Core Java (Must Know)
These questions test if you truly understand the language.
Key Questions
What is the difference between HashMap and ConcurrentHashMap?
Explain equals() vs hashCode() contract.
Difference between final, finally, finalize.
What happens when you override equals but not hashCode?
Difference between ArrayList and LinkedList.
What is immutability? Why is
Stringimmutable?What are Java Records and when should you use them?
Difference between Comparable vs Comparator.
What is Optional and when should it NOT be used?
Classic Trick Question
Why is HashMap not thread safe?
Expected answer:
race conditions during resize
lost updates
infinite loops in older JDK versions
2. Java Memory Model & JVM
Senior engineers are expected to know JVM behavior.
Key Questions
What are the different memory regions in JVM?
Answer:
Heap
Stack
Metaspace
Code Cache
Native Memory
Important JVM Question
How does Garbage Collection work?
Explain:
Young Generation
Old Generation
Minor GC
Major GC
Advanced Question
What is Stop-the-World?
Answer:
All application threads pause during GC.
JVM Performance Question
How do you troubleshoot memory leak in Java?
Answer:
Tools:
jmap
jvisualvm
heap dump
MAT (Memory Analyzer Tool)
3. Concurrency (VERY Important for Senior Role)
Key Questions
Difference between Runnable vs Callable
What is Future vs CompletableFuture
What is volatile keyword
What is synchronized vs Lock
What is deadlock
Important Question
What is ThreadLocal?
Answer:
ThreadLocal provides thread-specific variables.
Example:
ThreadLocal<SimpleDateFormat> formatter = ThreadLocal.withInitial(
() -> new SimpleDateFormat("yyyy-MM-dd")
);Used heavily in:
logging
transaction context
security context
Classic Interview Question
Difference between wait() vs sleep()
sleep() | wait() |
|---|---|
Thread class | Object class |
does NOT release lock | releases lock |
time based | signal based |
4. Serialization & APIs
Important Question
How do microservices typically serialize data?
Answer:
Most common:
JSON (Jackson)
Alternatives:
Protocol Buffers
Avro
Thrift
Follow-up Question
Why is JSON slower?
Answer:
text format
larger payload
parsing overhead
Binary formats like Protobuf are faster.
5. Spring Boot / Microservices
These questions appear in almost every interview.
Key Questions
What happens when a Spring Boot application starts?
What is Dependency Injection
Difference between @Component, @Service, @Repository
What is Spring Bean lifecycle
Important Question
What is the difference between @RestController vs @Controller
Annotation | Purpose |
|---|---|
@Controller | MVC view |
@RestController | REST API |
Microservices Questions
What is service discovery
What is API Gateway
What is circuit breaker
What is idempotency
6. Security Questions
Especially important in banking interviews.
Common Questions
What is CORS
What is CSRF
What is OAuth2
What is JWT
Important Question
Where should SSL terminate?
Answer:
Common architectures:
1️⃣ At Load Balancer (ALB)
2️⃣ At API Gateway
3️⃣ Inside container
Each has different security tradeoffs.
7. System Design (Senior Level)
These questions determine if you are architect level.
Example Questions
Design URL shortener
Design rate limiting service
Design distributed cache
Design log aggregation system
Microservices Design Question
How do you prevent cascading failure?
Answer:
Circuit Breaker
Bulkhead
Retry
Timeout
8. Docker & Cloud (Very common now)
Questions
What is multi-stage Docker build
What is Docker layer caching
What is sidecar container
Difference between container vs VM
9. Database Questions
Key Questions
Difference between optimistic vs pessimistic locking
What is ACID
What is transaction isolation level
What is N+1 query problem
10. Debugging / Real World Questions
These separate Senior vs Mid-level developers.
Examples
API latency suddenly increased → how do you debug?
Thread pool exhaustion → what do you check?
CPU spikes in production → how do you investigate?
GC running every few seconds → what does it mean?
Expected tools:
thread dump
heap dump
profiling
⭐ The 5 Questions That Almost ALWAYS Appear
If you remember only these, you will still perform well:
1️⃣ HashMap vs ConcurrentHashMap
2️⃣ Java Memory Model
3️⃣ CompletableFuture vs Future
4️⃣ Spring Bean lifecycle
5️⃣ How to debug memory leak in production
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