Add rate limiting code

implementations/java/rate_limiting/FixedWindowCounter.java

@@ -0,0 +1,33 @@
+package implementations.java.rate_limiting;
+
+import java.time.Instant;
+
+public class FixedWindowCounter {
+    private final long windowSizeInSeconds;  // Size of each window in seconds
+    private final long maxRequestsPerWindow; // Maximum number of requests allowed per window
+    private long currentWindowStart;         // Start time of the current window
+    private long requestCount;               // Number of requests in the current window
+
+    public FixedWindowCounter(long windowSizeInSeconds, long maxRequestsPerWindow) {
+        this.windowSizeInSeconds = windowSizeInSeconds;
+        this.maxRequestsPerWindow = maxRequestsPerWindow;
+        this.currentWindowStart = Instant.now().getEpochSecond();
+        this.requestCount = 0;
+    }
+
+    public synchronized boolean allowRequest() {
+        long now = Instant.now().getEpochSecond();
+        
+        // Check if we've moved to a new window
+        if (now - currentWindowStart >= windowSizeInSeconds) {
+            currentWindowStart = now;  // Start a new window
+            requestCount = 0;          // Reset the count for the new window
+        }
+
+        if (requestCount < maxRequestsPerWindow) {
+            requestCount++;  // Increment the count for this window
+            return true;     // Allow the request
+        }
+        return false;  // We've exceeded the limit for this window, deny the request
+    }    
+}