Top JAVA and Spring Boot Interview Questions in 2026
Java Core:
- How is Java platform independent (JVM vs JRE)?
- What is a ClassLoader?
- Difference: ClassNotFoundException vs NoClassDefFoundError
- String Pool, intern(), == vs equals()
OOP Concepts:
- What is Encapsulation?
- Serializable interface
- Can a class be static, final, or private?
- OOP vs scripting languages
Exceptions:
- Superclass of all exceptions
- Error vs Exception
- Checked vs Unchecked
- Try-with-resources
- Exception propagation
Java Language Features:
- AutoBoxing and Unboxing
- Key Java 8 features (Streams, Lambdas, Functional interfaces)
- Lambda functions and why they are useful
- Why interfaces cannot have constructors
- Performance: for-loop vs for-each
Other important topics
- Pass by value and Pass by reference
- Static variable working
- Concurrent Hashmap
- Hash Collusion
- Hashing Functions
- Thread, start, run, wait, stop
- ArrayList Interal implementation
- LinkedList vs ArrayList
Concurrency:
- Code using 5 threads printing in sequence
- Volatile vs Atomic
Design Patterns:
- Builder pattern (usage + example)
- Singleton implementation
- Factory pattern concept
- Why design patterns are needed
- Patterns you know and how to invent one
Spring and Spring Boot:
- What is Spring Boot and why it’s popular
- Spring annotations: @Component, @Bean, @Qualifier, @Value
- @Controller vs @RestController
- @Mock vs @InjectMocks
- OAuth basics and role-based access
- Spring Data JPA main interfaces
- Exception handling in controllers
- Transaction propagation
- application.properties purpose
- How Spring/Hibernate generates SQL
- MVC: Controller vs Service vs Repository
- Pagination and unique constraints
Collections:
- How HashMap works internally
- HashMap vs Hashtable vs ConcurrentHashMap
- List vs Set vs Map (when to use which)
- What is LinkedList
- Comparable interface
- How hashCode is generated
- Iterable interface
Streams API:
- What Streams API is
- Ways to create Streams
- Streams vs Collections
- Intermediate vs terminal operations
- map vs flatMap
- collect(), findFirst(), findAny()
- Exception handling in Streams
- Converting Stream to Collection
- Sequential vs parallel streams
Functional Programming:
- What is a functional interface
- Can it have default and static methods
- Common examples (Runnable, Comparator, Callable)
Learning Resource for backend engineers:
Spring Boot from Basics to Advanced:
JAVA from Basics to Advanced:
High Level Design from Basics to Advanced:
Low Level Design from Basics to Advanced: