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    • LLD: Java
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        DAY 1 — Core Foundations (SDE-2 Level)
      • DAY 2 — State, Time & Domain Correctness
      • DAY 3 — Extensibility & Failure Handling (SDE-2 Core Signal)
      • DAY 4 — Concurrency, Scheduling & Consistency (SDE-2 Critical)
      • DAY 5 — Workflow Design & Config-Driven Systems (SDE-2+)
      • DAY 6 — Eventing & Usage-Based Systems (Advanced SDE-2)
      • DAY 7 — Interview Simulation (Hard, Production-Grade)
      • Hidden Edge Cases Interviewers Expect (Day-wise)
      • Detailed Testing file:
    • Assignment:
      • If else Practice Set
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DAY 1 — Core Foundations (SDE-2 Level)
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DAY 1 — Core Foundations (SDE-2 Level)

⁠

🟢 DAY 1 — Core Foundations (SDE-2 Level)

Concepts Introduced Today

You’ll practice:
Time-based logic
Thread safety
Clear API design
Edge-case driven coding
No advanced patterns yet — focus is correctness + clarity.
⁠

✅ Problem 1: Rate Limiter (Sliding Window, Single Node)

⁠

🔹 New Concept (Short Explanation)

Sliding Window Rate Limiter

Instead of fixed buckets:
Track timestamps of requests
Allow request if count in last windowSize ≤ limit
Example:
Limit = 3 requests / 10 seconds

Requests at times:
t=1 -> allowed
t=2 -> allowed
t=3 -> allowed
t=9 -> ❌ blocked (3 already in window)
t=11 -> allowed (t=1 expired)
Key challenges:
Time handling
Removing expired entries
Thread safety
⁠

📌 Problem Statement

Design an in-memory rate limiter.

Requirements

Per-user rate limiting
Sliding window algorithm
Thread-safe
Millisecond precision
⁠

Example

limit = 2 requests / 5 seconds

request("user1") -> true
request("user1") -> true
request("user1") -> false
(wait 5 seconds)
request("user1") -> true
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🧩 Java Interface (DO NOT MODIFY)

public interface RateLimiter {
boolean allowRequest(String userId);
}
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🧪 Driver Code (Auto Tests + Edge Cases)

Java93 lines
⁠

🎯 What Interviewer Evaluates

Correct sliding window logic
Synchronization strategy
Clean data structures
No memory leaks
⁠

✅ Problem 2: In-Memory Key Value Store with TTL

⁠

🔹 New Concept (Short Explanation)

TTL (Time To Live)

Each key expires after a given duration.
Approach:
Store value + expiryTime
On get, check expiry
No background cleanup (lazy deletion)
⁠

📌 Problem Statement

Design an in-memory key-value store with TTL support.
⁠

Operations

put(key, value, ttlMillis)
get(key) -> value or null
⁠

Example

put("a", "1", 2000)
get("a") -> "1"
(wait 2 seconds)
get("a") -> null

⁠

🧩 Java Interface (DO NOT MODIFY)

public interface KeyValueStore {
void put(String key, String value, long ttlMillis);
String get(String key);
}
⁠

🧪 Driver Code (Auto Tests + Edge Cases)

Java91 lines
⁠

🎯 What Interviewer Evaluates

Expiry correctness
Clean modeling
Edge-case handling
Simplicity over overengineering
⁠

🧠 Day 1 Takeaways

By end of Day 1, you should be comfortable with:
Translating requirements → code
Handling time-based logic
Writing test-passing, interview-ready Java
⁠
Solution:
Problem 1:
package day1P1;

import java.time.Instant;
import java.util.ArrayDeque;
import java.util.Map;
import java.util.Queue;
import java.util.concurrent.ConcurrentHashMap;


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