Java System Design Interview Questions (With Code)
System design interviews are where most Java developers struggle — not because they don’t know Java, but because they don’t know how to design real systems using Java.
This guide focuses on Low-Level Design (LLD) + backend system design — the type asked in product companies, senior roles, and architect interviews.
- Real interview questions (not academic)
- Design thinking before code
- Clean Java implementations
- Trade-offs & follow-up questions explained
1. How to Approach Java System Design Interviews
Interviewers evaluate how you think, not how fast you code.
- Clarify requirements
- Identify scale & constraints
- Choose correct data structures
- Discuss trade-offs
- Write clean, extensible Java code
2. Design an LRU Cache (Classic)
Requirements
- O(1) get & put
- Evict least recently used item
Design
- HashMap for fast lookup
- Doubly Linked List for ordering
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 n = map.get(key);
remove(n);
insert(n);
return n.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 n) {
map.remove(n.key);
n.prev.next = n.next;
n.next.prev = n.prev;
}
private void insert(Node n) {
map.put(n.key, n);
n.next = head.next;
n.prev = head;
head.next.prev = n;
head.next = n;
}
}
- How to make it thread-safe?
- How to add TTL?
3. Design a Rate Limiter (Token Bucket)
Use Case
Prevent API abuse, throttle users.
Design Choice
- Token Bucket algorithm
- Synchronized access
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;
}
}
}
4. Design a Thread-Safe Queue (Enqueue / Dequeue)
Why Asked
- Producer-consumer pattern
- Thread coordination
class BlockingQueue {
private final Queue queue = new LinkedList<>();
private final int capacity;
public BlockingQueue(int capacity) {
this.capacity = capacity;
}
public synchronized void enqueue(T item) throws InterruptedException {
while (queue.size() == capacity) wait();
queue.add(item);
notifyAll();
}
public synchronized T dequeue() throws InterruptedException {
while (queue.isEmpty()) wait();
T item = queue.poll();
notifyAll();
return item;
}
}
BlockingQueue as the production solution.
5. Design an In-Memory Cache with TTL
Requirements
- Fast access
- Automatic expiration
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();
}
}
6. Design a Logger (Thread-Safe)
class Logger {
private static volatile Logger instance;
private Logger() {}
public static Logger getInstance() {
if (instance == null) {
synchronized (Logger.class) {
if (instance == null)
instance = new Logger();
}
}
return instance;
}
public synchronized void log(String msg) {
System.out.println(msg);
}
}
7. Design a Notification System (LLD)
Approach
- Strategy pattern
- Extensible channels
interface Notifier {
void send(String message);
}
class EmailNotifier implements Notifier {
public void send(String message) {
System.out.println("Email: " + message);
}
}
class SMSNotifier implements Notifier {
public void send(String message) {
System.out.println("SMS: " + message);
}
}
8. Common System Design Mistakes
- Over-engineering too early
- Ignoring concurrency
- No discussion of trade-offs
- Hard-coding instead of abstractions
9. How Interviewers Evaluate You
| Aspect | What They Look For |
|---|---|
| Design | Correct abstractions |
| Code | Clean, readable Java |
| Scalability | Future growth awareness |
| Trade-offs | Balanced decisions |
10. Summary
System design interviews are about building maintainable, scalable systems — not just passing test cases.
- Design first, code second
- Explain trade-offs clearly
- Use Java strengths wisely
🏗️ What to Master Alongside Java System Design?
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