Added Singleton java quiz

Author: nilandev

.gitignore

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+### IntelliJ IDEA ###
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store
+
+*.iml
+*.idea

singleton-design-pattern/.gitignore

@@ -0,0 +1,32 @@
+### IntelliJ IDEA ###
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store
+
+*.iml
+*.idea

singleton-design-pattern/src/com/stacktips/designpatterns/RateLimiter.java

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+package com.stacktips.designpatterns;
+
+import java.util.HashMap;
+import java.util.LinkedList;
+import java.util.Map;
+import java.util.Queue;
+
+class RateLimiter {
+
+    private static RateLimiter instance;
+
+    private Map<String, Queue<Integer>> userRequests;
+
+    private static final int MAX_REQUESTS_PER_MINUTE = 3;
+
+    private RateLimiter() {
+        userRequests = new HashMap<>();
+    }
+
+    public static RateLimiter getInstance() {
+        if (instance == null) {
+            synchronized (RateLimiter.class) {
+                if (instance == null) {
+                    instance = new RateLimiter();
+                }
+            }
+        }
+        return instance;
+    }
+
+    public synchronized boolean allowRequest(int timestamp, String userId) {
+        int currentMinute = timestamp / 60;
+
+        Queue<Integer> requests = userRequests.computeIfAbsent(userId, k -> new LinkedList<>());
+        while (!requests.isEmpty() && requests.peek() / 60 < currentMinute) {
+            requests.poll();
+        }
+
+        if (requests.size() < MAX_REQUESTS_PER_MINUTE) {
+            requests.offer(timestamp);
+            return true;
+        }
+
+        return false;
+    }
+}

singleton-design-pattern/src/com/stacktips/designpatterns/RateLimiterDemo.java

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+package com.stacktips.designpatterns;
+
+import java.util.Scanner;
+
+public class RateLimiterDemo {
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+        RateLimiter rateLimiter = RateLimiter.getInstance();
+
+        while (true) {
+            System.out.print("Enter user ID (or 'exit' to quit): ");
+            String input = scanner.nextLine();
+
+            if (input.equalsIgnoreCase("exit")) {
+                break;
+            }
+
+            int currentTimestamp = (int) (System.currentTimeMillis() / 1000);
+            if (rateLimiter.allowRequest(currentTimestamp, input)) {
+                System.out.println("Hello! Request accepted for user " + input);
+            } else {
+                System.out.println("Rate limit exceeded for user " + input + ". Try again later.");
+            }
+        }
+
+        scanner.close();
+        System.out.println("Program terminated.");
+    }
+}