations using the @interface keyword.

@interface MyAnnotation {
    String value();
    int version() default 1;
}

@MyAnnotation(value = "Test", version = 2)
public void myMethod() { }

Full Example

import java.lang.annotation.*;
import java.lang.reflect.*;

// 1. Define valid targets (Method) and retention (Runtime)
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface MyInfo {
    String author();

String date();

int version() default 1;
}

class DemoClass {

// 2. Use the annotation
    @MyInfo(author = "Anish", date = "2023-10-27", version = 2)
    public void printMessage() {
        System.out.println("Hello from DemoClass");
    }

@Deprecated
    public void oldMethod() {
        System.out.println("This is old.");
    }
}

public class AnnotationExample {
    public static void main(String[] args) throws Exception {

DemoClass obj = new DemoClass();
        obj.printMessage();

System.out.println("\n--- Reading Annotations via Reflection ---");
        Method m = obj.getClass().getMethod("printMessage");

if (m.isAnnotationPresent(MyInfo.class)) {
            MyInfo info = m.getAnnotation(MyInfo.class);
            System.out.println("Author: " + info.author());
            System.out.println("Date: " + info.date());
            System.out.println("Version: " + info.version());
        }
    }
}

Output

Click Run to execute your code

Summary

Annotations start with @.
Use @Override to prevent bugs when overriding methods.
Annotations can be processed at runtime using Reflection (next lesson).

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Built-in Annotations

Creating Custom Annotations

Full Example

Summary

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