Add thread-safe ConcurrentHashMap implementation with coverage tests

PritiAryal · PritiAryal · commit 47d7f4d7cd11 · 2026-02-22T23:13:21.000-05:00
- Implemented `ConcurrentHashMap` using separate chaining with linked lists for collision handling.
- Ensured thread safety with bucket-level locking using `ReentrantLock`.
- Added methods: `put`, `get`, `remove`, and `containsKey`.
- Supported `null` keys and values.
- Added unit tests in `ConcurrentHashMapTest`:
  - Verified basic operations, null key handling, and edge cases.
  - Simulated concurrent access with multiple threads.
diff --git a/src/main/java/com/thealgorithms/datastructures/hashmap/hashing/ConcurrentHashMap.java b/src/main/java/com/thealgorithms/datastructures/hashmap/hashing/ConcurrentHashMap.java
@@ -0,0 +1,189 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * A thread-safe implementation of a HashMap using separate chaining with linked lists
+ * and ReentrantLocks for concurrency control.
+ *
+ * @param <K> the type of keys maintained by this map
+ * @param <V> the type of mapped values
+ */
+@SuppressWarnings("rawtypes")
+public class ConcurrentHashMap<K, V> {
+    private final int hashSize;
+    private final Bucket<K, V>[] buckets;
+    private final ReentrantLock[] locks;
+
+    /**
+     * Constructs a ConcurrentHashMap with the specified hash size.
+     *
+     * @param hashSize the number of buckets in the hash map
+     */
+    @SuppressWarnings("unchecked")
+    public ConcurrentHashMap(int hashSize) {
+        this.hashSize = hashSize;
+        this.buckets = new Bucket[hashSize];
+        this.locks = new ReentrantLock[hashSize];
+        for (int i = 0; i < hashSize; i++) {
+            buckets[i] = new Bucket<>();
+            locks[i] = new ReentrantLock();
+        }
+    }
+
+    /**
+     * Computes the hash code for the specified key.
+     * Null keys are hashed to bucket 0.
+     *
+     * @param key the key for which the hash code is to be computed
+     * @return the hash code corresponding to the key
+     */
+    private int computeHash(K key) {
+        if (key == null) {
+            return 0; // Use a special bucket (e.g., bucket 0) for null keys
+        }
+        int hash = key.hashCode() % hashSize;
+        return hash < 0 ? hash + hashSize : hash;
+    }
+
+    /**
+     * Inserts the specified key-value pair into the hash map.
+     * If the key already exists, the value is updated.
+     *
+     * @param key   the key to be inserted
+     * @param value the value to be associated with the key
+     */
+    public void put(K key, V value) {
+        int hash = computeHash(key);
+        locks[hash].lock();
+        try {
+            buckets[hash].put(key, value);
+        } finally {
+            locks[hash].unlock();
+        }
+    }
+
+    /**
+     * Retrieves the value associated with the specified key.
+     *
+     * @param key the key whose associated value is to be returned
+     * @return the value associated with the specified key, or null if the key does not exist
+     */
+    public V get(K key) {
+        int hash = computeHash(key);
+        locks[hash].lock();
+        try {
+            return buckets[hash].get(key);
+        } finally {
+            locks[hash].unlock();
+        }
+    }
+
+    /**
+     * Removes the key-value pair associated with the specified key from the hash map.
+     *
+     * @param key the key whose key-value pair is to be removed
+     */
+    public void remove(K key) {
+        int hash = computeHash(key);
+        locks[hash].lock();
+        try {
+            buckets[hash].remove(key);
+        } finally {
+            locks[hash].unlock();
+        }
+    }
+
+    /**
+     * Checks if the hash map contains the specified key.
+     *
+     * @param key the key to check
+     * @return true if the key exists, false otherwise
+     */
+    public boolean containsKey(K key) {
+        int hash = computeHash(key);
+        locks[hash].lock();
+        try {
+            return buckets[hash].containsKey(key);
+        } finally {
+            locks[hash].unlock();
+        }
+    }
+
+    /**
+     * A nested static class representing a bucket in the hash map.
+     * Each bucket uses a linked list to store key-value pairs.
+     *
+     * @param <K> the type of keys maintained by this bucket
+     * @param <V> the type of mapped values
+     */
+    private static class Bucket<K, V> {
+        private Node<K, V> head;
+
+        public void put(K key, V value) {
+            Node<K, V> node = findNode(key);
+            if (node != null) {
+                node.value = value;
+            } else {
+                Node<K, V> newNode = new Node<>(key, value);
+                newNode.next = head;
+                head = newNode;
+            }
+        }
+
+        public V get(K key) {
+            Node<K, V> node = findNode(key);
+            return node != null ? node.value : null;
+        }
+
+        public void remove(K key) {
+            if (head == null) {
+                return;
+            }
+            if ((key == null && head.key == null) || (head.key != null && head.key.equals(key))) {
+                head = head.next;
+                return;
+            }
+            Node<K, V> current = head;
+            while (current.next != null) {
+                if ((key == null && current.next.key == null) || (current.next.key != null && current.next.key.equals(key))) {
+                    current.next = current.next.next;
+                    return;
+                }
+                current = current.next;
+            }
+        }
+
+        public boolean containsKey(K key) {
+            return findNode(key) != null;
+        }
+
+        private Node<K, V> findNode(K key) {
+            Node<K, V> current = head;
+            while (current != null) {
+                if ((key == null && current.key == null) || (current.key != null && current.key.equals(key))) {
+                    return current;
+                }
+                current = current.next;
+            }
+            return null;
+        }
+    }
+
+    /**
+     * A nested static class representing a node in the linked list.
+     *
+     * @param <K> the type of key maintained by this node
+     * @param <V> the type of value maintained by this node
+     */
+    private static class Node<K, V> {
+        private final K key;
+        private V value;
+        private Node<K, V> next;
+
+        public Node(K key, V value) {
+            this.key = key;
+            this.value = value;
+        }
+    }
+}
diff --git a/src/test/java/com/thealgorithms/datastructures/hashmap/hashing/ConcurrentHashMapTest.java b/src/test/java/com/thealgorithms/datastructures/hashmap/hashing/ConcurrentHashMapTest.java
@@ -0,0 +1,140 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class ConcurrentHashMapTest {
+
+    @Test
+    void testPutAndGet() {
+        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(10);
+        map.put(1, "Value1");
+        map.put(2, "Value2");
+        map.put(3, "Value3");
+
+        assertEquals("Value1", map.get(1));
+        assertEquals("Value2", map.get(2));
+        assertEquals("Value3", map.get(3));
+        assertNull(map.get(4)); // Non-existent key
+    }
+
+    @Test
+    void testUpdateValue() {
+        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(10);
+        map.put(1, "Value1");
+        map.put(1, "UpdatedValue1");
+
+        assertEquals("UpdatedValue1", map.get(1)); // Verify updated value
+    }
+
+    @Test
+    void testRemove() {
+        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(10);
+        map.put(1, "Value1");
+        map.put(2, "Value2");
+
+        map.remove(1);
+        assertNull(map.get(1)); // Verify removal
+        assertEquals("Value2", map.get(2)); // Ensure other keys are unaffected
+    }
+
+    @Test
+    void testContainsKey() {
+        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(10);
+        map.put(1, "Value1");
+        map.put(2, "Value2");
+
+        assertTrue(map.containsKey(1));
+        assertTrue(map.containsKey(2));
+        assertFalse(map.containsKey(3)); // Non-existent key
+    }
+
+    @Test
+    void testNullKey() {
+        ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>(10);
+        map.put(null, "NullValue");
+
+        assertEquals("NullValue", map.get(null)); // Verify null key handling
+        map.remove(null);
+        assertNull(map.get(null)); // Verify null key removal
+    }
+
+    @Test
+    void testConcurrency() throws InterruptedException {
+        ConcurrentHashMap<Integer, Integer> map = new ConcurrentHashMap<>(10);
+
+        Thread writer1 = new Thread(() -> {
+            for (int i = 0; i < 50; i++) {
+                map.put(i, i * 10);
+            }
+        });
+
+        Thread writer2 = new Thread(() -> {
+            for (int i = 50; i < 100; i++) {
+                map.put(i, i * 10);
+            }
+        });
+
+        Thread reader = new Thread(() -> {
+            for (int i = 0; i < 100; i++) {
+                map.get(i);
+            }
+        });
+
+        writer1.start();
+        writer2.start();
+        reader.start();
+
+        writer1.join();
+        writer2.join();
+        reader.join();
+
+        for (int i = 0; i < 100; i++) {
+            assertEquals(i * 10, map.get(i));
+        }
+    }
+
+    @Test
+    void testRemoveNonExistentKey() {
+        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(10);
+        map.put(1, "Value1");
+        map.remove(2); // Attempt to remove a non-existent key
+
+        assertEquals("Value1", map.get(1)); // Ensure existing key remains
+        assertNull(map.get(2)); // Confirm non-existent key remains null
+    }
+
+    @Test
+    void testEmptyMap() {
+        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>(10);
+        assertNull(map.get(1)); // Test get on empty map
+        assertFalse(map.containsKey(1)); // Test containsKey on empty map
+    }
+
+    @Test
+    void testMultipleThreadsSameKey() throws InterruptedException {
+        ConcurrentHashMap<Integer, Integer> map = new ConcurrentHashMap<>(10);
+
+        Thread writer1 = new Thread(() -> {
+            for (int i = 0; i < 100; i++) {
+                map.put(1, i);
+            }
+        });
+
+        Thread writer2 = new Thread(() -> {
+            for (int i = 100; i < 200; i++) {
+                map.put(1, i);
+            }
+        });
+
+        writer1.start();
+        writer2.start();
+
+        writer1.join();
+        writer2.join();
+
+        assertNotNull(map.get(1)); // Ensure key exists
+        assertTrue(map.get(1) >= 0 && map.get(1) < 200); // Value should be within range
+    }
+}
