Question 1

Coin Change Problem - minimum number of coins required to make change

Accepted Answer

**DP:** `dp[i]` = min coins for amount i. `dp[0]=0`; for each coin c: `dp[i]=min(dp[i], 1+dp[i-c])`. O(amount·coins). **Greedy:** Only for canonical systems (US). **Why:** Classic DP; optimization. **Edge:** amount 0; no solution....

Question 2

Explain and implement Semaphore in Java

Accepted Answer

**Semaphore:** Limits concurrent access. `Semaphore(n)`; acquire/release or tryAcquire. **Use:** Rate limiting; pool access. **Implementation:** try/finally for release. **Why:** Backpressure; resource protection. **Production:** Fairness for ordering....

Question 3

Multithreading and Synchronization in Java - write code to manage synchronized threads

Accepted Answer

**Why Synchronization:** Multiple threads accessing shared mutable state cause race conditions. synchronized guarantees mutual exclusion; ReentrantLock offers finer control (tryLock, fairness).

**Scalability:** synchronized is coarse—contention becomes bottleneck. Prefer: (1) ConcurrentHashMap for concurrent maps. (2) AtomicInteger/Long for counters. (3) Lock-free structures where possible....

Question 4

Partitioning a Linked List based on a value

Accepted Answer

**Why Partitioning:** Separates elements by predicate—foundation for quicksort on linked lists, routing (high/low priority), and stream processing (route by key).

**Two-Pointer Approach:** Maintain two lists: less_than and ge_than. Single pass, O(n), O(1) extra (reuse nodes). Stable partition preserves relative order within each group.

**Scalability:** In-memory only—linked lists aren't used at big-data scale....

Question 5

The transient Keyword in Java

Accepted Answer

**Why Transient:** Excludes field from serialization. Use for: computed/cached values, sensitive data (passwords), non-Serializable (Thread, Socket).

**Behavior:** On deserialize, transient fields get default (null, 0, false). Custom logic via writeObject/readObject.

**Production:** Mark passwords, connection handles, threads. For Spark: broadcast variables and accumulators are serialized—ensure DTOs don't include non-serializable refs....

Question 6

Write a simple service and controller class in Spring Boot for REST API

Accepted Answer

**Layers:** Controller = HTTP, validation, DTOs. Service = business logic, transactions. Repository = data access. Thin controller, fat service.

@RestController @RequestMapping("/api")
class DataController {
    @Autowired DataService service;
    @GetMapping("/items/{id}")
    public ResponseEntity<Item> get(@PathVariable Long id) {
        return ResponseEntity.ok(service.findById(id));
    }
}
@Service
class DataService { public Item findById(Long id) { return repo.findById(id).orElseThrow()...

Question 7

Write code to manually invoke garbage collection in Java

Accepted Answer

**System.gc() / Runtime.getRuntime().gc():** Hint only—JVM may ignore. Use sparingly: before critical low-memory section, after releasing large objects, or in tests.

**Production:** Tune heap (-Xmx) over manual GC. For G1: -XX:+ExplicitGCInvokesConcurrent....

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