interview question of Java Backend

Interview Questions

Core Java (OOPs, Collections, Concurrency Basics)

Explain difference between HashMap, HashTable, and ConcurrentHashMap.

Feature	HashMap	Hashtable	ConcurrentHashMap
Thread Safety	Not thread-safe	Thread-safe (synchronized methods)	Thread-safe (lock segmentation / fine-grained locking)
Null Keys/Values	Allows one null key and multiple null values	Does not allow any null keys/values	Does not allow null keys/values
Performance	Faster (no synchronization overhead)	Slower (entire table is locked)	Better concurrency — multiple threads can read/write different segments simultaneously
Use Case	Single-threaded apps	Legacy thread-safe code	High-performance concurrent apps

Example:
If multiple threads are accessing and modifying a map, prefer ConcurrentHashMap. For non-concurrent cases, use HashMap. Avoid Hashtable in modern code.

How does equals() and hashCode() work together in collections?

• hashCode(): Returns an integer hash value used to place objects in hash-based collections (like HashMap, HashSet).
• equals(): Compares two objects for logical equality.

Contract:

• If two objects are equal according to equals(), they must have the same hashCode().
• If two objects have the same hashCode(), they may or may not be equal.

Why it matters:
Collections like HashMap use hashCode() to find a bucket, and then equals() to check if the key already exists.
Example:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Person)) return false;
    Person p = (Person) o;
    return id == p.id;
}

@Override
public int hashCode() {
    return Objects.hash(id);
}

Difference between shallow copy vs deep copy in Java.

Type	Description	Example
Shallow Copy	Copies only object references (not the objects they refer to). Both objects share the same nested objects.	clone() without custom implementation
Deep Copy	Copies everything — including new instances of nested objects. Changes in one object don’t affect the other.	Implemented manually or via serialization

Example:

// Shallow copy
Employee e2 = e1.clone(); // e1.address and e2.address point to same object

// Deep copy
Employee e2 = new Employee(e1);
e2.address = new Address(e1.address);

How do you prevent deadlock in multithreaded applications?

Deadlock occurs when two or more threads are waiting on each other’s locks.

Ways to prevent it:

1. Lock ordering – Always acquire locks in a consistent order.
2. Try-lock with timeout – Use tryLock() from ReentrantLock to avoid waiting forever.
3. Avoid nested locks – Reduce code sections that acquire multiple locks.
4. Use concurrent utilities – Prefer ConcurrentHashMap, BlockingQueue, etc., which manage synchronization internally.
5. Minimize synchronized blocks – Keep critical sections small.

Example:

if (lock1.tryLock(50, TimeUnit.MILLISECONDS)) {
    if (lock2.tryLock(50, TimeUnit.MILLISECONDS)) {
        // work
    }
}

Explain volatile vs synchronized — when to use each.

Feature	volatile	synchronized
Purpose	Guarantees visibility of changes across threads	Guarantees atomicity and visibility
Use Case	When multiple threads read/write a variable, and writes are independent	When operations on shared data must be atomic
Locking	No locking (lighter, faster)	Uses intrinsic lock (can block threads)
Scope	Variable-level only	Code block or method
Example Use	Status flags, configuration values	Counters, complex state updates

Example:

volatile boolean running = true; // visibility only

synchronized void increment() {  // atomic operation
    count++;
}

Rule of thumb:

• Use volatile for simple flags or status updates.
• Use synchronized (or locks) for compound operations that must be atomic.

Spring & Spring Boot

Difference between @Component, @Service, @Repository, and @Controller.

What is Spring Boot auto-configuration? How does it work internally?

How do you implement exception handling globally in Spring Boot REST APIs?

Explain difference between @RequestParam, @PathVariable, and @RequestBody.

How do you implement profiles in Spring Boot for multiple environments?

JPA / Hibernate

What are entity states (Transient, Persistent, Detached, Removed) in Hibernate?

Explain lazy loading and how to avoid LazyInitializationException.

Difference between Criteria API and JPQL.

What is the difference between CascadeType.ALL and orphanRemoval?

How do you implement pagination and sorting in Spring Data JPA?

Microservices & Architecture

Explain difference between monolithic and microservices architectures.

How do you implement inter-service communication? (REST, Kafka, gRPC)

What is API Gateway and why do we need it?

How do you manage configuration in multiple microservices?

Explain circuit breaker pattern (Hystrix/Resilience4j) with example.

Cloud & DevOps Basics

What is the difference between Docker image and Docker container?

How does Kubernetes handle scaling and service discovery?

Explain difference between Blue-Green and Rolling deployments.

How do you monitor Spring Boot microservices in production (Actuator, ELK, Prometheus)?

What is CI/CD? How have you implemented it in your projects?

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SQL / Database Knowledge

Write a SQL query to get department-wise average salary.

Difference between INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN.

How do you optimize slow-running SQL queries?

Explain ACID properties and isolation levels in databases.

How do you use indexes, and what are clustered vs non-clustered indexes?

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DSA & Coding (GlobalLogic Special Focus)

Write a program to reverse a linked list in Java (iterative + recursive).

Implement a program to find the first non-repeating character in a string.

Write a Java program to find the longest common prefix among a set of strings.

Implement LRU Cache in Java.

Write a REST API to fetch last N orders of a customer (Spring Boot + JPA).

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Behavioral / Project Discussion

Tell me about a real-time issue you solved in production.