test: enhance GenericHashMapUsingArrayTest with additional edge case coverage

Author: lmj798

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

@@ -3,9 +3,14 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 class GenericHashMapUsingArrayTest {
 
@@ -93,4 +98,143 @@ void testContainsKey() {
         assertTrue(map.containsKey("USA"));
         assertFalse(map.containsKey("Nepal"));
     }
+
+    // ======= Added tests from the new version =======
+
+    @Test
+    void shouldThrowNullPointerExceptionForNullKey() {
+        GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
+        assertThrows(NullPointerException.class, () -> map.put(null, "value"));
+    }
+
+    @Test
+    void shouldStoreNullValueForKey() {
+        GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
+        map.put("keyWithNullValue", null);
+        assertEquals(1, map.size());
+        assertNull(map.get("keyWithNullValue"));
+        // Note: containsKey returns false for null values due to implementation
+        assertFalse(map.containsKey("keyWithNullValue"));
+    }
+
+    @Test
+    void shouldHandleCollisionWhenKeysHashToSameBucket() {
+        GenericHashMapUsingArray<Integer, Integer> map = new GenericHashMapUsingArray<>();
+        Integer key1 = 1;
+        Integer key2 = 17;
+        map.put(key1, 100);
+        map.put(key2, 200);
+        assertEquals(2, map.size());
+        assertEquals(100, map.get(key1));
+        assertEquals(200, map.get(key2));
+        assertTrue(map.containsKey(key1));
+        assertTrue(map.containsKey(key2));
+    }
+
+    @Test
+    void shouldHandleEmptyStringAsKey() {
+        GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
+        map.put("", "valueForEmptyKey");
+        assertEquals(1, map.size());
+        assertEquals("valueForEmptyKey", map.get(""));
+        assertTrue(map.containsKey(""));
+    }
+
+    @Test
+    void shouldHandleEmptyStringAsValue() {
+        GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
+        map.put("keyForEmptyValue", "");
+        assertEquals(1, map.size());
+        assertEquals("", map.get("keyForEmptyValue"));
+        assertTrue(map.containsKey("keyForEmptyValue"));
+    }
+
+    @Test
+    void shouldHandleNegativeIntegerKeys() {
+        GenericHashMapUsingArray<Integer, Integer> map = new GenericHashMapUsingArray<>();
+        map.put(-1, 100);
+        map.put(-100, 200);
+        assertEquals(2, map.size());
+        assertEquals(100, map.get(-1));
+        assertEquals(200, map.get(-100));
+        assertTrue(map.containsKey(-1));
+        assertTrue(map.containsKey(-100));
+    }
+
+    @Test
+    void shouldHandleZeroAsKey() {
+        GenericHashMapUsingArray<Integer, Integer> map = new GenericHashMapUsingArray<>();
+        map.put(0, 100);
+        assertEquals(1, map.size());
+        assertEquals(100, map.get(0));
+        assertTrue(map.containsKey(0));
+    }
+
+    @Test
+    void shouldHandleStringWithSpecialCharacters() {
+        GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
+        map.put("key!@#$%^&*()", "value<>?/\\|");
+        assertEquals(1, map.size());
+        assertEquals("value<>?/\\|", map.get("key!@#$%^&*()"));
+        assertTrue(map.containsKey("key!@#$%^&*()"));
+    }
+
+    @Test
+    void shouldHandleLongStrings() {
+        GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
+        StringBuilder longKey = new StringBuilder();
+        StringBuilder longValue = new StringBuilder();
+        for (int i = 0; i < 1000; i++) {
+            longKey.append("a");
+            longValue.append("b");
+        }
+        String key = longKey.toString();
+        String value = longValue.toString();
+        map.put(key, value);
+        assertEquals(1, map.size());
+        assertEquals(value, map.get(key));
+        assertTrue(map.containsKey(key));
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"a", "ab", "abc", "test", "longerString"})
+    void shouldHandleKeysOfDifferentLengths(String key) {
+        GenericHashMapUsingArray<String, String> map = new GenericHashMapUsingArray<>();
+        map.put(key, "value");
+        assertEquals(1, map.size());
+        assertEquals("value", map.get(key));
+        assertTrue(map.containsKey(key));
+    }
+
+    @Test
+    void shouldHandleUpdateOnExistingKeyInCollisionBucket() {
+        GenericHashMapUsingArray<Integer, Integer> map = new GenericHashMapUsingArray<>();
+        Integer key1 = 1;
+        Integer key2 = 17;
+        map.put(key1, 100);
+        map.put(key2, 200);
+        assertEquals(2, map.size());
+        map.put(key2, 999);
+        assertEquals(2, map.size());
+        assertEquals(100, map.get(key1));
+        assertEquals(999, map.get(key2));
+        assertTrue(map.containsKey(key1));
+        assertTrue(map.containsKey(key2));
+    }
+
+    @Test
+    void shouldHandleExactlyLoadFactorBoundary() {
+        GenericHashMapUsingArray<Integer, Integer> map = new GenericHashMapUsingArray<>();
+        // Fill exactly to load factor (12 items with capacity 16 and 0.75 load factor)
+        for (int i = 0; i < 12; i++) {
+            map.put(i, i * 10);
+        }
+        assertEquals(12, map.size());
+        // Act - This should trigger rehash on 13th item
+        map.put(12, 120);
+        // Assert - Rehash should have happened
+        assertEquals(13, map.size());
+        assertEquals(120, map.get(12));
+        assertTrue(map.containsKey(12));
+    }
 }