Java and Spring Boot Interview Questions and Answers (Top 60+)

Here are the parts of the tutorial series:

1. Difference between Java 8, 17, and 21

Java 8 (2014):

Introduced Lambdas, Streams, Functional Interfaces
Default and static methods in interfaces
Optional class for better null handling
New Date and Time API (java.time.*)
Nashorn JavaScript Engine
Parallel array operations

Java 17 (2021 - LTS):

Sealed classes
Pattern Matching for instanceof
New macOS rendering pipeline (Metal)
Deprecation/removal of older APIs (like Applet API)
Strong encapsulation of JDK internals

Java 21 (2023 - LTS):

Virtual Threads (Project Loom)
Record patterns, unnamed patterns
Pattern matching for switch
String templates (Preview)
Sequenced Collections
Structured concurrency (Preview)

Java 17 and 21 focus more on performance, language enhancements, and concurrency improvements.

2. Java 8 Features

Lambda Expressions
Functional Interfaces
Streams API
Default and Static Methods in Interfaces
New Date and Time API
Optional Class
Nashorn JavaScript Engine
Method References
Repeatable Annotations

3. Why Should We Use Predicate?

Predicate<T> is a functional interface with a method boolean test(T t).
Used for filtering and matching conditions (e.g., in Streams).
Promotes clean, declarative style code.

Example:

List<String> names = List.of("John", "Jane", "Doe");
names.stream().filter(name -> name.startsWith("J")).forEach(System.out::println);

4. What is a Functional Interface?

An interface with exactly one abstract method.
Can have multiple default or static methods.
Used with Lambda expressions.

Example:

@FunctionalInterface
interface MyFunction {
    void apply();
}

5. Default Methods in Interfaces

Introduced in Java 8 to allow methods with body in interfaces.
Helps in interface evolution without breaking implementation classes.

Calling Default/Static Method Example:

interface Demo {
    default void show() {
        System.out.println("Default Method");
    }

    static void display() {
        System.out.println("Static Method");
    }
}

class Test implements Demo {}

new Test().show();
Demo.display();

6. Can We Create Object of an Interface?

No, interfaces cannot be instantiated directly.
They can be implemented by classes or used via anonymous inner classes/lambdas.

7. What is a Lambda Expression?

A short block of code which takes in parameters and returns a value.
Syntax: (parameters) -> expression

Example:

Runnable r = () -> System.out.println("Hello World");
r.run();

8. Difference Between Abstract Class and Interface

Feature	Abstract Class	Interface
Inheritance	Single	Multiple
Access Modifiers	Can use	Only public
Fields	Yes	Only static final
Constructors	Yes	No
Default Impl	Yes	Yes (Java 8+)

9. Can We Create Object of Abstract Class?

No, but we can call concrete methods via subclass.

Example:

abstract class Animal {
    void sound() { System.out.println("Sound"); }
}

class Dog extends Animal {}
new Dog().sound();

10. What is Method Overriding?

Redefining a superclass method in a subclass with the same signature.
Used for runtime polymorphism.

11. What if Return Type is Changed in Overriding?

Must be covariant: child return type must be subtype of parent return type.
Changing from int to String causes compile-time error.

12. What is Method Overloading?

Same method name with different parameters.

Example:

void print(int a) {}
void print(String a) {}

13. Does Overloading Apply in Inheritance?

Yes. Overloaded methods can be inherited and further overloaded.

14. What is a Stream?

Introduced in Java 8 for processing collections in a functional way.
Operations: filter, map, reduce, collect, sorted.
Types: Sequential, Parallel

15. Map vs FlatMap

map transforms each element.
flatMap flattens nested structure.

Example:

list.stream().map(String::toUpperCase);
listOfLists.stream().flatMap(List::stream);

16. What is a Singleton Class? How Can We Make It?

A class that allows only one instance.
Ensures controlled access to the only instance.

Example:

public class Singleton {
    private static final Singleton instance = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() {
        return instance;
    }
}

17. What is a Synchronized Keyword?

Used to prevent thread interference and memory consistency errors.
Applies lock to method or block so only one thread can execute.

18. notify vs notifyAll

notify() wakes a single waiting thread.
notifyAll() wakes all waiting threads.
Both used in inter-thread communication with wait().

19. Thread Lifecycle

New
Runnable
Running
Waiting
Timed Waiting
Terminated

20. What is Runnable? How Many Methods?

Functional interface to define a task for threads.
Has one method: void run();

21. Can We Call start() on a Thread Multiple Times?

No. Once a thread is started, calling start() again throws IllegalThreadStateException.

22. What is an Immutable Class?

State cannot be modified after creation.
All fields final, private constructor, no setters.

23. What is final, finally, and finalize?

final: constant or unchangeable (variable, method, class)
finally: block always executed after try-catch
finalize(): method called before object is garbage collected (deprecated)

24. What is try-with-resources?

Introduced in Java 7.
Simplifies closing of resources (like Streams, Connections).
Automatically closes resources implementing AutoCloseable.

Example:

try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    System.out.println(br.readLine());
} catch (IOException e) {
    e.printStackTrace();
}

25. What if I Declare NullPointerException in Catch and It Is Thrown?

It will be caught if it's declared correctly.

try {
    String s = null;
    s.length();
} catch (NullPointerException e) {
    System.out.println("Caught NPE");
}

26. What Is an Unchecked Exception? Give an Example

Runtime exceptions, not checked at compile time.
Example: NullPointerException, ArrayIndexOutOfBoundsException

27. How to Make a Custom Exception?

class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

28. How to Handle Exceptions in Spring Boot?

Using @ControllerAdvice with @ExceptionHandler

@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(CustomException.class)
    public ResponseEntity<String> handleException(CustomException e) {
        return new ResponseEntity<>(e.getMessage(), HttpStatus.BAD_REQUEST);
    }
}

29. What is 400 Status Code?

Bad Request: The server cannot process the request due to client error.

30. What is @RestController?

Combines @Controller + @ResponseBody
Used to build RESTful APIs.
Returns JSON/XML instead of views.

31. Difference Between @Controller and @RestController?

Feature	@Controller	@RestController
Purpose	Web MVC (HTML/JSP)	REST API (JSON/XML)
@ResponseBody	Must be added manually	Implicitly present

32. What Are Idempotent Methods?

Same result no matter how many times called.
HTTP Methods: GET, PUT, DELETE are idempotent; POST is not.

33. What Are Stereotype Annotations?

Spring annotations that define roles:
- @Component
- @Service
- @Repository
- @Controller

34. What Do We Use @SpringBootApplication For?

Combines @Configuration, @EnableAutoConfiguration, and @ComponentScan
Entry point for Spring Boot application.

35. Is @EnableConfiguration IOC Configuration?

@EnableAutoConfiguration is more accurate.
Part of enabling Spring Boot's inversion of control and auto-config.

36. What is Dependency Injection?

Design pattern to inject dependencies at runtime.
Reduces tight coupling, improves testability.
Done using @Autowired, constructor injection, or setter.

37. Why Use @Autowired for Dependency Injection?

Automatically resolves and injects beans.
Eliminates the need for explicit instantiation.

38. Where Are Beans Stored in Spring Boot?

ApplicationContext container.
Retrieved using @Autowired or context.getBean().

39. What is IOC and Its Types? How to Achieve It?

Inversion of Control: Delegating creation of objects to container.
Types:
- Constructor Injection
- Setter Injection
Achieved via Spring Beans and Dependency Injection.

40. @Bean vs @Component

Feature	@Bean	@Component
Placement	Used in @Configuration class	Used on class itself
Usage	Manual bean declaration	Auto-detection by Spring

41. What is @Transactional and When to Use?

Marks method or class as transactional.
Used for database consistency: commit or rollback as a unit.

42. What is OAuth Authentication?

Open standard for token-based authorization.
Used for delegated access (e.g., Google/Facebook login).

43. How to Authenticate Multiple Microservices Using OAuth?

Use centralized OAuth provider.
Services verify token with provider (e.g., Keycloak, Auth0).

44. What is API Gateway Design Pattern?

Entry point for microservices.
Handles routing, rate limiting, authentication, logging.
Example: Netflix Zuul, Spring Cloud Gateway

45. Authentication vs Authorization

Authentication	Authorization
Who you are	What you can do
Performed first	Performed after auth

46. In JPA, Flow of Getting All Data from a Table

Define Entity Class with @Entity
Create Repository Interface extending JpaRepository
Use method like findAll()

List<Employee> employees = employeeRepository.findAll();

47. How to Handle Pagination in JPA?

Use Pageable interface from Spring Data:

Page<Employee> page = employeeRepository.findAll(PageRequest.of(0, 10));

48. What is Circuit Breaker Pattern and Its Types?

Prevents system failure from cascading when service fails.
Types:
- Closed: Requests pass through
- Open: Requests blocked
- Half-Open: Limited test requests

Example: Resilience4j, Hystrix

49. What is a Discovery Server? How to Achieve in Spring Boot?

Used to register and discover microservices.
Example: Eureka Server
Add dependency spring-cloud-starter-netflix-eureka-server

50. How Many Indexes Can Be Provided in a Table?

Technically unlimited, but performance and RDBMS constraints exist.
Best practice: Use indexes wisely based on query patterns.

51. Can We Create Multiple Primary Keys in a Table?

No. Only one primary key, but it can be composite (multiple columns).

52. What is a Composite Key?

Primary key using more than one column.

@Embeddable
class OrderId implements Serializable {
    private Long orderId;
    private Long productId;
}

@Entity
@IdClass(OrderId.class)
class Order {
    @Id private Long orderId;
    @Id private Long productId;
}

53. Primary Key vs Unique Key

Feature	Primary Key	Unique Key
Null Values	Not allowed	Allowed (one null)
Count	Only one	Multiple allowed
Purpose	Entity identity	Enforce uniqueness

54. How to Get Second Salary in PostgreSQL?

SELECT DISTINCT salary
FROM employee
ORDER BY salary DESC
OFFSET 1 LIMIT 1;

55. How to Get Second Salary Using JPA?

Use @Query:

@Query("SELECT DISTINCT e.salary FROM Employee e ORDER BY e.salary DESC")
List<Double> findSalaries(Pageable pageable);

Then call with offset:

findSalaries(PageRequest.of(1, 1));

56. What is Self Join? When to Use? Example?

Join table with itself.
Useful for hierarchical data (e.g., employees & managers).

SELECT A.name, B.name AS manager
FROM employee A
JOIN employee B ON A.manager_id = B.id;

57. Difference Between UNION and UNION ALL

Feature	UNION	UNION ALL
Duplicates	Removes	Keeps
Performance	Slower	Faster

58. How to Get Common Data from Two Tables? Which Join to Use?

Use INNER JOIN

SELECT *
FROM table1 t1
JOIN table2 t2 ON t1.id = t2.id;

59. How Many Joins Are There?

INNER JOIN
LEFT JOIN (LEFT OUTER)
RIGHT JOIN (RIGHT OUTER)
FULL JOIN (FULL OUTER)
SELF JOIN
CROSS JOIN

60. Difference Between Agile and Waterfall

Feature	Agile	Waterfall
Approach	Iterative & Incremental	Sequential
Flexibility	High	Low
Feedback	Continuous	End of development
Testing	Concurrent	After development

61. What is a Feature Branch in Git?

A dedicated branch to develop a specific feature.
Isolated from main/master branch.
Allows team to work independently without affecting production code.