Add Immutable HashMap implementation (#7183)

* Add immutable HashMap implementation

* Add immutable HashMap implementation

* Applied clang-format

---------

Co-authored-by: Koushik Sai <nyamathabadkoushik@gmail.com>

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/ImmutableHashMapTest.java

@@ -0,0 +1,55 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class ImmutableHashMapTest {
+
+    @Test
+    void testEmptyMap() {
+        ImmutableHashMap<String, Integer> map = ImmutableHashMap.<String, Integer>empty();
+
+        assertEquals(0, map.size());
+        assertNull(map.get("A"));
+    }
+
+    @Test
+    void testPutDoesNotModifyOriginalMap() {
+        ImmutableHashMap<String, Integer> map1 = ImmutableHashMap.<String, Integer>empty();
+
+        ImmutableHashMap<String, Integer> map2 = map1.put("A", 1);
+
+        assertEquals(0, map1.size());
+        assertEquals(1, map2.size());
+        assertNull(map1.get("A"));
+        assertEquals(1, map2.get("A"));
+    }
+
+    @Test
+    void testMultiplePuts() {
+        ImmutableHashMap<String, Integer> map = ImmutableHashMap.<String, Integer>empty().put("A", 1).put("B", 2);
+
+        assertEquals(2, map.size());
+        assertEquals(1, map.get("A"));
+        assertEquals(2, map.get("B"));
+    }
+
+    @Test
+    void testContainsKey() {
+        ImmutableHashMap<String, Integer> map = ImmutableHashMap.<String, Integer>empty().put("X", 100);
+
+        assertTrue(map.containsKey("X"));
+        assertFalse(map.containsKey("Y"));
+    }
+
+    @Test
+    void testNullKey() {
+        ImmutableHashMap<String, Integer> map = ImmutableHashMap.<String, Integer>empty().put(null, 50);
+
+        assertEquals(50, map.get(null));
+    }
+}