Create ConcurrentHashMapDemo.java

rahulXbarnwal · web-flow · commit 36315f31061b · 2026-03-15T18:00:41.000+05:30
diff --git a/src/learnCollections/ConcurrentHashMapDemo.java b/src/learnCollections/ConcurrentHashMapDemo.java
@@ -0,0 +1,104 @@
+package learnCollections;
+
+import java.util.concurrent.ConcurrentHashMap;
+
+/*
+                    ConcurrentHashMap
+                           |
+   ---------------------------------------------------------
+   |        |        |        |        |        |        |
+ Segment0 Segment1 Segment2 Segment3 Segment4 Segment5 Segment6
+   |        |        |        |        |        |        |
+ HashMap  HashMap  HashMap  HashMap  HashMap  HashMap  HashMap
+
+
+- 16 segments by default
+- Does not guarantee any ordering of keys
+
+CASE 1: Two threads writing to different segments
+
+            KeyA.hash → Segment3
+            KeyB.hash → Segment10
+
+            Thread A → put(KeyA)
+            Thread B → put(KeyB)
+
+            KeyA → Segment3
+            KeyB → Segment10
+
+
+            Thread A
+              |
+              v
+            Segment3 (LOCK)
+              |
+              v
+            HashMap
+
+
+            Thread B
+              |
+              v
+            Segment10 (LOCK)
+              |
+              v
+            HashMap
+
+            Segment3 lock ≠ Segment10 lock
+
+            Thread A and Thread B run simultaneously
+
+
+CASE 2: Two threads writing to the same segment
+
+            KeyA.hash → Segment3
+            KeyB.hash → Segment3
+
+            Thread A → put(KeyA)
+            Thread B → put(KeyB)
+
+            KeyA → Segment3
+            KeyB → Segment3
+
+            Thread A
+              |
+              v
+            Segment3 (LOCK ACQUIRED)
+              |
+              v
+            HashMap
+
+            Thread B
+              |
+              v
+            Segment3 (WAITING FOR LOCK)
+
+            Thread B must wait until Thread A releases lock
+ */
+
+
+            
+/*
+        Map                         | Thread-safe | Sorted
+        --------------------------------------------------
+        HashMap                     | No          | No
+        ConcurrentHashMap           | Yes         | No
+        TreeMap                     | No          | Yes
+        ConcurrentSkipListMap       | Yes         | Yes
+*/
+
+public class ConcurrentHashMapDemo {
+    public static void main(String[] args) {
+        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
+        // Java 7 --> segment based locking --> 16 segments --> smaller hashmaps
+        // Only the segment being written to or read from is locked
+        // read: do not require locking unless there is a write operation happening on the same segment
+        // write: lock
+
+        // Java 8 --> no segmentation
+        //        --> Compare-And-Swap approach --> no locking except resizing or collision
+        // Thread A last saw --> x = 42
+        // Thread A work --> x to 50
+        // if x is still 42, then change it to 50, else don't change and retry
+    }
+}
