If you’re a senior Java developer than your next role demands deep knowledge and confidence in solving complex problems. Most interviews for experienced professionals go beyond basic Java syntax—they focus on architecture, design principles, multithreading, memory management, performance tuning, and best practices. In this blog, we present the most searched Java Interview Questions for 10 Years Experience, along with comprehensive answers and examples.
Top 20 Java Interview Questions for 10 Years Experience
1. Explain the differences between HashMap, Hashtable, and ConcurrentHashMap. When should each be used?
Answer:
| Feature | HashMap | Hashtable | ConcurrentHashMap |
|---|---|---|---|
| Thread-Safe | No | Yes (synchronized) | Yes (uses segmentation) |
| Performance | Fast | Slower | Faster than Hashtable |
| Null Keys/Values | 1 null key, many null values | No null keys/values | Allows null values but not keys |
Usage:
- Use HashMap when thread safety is not required.
- Use Hashtable when working with legacy code.
- Use ConcurrentHashMap in multi-threaded environments where high concurrency is expected.
2. What are strong, weak, soft, and phantom references in Java?
Answer:
Java provides different types of references to support effective memory management:
- Strong Reference: Default reference type. Objects won’t be garbage collected as long as a strong reference exists.
- Soft Reference: Useful for caching. Collected only when JVM is low on memory.
- Weak Reference: Objects are collected in the next GC cycle even if referenced.
- Phantom Reference: Used for scheduling post-mortem cleanup actions.
These references are part of java.lang.ref package and are crucial for advanced memory tuning.
3. Describe Java Memory Model (JMM) and its role in concurrency.
Answer:
The Java Memory Model defines how threads interact through memory and what behaviors are allowed in concurrent execution. It ensures visibility, ordering, and atomicity of variables shared between threads.
Key concepts:
- Volatile: Ensures visibility, not atomicity.
- Happens-before relationship: Guarantees ordering.
- Synchronized blocks: Ensure mutual exclusion and memory consistency.
JMM prevents issues like stale data or instruction reordering in multi-threaded programs.
4. What are common causes of memory leaks in Java? How do you identify and fix them?
Answer:
Common causes:
- Unclosed resources (Streams, JDBC connections)
- Static references holding large objects
- Inner classes with implicit outer class references
- Listeners or callbacks that are not deregistered
Identification tools:
- VisualVM
- Eclipse MAT
- JConsole
- Heap dump analysis
Fixes:
- Proper resource management (try-with-resources)
- Weak references for listeners
- Manual nullification and object cleanup
5. What are some advanced Java features introduced after Java 8 that you use?
Answer:
With 10 years of experience, familiarity with features beyond Java 8 is essential:
- Java 9: Modules (Project Jigsaw), JShell
- Java 10: Local-variable type inference (
var) - Java 11: New String methods,
HttpClientAPI - Java 14+: Records, Pattern Matching (preview), Switch Expressions
Understanding and applying these features improves code readability and maintainability in modern applications. Read more on Java 8 Features
6. How would you implement a custom class loader and why is it needed?
Answer:
Custom class loaders allow dynamic loading of classes at runtime, useful in plugin architectures or app servers.
Steps:
- Extend
ClassLoader. - Override
findClass(String name). - Load class bytes and define class using
defineClass.
Use cases:
- Application isolation (e.g., OSGi)
- Hot deployment
- Security sandboxing
7. Explain Fork/Join Framework and how it compares to ExecutorService.
Answer:
Fork/Join Framework is designed for divide-and-conquer tasks that can be split recursively. Introduced in Java 7 via ForkJoinPool.
ForkJoin vs ExecutorService:
| Feature | ForkJoinPool | ExecutorService |
|---|---|---|
| Best Use Case | Recursive tasks | Independent tasks |
| Thread Usage | Work-stealing | Fixed/Custom pool |
| Example Class | RecursiveTask | Callable, Runnable |
8. What’s the difference between composition and inheritance? Which is preferred and why?
Answer:
- Inheritance: “Is-a” relationship. Subclass extends parent.
- Composition: “Has-a” relationship. One class contains an instance of another.
Preferred: Composition, due to better encapsulation, loose coupling, and more flexibility.
9. How do you handle high-concurrency requirements in Java applications?
Answer:
Techniques include:
- Thread pools via
ExecutorService - Concurrent collections (
ConcurrentHashMap,BlockingQueue) - Lock-free algorithms using
AtomicInteger,AtomicReference - Back-pressure and rate limiting
- Reactive programming (Project Reactor, RxJava)
Proper synchronization, load testing, and profiling are essential to ensure scalability.
10. What is the difference between checked and unchecked exceptions? When should custom exceptions be used?
Answer:
| Type | Checked Exception | Unchecked Exception |
|---|---|---|
| Inheritance | Extends Exception | Extends RuntimeException |
| Handling | Must be declared or handled | Optional |
| Use Cases | External conditions (e.g. IO) | Programming errors (e.g. NPE) |
Custom Exceptions: Use them to represent application-specific errors for clarity and better error handling.
11. How does Garbage Collection work in Java? Name some GC algorithms.
Answer:
Java GC automatically reclaims unused memory.
Phases:
- Mark
- Sweep
- Compact
Algorithms:
- Serial GC
- Parallel GC
- CMS (Concurrent Mark-Sweep)
- G1 (Garbage First)
- ZGC and Shenandoah (low-latency)
Tuning involves JVM options like -Xms, -Xmx, -XX:+UseG1GC, and GC logs for profiling.
12. What is immutability in Java and why is it important?
Answer:
An immutable object cannot be changed after creation. Examples: String, Integer.
Benefits:
- Thread safety
- Cache safety
- Predictable behavior
Implementation:
- Declare class
final - Make fields
private final - No setters
- Defensive copies of mutable objects
13. What are design patterns you often use in enterprise Java applications?
Answer:
Frequently used patterns:
- Singleton: Ensure a class has one instance
- Factory Method: Create objects without exposing instantiation logic
- Builder: Construct complex objects step-by-step
- Strategy: Allow interchangeable behaviors
- Decorator: Add responsibilities dynamically
Understanding these patterns enables better system design and code reusability.
14. What is the difference between final, finally, and finalize?
Answer:
final: Keyword used to mark constants, prevent method overriding or inheritance.finally: Block used to ensure execution aftertry-catch.finalize(): Method called before object is garbage collected (deprecated in Java 9+).
15. What are the SOLID principles in Java? Explain with examples.
Answer:
SOLID is a set of design principles that help in building maintainable and scalable software.
- S: Single Responsibility Principle (SRP) – A class should have only one reason to change.
Example: Splitting file operations and logging into separate classes. - O: Open/Closed Principle – Open for extension, closed for modification.
Example: Use abstract classes/interfaces so behavior can be extended without modifying existing code. - L: Liskov Substitution Principle – Subtypes must be substitutable for their base types.
Example: Derived classes must not throw exceptions unexpected by clients of the base class. - I: Interface Segregation Principle – Prefer many client-specific interfaces over one general-purpose interface.
Example: Avoid bloated interfaces; use fine-grained interfaces instead. - D: Dependency Inversion Principle – Depend on abstractions, not concretions.
Example: Use interfaces and inject dependencies via constructor or setter (e.g., Spring DI).
16. How do you prevent Deadlock in Java?
Answer:
Deadlock occurs when two or more threads wait for each other indefinitely.
Prevention Techniques:
- Always acquire locks in a consistent global order.
- Use timeout while acquiring locks (
tryLockfromReentrantLock). - Prefer using higher-level concurrency utilities (
ExecutorService,Semaphore, etc.). - Avoid nested locks where possible.
Detection: Use tools like jstack, VisualVM, or thread dumps.
17. What is the role of the transient keyword in Java?
Answer:
The transient keyword prevents a variable from being serialized. That means the variable declared as transiet will not be saved into database.
class User implements Serializable {
private String name;
private transient String password; // will not be serialized
}Use case: Sensitive information like passwords, tokens, or file handles should be marked transient.
18. What is the difference between Executor, ExecutorService, and ScheduledExecutorService?
Answer:
| Interface | Description |
|---|---|
Executor | Basic interface for launching tasks |
ExecutorService | Extends Executor with lifecycle methods like shutdown() and invokeAll() |
ScheduledExecutorService | Supports scheduled and periodic task execution |
Example:
ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
scheduler.scheduleAtFixedRate(() -> System.out.println("Running..."), 0, 5, TimeUnit.SECONDS);19. How do you handle thread-safety in Singleton design pattern?
Answer:
Several ways to create thread-safe Singletons:
- Eager Initialization:
public class Singleton {
private static final Singleton instance = new Singleton();
private Singleton() {}
public static Singleton getInstance() {
return instance;
}
}- Lazy Initialization with
synchronized(not optimal):
public static synchronized Singleton getInstance() { ... }- Double-Checked Locking with volatile:
private static volatile Singleton instance;
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}- Enum-based Singleton (Recommended):
public enum Singleton {
INSTANCE;
}20. What are varargs in Java? What are its limitations?
Answer:
Varargs allow passing a variable number of arguments to a method:
public void log(String... messages) {
for (String msg : messages) {
System.out.println(msg);
}
}Limitations:
- Only one varargs parameter is allowed per method.
- Must be the last parameter in the method signature.
- Can lead to ambiguity with method overloading.
Conclusion
These are some of the most commonly asked Java Interview Questions for 10 Years Experience in 2025, frequently appearing in Google searches and real-world interviews. To stand out in a senior Java role, focus on deep conceptual clarity, clean coding practices, performance optimization, and system design principles. Download Java.
As an experienced developer, your goal is not just to solve problems but to solve them well—with scalability, maintainability, and elegance in mind.
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