Annotations

Annotations provide metadata about the program.

Built-in Annotations

@Override

Indicates a method overrides a superclass method.

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

@Deprecated

Marks code as outdated.

@Deprecated
public void oldMethod() {
    // Don't use this!
}

@SuppressWarnings

Suppresses compiler warnings.

@SuppressWarnings("unchecked")
List list = new ArrayList();

Meta-Annotations

Used to create custom annotations.

@Retention

How long the annotation is retained.

@Retention(RetentionPolicy.RUNTIME) // Available at runtime
@Retention(RetentionPolicy.SOURCE)  // Discarded by compiler
@Retention(RetentionPolicy.CLASS)   // In .class file, not at runtime

@Target

Where the annotation can be applied.

@Target(ElementType.METHOD)  // Only on methods
@Target(ElementType.TYPE)    // On classes/interfaces
@Target(ElementType.FIELD)   // On fields

Custom Annotations

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Test {
    int priority() default 1;
    String author();
}

// Usage
@Test(priority = 2, author = "Alice")
public void myTest() {
    // Test code
}

Processing Annotations with Reflection

Method[] methods = MyClass.class.getDeclaredMethods();
for (Method method : methods) {
    if (method.isAnnotationPresent(Test.class)) {
        Test test = method.getAnnotation(Test.class);
        System.out.println("Priority: " + test.priority());
        System.out.println("Author: " + test.author());
    }
}

Quiz