refactor: HashMap (#5426)

* refactor: HashMap

* checkstyle: fix formatting

* refactor: remove redundant comments

---------

Co-authored-by: alxkm <alx@alx.com>

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

@@ -0,0 +1,146 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+public class HashMapTest {
+
+    @Test
+    public void testInsertAndSearch() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        hashMap.insert(15, "Value15");
+        hashMap.insert(25, "Value25");
+        hashMap.insert(35, "Value35");
+
+        assertEquals("Value15", hashMap.search(15));
+        assertEquals("Value25", hashMap.search(25));
+        assertEquals("Value35", hashMap.search(35));
+        assertNull(hashMap.search(45));
+    }
+
+    @Test
+    public void testDelete() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        hashMap.insert(15, "Value15");
+        hashMap.insert(25, "Value25");
+        hashMap.insert(35, "Value35");
+
+        assertEquals("Value25", hashMap.search(25));
+        hashMap.delete(25);
+        assertNull(hashMap.search(25));
+    }
+
+    @Test
+    public void testDisplay() {
+        HashMap<Integer, String> hashMap = new HashMap<>(5);
+        hashMap.insert(15, "Value15");
+        hashMap.insert(25, "Value25");
+        hashMap.insert(35, "Value35");
+        hashMap.display();
+    }
+
+    @Test
+    public void testInsertNullKey() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        hashMap.insert(null, "NullValue");
+        assertEquals("NullValue", hashMap.search(null));
+    }
+
+    @Test
+    public void testInsertNullValue() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        hashMap.insert(15, null);
+        assertNull(hashMap.search(15));
+    }
+
+    @Test
+    public void testUpdateExistingKey() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        hashMap.insert(15, "Value15");
+        hashMap.insert(15, "UpdatedValue15");
+
+        assertEquals("UpdatedValue15", hashMap.search(15));
+    }
+
+    @Test
+    public void testHandleCollisions() {
+        HashMap<Integer, String> hashMap = new HashMap<>(3);
+        // These keys should collide if the hash function is modulo 3
+        hashMap.insert(1, "Value1");
+        hashMap.insert(4, "Value4");
+        hashMap.insert(7, "Value7");
+
+        assertEquals("Value1", hashMap.search(1));
+        assertEquals("Value4", hashMap.search(4));
+        assertEquals("Value7", hashMap.search(7));
+    }
+
+    @Test
+    public void testSearchInEmptyHashMap() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        assertNull(hashMap.search(10));
+    }
+
+    @Test
+    public void testDeleteNonExistentKey() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        hashMap.insert(15, "Value15");
+        hashMap.delete(25);
+
+        assertEquals("Value15", hashMap.search(15));
+        assertNull(hashMap.search(25));
+    }
+
+    @Test
+    public void testInsertLargeNumberOfElements() {
+        HashMap<Integer, String> hashMap = new HashMap<>(10);
+        for (int i = 0; i < 100; i++) {
+            hashMap.insert(i, "Value" + i);
+        }
+
+        for (int i = 0; i < 100; i++) {
+            assertEquals("Value" + i, hashMap.search(i));
+        }
+    }
+
+    @Test
+    public void testDeleteHeadOfBucket() {
+        HashMap<Integer, String> hashMap = new HashMap<>(3);
+        hashMap.insert(1, "Value1");
+        hashMap.insert(4, "Value4");
+        hashMap.insert(7, "Value7");
+
+        hashMap.delete(1);
+        assertNull(hashMap.search(1));
+        assertEquals("Value4", hashMap.search(4));
+        assertEquals("Value7", hashMap.search(7));
+    }
+
+    @Test
+    public void testDeleteTailOfBucket() {
+        HashMap<Integer, String> hashMap = new HashMap<>(3);
+        hashMap.insert(1, "Value1");
+        hashMap.insert(4, "Value4");
+        hashMap.insert(7, "Value7");
+
+        hashMap.delete(7);
+        assertNull(hashMap.search(7));
+        assertEquals("Value1", hashMap.search(1));
+        assertEquals("Value4", hashMap.search(4));
+    }
+
+    @Test
+    public void testDeleteMiddleElementOfBucket() {
+        HashMap<Integer, String> hashMap = new HashMap<>(3);
+        hashMap.insert(1, "Value1");
+        hashMap.insert(4, "Value4");
+        hashMap.insert(7, "Value7");
+
+        hashMap.delete(4);
+        assertNull(hashMap.search(4));
+        assertEquals("Value1", hashMap.search(1));
+        assertEquals("Value7", hashMap.search(7));
+    }
+}