Advanced Java Programs Asked in Interviews (Real-World)
Most Java interview failures don’t happen because candidates don’t know syntax — they happen because candidates can’t design real-world programs.
This article focuses on advanced, practical Java programs that are frequently asked in product-based companies, system design rounds, and senior interviews.
What makes these programs different:
- Real-world relevance (not textbook problems)
- Design + code + reasoning
- Thread-safety and performance considerations
- Follow-up interview questions explained
1. LRU Cache Implementation (Most Asked)
Problem: Design an LRU (Least Recently Used) cache with O(1) get and put operations.
Core Idea
- HashMap → O(1) lookup
- Doubly Linked List → maintain access order
class LRUCache {
private final int capacity;
private final Map map = new HashMap<>();
private final Node head = new Node(0, 0);
private final Node tail = new Node(0, 0);
static class Node {
int key, value;
Node prev, next;
Node(int k, int v) { key = k; value = v; }
}
public LRUCache(int capacity) {
this.capacity = capacity;
head.next = tail;
tail.prev = head;
}
public int get(int key) {
if (!map.containsKey(key)) return -1;
Node node = map.get(key);
remove(node);
insert(node);
return node.value;
}
public void put(int key, int value) {
if (map.containsKey(key)) remove(map.get(key));
if (map.size() == capacity) remove(tail.prev);
insert(new Node(key, value));
}
private void remove(Node node) {
map.remove(node.key);
node.prev.next = node.next;
node.next.prev = node.prev;
}
private void insert(Node node) {
map.put(node.key, node);
node.next = head.next;
node.prev = head;
head.next.prev = node;
head.next = node;
}
}
Interview follow-up: How would you make this thread-safe?
Answer: synchronized blocks or ReadWriteLock.
2. Producer–Consumer Using BlockingQueue (Enqueue / Dequeue)
This tests your understanding of queues, concurrency, enqueue/dequeue semantics.
Why Interviewers Love This
- Thread coordination
- Blocking behavior
- Correct use of Java concurrency utilities
BlockingQueue queue = new ArrayBlockingQueue<>(5);
// Producer
Runnable producer = () -> {
try {
for (int i = 1; i <= 10; i++) {
queue.put(i); // enqueue
System.out.println("Produced: " + i);
}
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
};
// Consumer
Runnable consumer = () -> {
try {
while (true) {
Integer item = queue.take(); // dequeue
System.out.println("Consumed: " + item);
}
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
};
Never use wait/notify in interviews unless asked explicitly.
BlockingQueue is the correct modern solution.
3. Thread-Safe Singleton (All Correct Ways)
Interviewers often reject candidates who give only one version.
Best Solution (Enum)
public enum Singleton {
INSTANCE;
}
Lazy + Thread-Safe (Double-Checked Locking)
class Singleton {
private static volatile Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null)
instance = new Singleton();
}
}
return instance;
}
}
4. Rate Limiter (Token Bucket – Real Systems)
Asked in system design + coding rounds.
class RateLimiter {
private final int capacity;
private int tokens;
private long lastRefillTime;
public RateLimiter(int capacity) {
this.capacity = capacity;
this.tokens = capacity;
this.lastRefillTime = System.nanoTime();
}
synchronized boolean allowRequest() {
refill();
if (tokens > 0) {
tokens--;
return true;
}
return false;
}
private void refill() {
long now = System.nanoTime();
if (now - lastRefillTime > 1_000_000_000L) {
tokens = capacity;
lastRefillTime = now;
}
}
}
Follow-up: How would you make this distributed?
Answer: Redis / bucket per user.
5. Custom Thread Pool (Core Concept)
class SimpleThreadPool {
private final BlockingQueue queue = new LinkedBlockingQueue<>();
public SimpleThreadPool(int n) {
for (int i = 0; i < n; i++) {
new Thread(() -> {
while (true) {
try {
queue.take().run();
} catch (Exception ignored) {}
}
}).start();
}
}
public void submit(Runnable task) {
queue.offer(task);
}
}
Used to test understanding of executor internals.
6. Immutable Class Design
final class ImmutableUser {
private final String name;
private final List roles;
public ImmutableUser(String name, List roles) {
this.name = name;
this.roles = List.copyOf(roles);
}
public String getName() { return name; }
public List getRoles() { return roles; }
}
7. Deadlock Example + Fix
synchronized (a) {
synchronized (b) {
// deadlock risk
}
}
Fix: consistent lock ordering or tryLock.
8. In-Memory Cache with TTL
class Cache {
private final Map map = new ConcurrentHashMap<>();
void put(String key, long ttlMs) {
map.put(key, System.currentTimeMillis() + ttlMs);
}
boolean isValid(String key) {
return map.containsKey(key) &&
map.get(key) > System.currentTimeMillis();
}
}
9. Why These Programs Matter
- They test design thinking
- They reflect real production problems
- They separate junior from senior candidates
Golden rule:
Write code that survives production, not whiteboards.
10. Summary
If you master these programs, you will:
- Crack coding + system design rounds
- Write safer concurrent Java code
- Think like a backend engineer, not a student