Java Annotations

1. What is an annotation in Java?

An annotation is a form of metadata that provides data about a program but is not part of the program itself.

2. How do you define an annotation?

You define an annotation using the @interface keyword.

public @interface MyAnnotation {
    String value();
}

3. What are the built-in annotations in Java?

Some built-in annotations include @Override, @Deprecated, and @SuppressWarnings.

4. What is the purpose of the @Override annotation?

The @Override annotation indicates that a method is intended to override a method in a superclass.

class Parent {
    void display() {}
}
class Child extends Parent {
    @Override
    void display() {} // Correctly overrides
}

5. What is the purpose of the @Deprecated annotation?

The @Deprecated annotation indicates that a method or class is outdated and should not be used.

@Deprecated
void oldMethod() {}

6. What is the purpose of the @SuppressWarnings annotation?

The @SuppressWarnings annotation is used to suppress compiler warnings for the annotated element.

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

7. How do you apply an annotation to a method?

You can apply an annotation to a method by placing it above the method declaration.

@MyAnnotation("Example")
void myMethod() {}

8. Can annotations have parameters?

Yes, annotations can have parameters, which are defined as methods in the annotation interface.

public @interface MyAnnotation {
    String value();
    int count() default 1; // Default value
}

9. What is a marker annotation?

A marker annotation is an annotation that does not have any elements and is used to mark a class or method.

public @interface MyMarkerAnnotation {}

10. What is a single-value annotation?

A single-value annotation is an annotation that has only one element, allowing you to use a shorthand syntax.

public @interface MySingleValue {
    String value();
}
@MySingleValue("Example") // Shorthand
void myMethod() {}

11. What is a repeatable annotation?

A repeatable annotation allows you to apply the same annotation multiple times to a single element.

@Repeatable(MyAnnotations.class)
public @interface MyAnnotation {
    String value();
}

12. How do you create a repeatable annotation?

You create a repeatable annotation by using the @Repeatable meta-annotation.

@Repeatable(MyAnnotations.class)
public @interface MyAnnotation {
    String value();
}

13. What is a meta-annotation?

A meta-annotation is an annotation that is applied to another annotation, such as @Retention or @Target.

14. What is the @Retention annotation?

The @Retention annotation specifies how long annotations with the annotated type are to be retained.

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

15. What are the retention policies?

The retention policies are SOURCE, CLASS, and RUNTIME.

16. What is the @Target annotation?

The @Target annotation specifies the kinds of program elements to which an annotation type is applicable.

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

17. What are the element types for @Target?

The element types include TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE.

18. What is the @Inherited annotation?

The @Inherited annotation indicates that an annotation type is automatically inherited.

@Inherited
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface MyInheritedAnnotation {}

19. How do you access annotations at runtime?

You can access annotations at runtime using reflection.

MyAnnotation annotation = MyClass.class.getAnnotation(MyAnnotation.class);

20. What is the getAnnotations() method?

The getAnnotations() method returns all annotations present on a class, method, or field.

Annotation[] annotations = MyClass.class.getAnnotations();

21. What is the isAnnotationPresent() method?

The isAnnotationPresent() method checks if a specific annotation is present on a class, method, or field.

boolean isPresent = MyClass.class.isAnnotationPresent(MyAnnotation.class);

22. How do you retrieve an annotation's value?

You can retrieve an annotation's value by calling the appropriate method on the annotation instance.

String value = annotation.value();

23. What is the Documented annotation?

The Documented annotation indicates that an annotation type is to be documented by the Javadoc tool.

24. What is the purpose of the RetentionPolicy enum?

The RetentionPolicy enum defines the three retention policies: SOURCE, CLASS, and RUNTIME.

25. How do you create a custom annotation?

You create a custom annotation by defining an interface with the @interface keyword and specifying its properties.

public @interface MyCustomAnnotation {
    String name();
    int value();
}

26. Can annotations be inherited?

Yes, annotations can be inherited if they are marked with the @Inherited meta-annotation.

27. What is the @FunctionalInterface annotation?

The @FunctionalInterface annotation indicates that an interface is intended to be a functional interface, which has exactly one abstract method.

@FunctionalInterface
public interface MyFunctionalInterface {
    void execute();
}

28. What is the difference between @Target(ElementType.TYPE) and @Target(ElementType.METHOD)?

@Target(ElementType.TYPE) indicates that the annotation can be applied to classes, interfaces, or enums, while @Target(ElementType.METHOD) indicates it can only be applied to methods.

29. How do you use annotations in Spring?

In Spring, annotations are used for configuration, such as @Component, @Autowired, and @RequestMapping.

30. What is the @Entity annotation in JPA?

The @Entity annotation specifies that a class is an entity and is mapped to a database table.

@Entity
public class User {
    @Id
    private Long id;
    private String name;
}

31. What is the @Table annotation in JPA?

The @Table annotation specifies the table to which an entity is mapped.

@Entity
@Table(name = "users")
public class User {}

32. What is the @Column annotation in JPA?

The @Column annotation specifies the column in the database table that is mapped to a field in the entity.

@Column(name = "user_name")
private String name;

33. What is the @Id annotation in JPA?

The @Id annotation specifies the primary key of an entity.

@Id
private Long id;

34. What is the @GeneratedValue annotation in JPA?

The @GeneratedValue annotation specifies that the value of the primary key will be generated automatically.

@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

35. What is the @OneToMany annotation in JPA?

The @OneToMany annotation defines a one-to-many relationship between two entities.

@OneToMany(mappedBy = "user")
private List<Order> orders;

36. What is the @ManyToOne annotation in JPA?

The @ManyToOne annotation defines a many-to-one relationship between two entities.

@ManyToOne
private User user;

37. What is the @ManyToMany annotation in JPA?

The @ManyToMany annotation defines a many-to-many relationship between two entities.

@ManyToMany
private List<Tag> tags;

38. What is the @Transactional annotation in Spring?

The @Transactional annotation is used to define the scope of a transaction in Spring.

@Transactional
public void saveUser(User user) {}

39. What is the @RequestMapping annotation in Spring?

The @RequestMapping annotation is used to map web requests to specific handler methods in a controller.

@RequestMapping("/users")
public String getUsers() {}

40. What is the @Autowired annotation in Spring?

The @Autowired annotation is used for automatic dependency injection in Spring.

@Autowired
private UserService userService;

41. What is the @Component annotation in Spring?

The @Component annotation indicates that a class is a Spring component and should be managed by the Spring container.

@Component
public class UserService {}

42. What is the @Service annotation in Spring?

The @Service annotation is a specialization of the @Component annotation, indicating that a class provides business services.

@Service
public class UserService {}

43. What is the @Repository annotation in Spring?

The @Repository annotation is a specialization of the @Component annotation, indicating that a class is a data access object (DAO).

@Repository
public class UserRepository {}

44. What is the @Configuration annotation in Spring?

The @Configuration annotation indicates that a class declares one or more @Bean methods and can be processed by the Spring container.

@Configuration
public class AppConfig {
    @Bean
    public UserService userService() {
        return new UserService();
    }
}

45. What is the @Bean annotation in Spring?

The @Bean annotation indicates that a method produces a bean to be managed by the Spring container.

@Bean
public UserService userService() {
    return new UserService();
}

46. What is the @Value annotation in Spring?

The @Value annotation is used to inject values into fields from property files or environment variables.

@Value("${app.name}")
private String appName;

47. What is the @PostConstruct annotation?

The @PostConstruct annotation is used to indicate a method that should be executed after dependency injection is complete.

@PostConstruct
public void init() {
    // initialization code
}

48. What is the @PreDestroy annotation?

The @PreDestroy annotation is used to indicate a method that should be executed just before the bean is removed from the context.

@PreDestroy
public void cleanup() {
    // cleanup code
}

49. How do you create a custom annotation in Spring?

You create a custom annotation by defining an interface with the @interface keyword and specifying its properties.

public @interface MyCustomAnnotation {
    String value();
}

50. How do you use annotations in unit testing with JUnit?

In JUnit, annotations like @Test, @Before, and @After are used to define test methods and setup/teardown methods.

import org.junit.Test;
import org.junit.Before;

public class MyTest {
    @Before
    public void setUp() {
        // setup code
    }

    @Test
    public void testMethod() {
        // test code
    }
}