400 Java Interview Questions with Answers 2026

Advanced Java & Concurrency: Multithreading, java.util.concurrent, Locks, and Parallel Streams.

Object-Oriented Design: SOLID Principles, GoF Design Patterns, and Refactoring.

Ecosystem & Frameworks: Spring Boot, Hibernate/JPA, Maven, JUnit, and REST APIs.

Performance & Security: Profiling, Memory Management, OWASP, and Microservices Resilience.

B) It ensures the variable's reference cannot be changed once assigned.

C) It forces the variable to be stored in the Metaspace rather than the Heap.

D) It automatically makes the variable thread-safe for all concurrent operations.

E) It prevents the class containing the variable from being subclassed.

F) It is a hint to the JIT compiler to inline the variable's value globally.

Correct Answer: B

Overall Explanation: The final keyword in Java is used to restrict the user. When applied to a variable, it means the value (for primitives) or the reference (for objects) cannot be reassigned after initialization.

Option A Incorrect: final only stops reassignment of the reference; the internal state of the object can still be modified.

Option B Correct: This is the definition of a final variable; the reference/value is constant once set.

Option C Incorrect: Metaspace stores class metadata, not instance or local variables.

Option D Incorrect: final helps with visibility in concurrency but does not make an object's methods or state changes thread-safe.

Option E Incorrect: This describes a final class, not a final variable.

Option F Incorrect: While the JIT may optimize final constants, "global inlining" is not a defined language rule for all final variables.

Question 2: In a high-concurrency environment, why might you prefer LongAdder over AtomicLong?

B) LongAdder provides a stronger guarantee of "happens-before" consistency.

C) LongAdder reduces contention by maintaining a variables-cell array for updates.

D) LongAdder is compatible with Java 5, whereas AtomicLong requires Java 8.

E) LongAdder allows for atomic multiplication and division operations.

F) LongAdder automatically serializes all requests to a single thread.

Correct Answer: C

Overall Explanation: Under high contention (many threads updating the same value), AtomicLong performance suffers due to "spinning" on CAS (Compare-And-Swap) failures. LongAdder distributes the load across multiple cells.

Option A Incorrect: LongAdder usually uses more memory because it maintains multiple cells to store partial sums.

Option B Incorrect: Both provide similar memory visibility guarantees; LongAdder is actually "eventually consistent" until sum() is called.

Option C Correct: This is the core mechanism of LongAdder to scale under high thread contention.

Option D Incorrect: LongAdder was introduced in Java 8; AtomicLong has been around much longer (Java 5).

Option E Incorrect: LongAdder is designed for additions/increments, not complex math like multiplication.

Option F Incorrect: Serializing requests would destroy performance; LongAdder is highly parallel.

Question 3: Which SOLID principle is most directly violated if a "Duck" class is forced to implement a fly() method even if it represents a "Rubber Duck"?

B) Open/Closed Principle

C) Liskov Substitution Principle

D) Interface Segregation Principle

E) Dependency Inversion Principle

F) Encapsulation Principle

Correct Answer: D

Overall Explanation: The Interface Segregation Principle (ISP) states that no client should be forced to depend on methods it does not use.

Option A Incorrect: SRP deals with the "reason to change" for a class, not the bloating of an interface.

Option B Incorrect: OCP focuses on extending behavior without modifying existing code.

Option C Incorrect: While LSP is also related (since a Rubber Duck can't truly substitute a Bird), the act of forcing an implementation of an irrelevant method is the definition of an ISP violation.

Option D Correct: Forcing a class to implement "dummy" or "throw exception" methods for functionality it doesn't need violates ISP.

Option E Incorrect: DIP deals with depending on abstractions rather than concretions.

Option F Incorrect: Encapsulation is a general OOP pillar about hiding data, not a SOLID-specific principle.

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