testing: improve test coverage DequeTest (#6429)

testing: improve test coverage DequeTest

src/test/java/com/thealgorithms/datastructures/queues/DequeTest.java

@@ -121,4 +121,77 @@ class DequeTest {
         assertNull(deque.pollFirst());
         org.junit.jupiter.api.Assertions.assertTrue(deque.isEmpty());
     }
+
+    @Test
+    void testMultipleAddFirst() {
+        Deque<Integer> deque = new Deque<>();
+        deque.addFirst(1);
+        deque.addFirst(2);
+        deque.addFirst(3);
+
+        assertEquals(3, deque.peekFirst(), "First element should be the last added to front");
+        assertEquals(1, deque.peekLast(), "Last element should be the first added to front");
+        assertEquals(3, deque.size(), "Size should reflect all additions");
+    }
+
+    @Test
+    void testMultipleAddLast() {
+        Deque<Integer> deque = new Deque<>();
+        deque.addLast(1);
+        deque.addLast(2);
+        deque.addLast(3);
+
+        assertEquals(1, deque.peekFirst(), "First element should be the first added to back");
+        assertEquals(3, deque.peekLast(), "Last element should be the last added to back");
+        assertEquals(3, deque.size(), "Size should reflect all additions");
+    }
+
+    @Test
+    void testSingleElementOperations() {
+        Deque<Integer> deque = new Deque<>();
+        deque.addFirst(1);
+
+        assertEquals(1, deque.peekFirst(), "Single element should be both first and last");
+        assertEquals(1, deque.peekLast(), "Single element should be both first and last");
+        assertEquals(1, deque.size());
+
+        assertEquals(1, deque.pollFirst(), "Should be able to poll single element from front");
+        org.junit.jupiter.api.Assertions.assertTrue(deque.isEmpty(), "Deque should be empty after polling single element");
+    }
+
+    @Test
+    void testSingleElementPollLast() {
+        Deque<Integer> deque = new Deque<>();
+        deque.addLast(1);
+
+        assertEquals(1, deque.pollLast(), "Should be able to poll single element from back");
+        org.junit.jupiter.api.Assertions.assertTrue(deque.isEmpty(), "Deque should be empty after polling single element");
+    }
+
+    @Test
+    void testMixedNullAndValues() {
+        Deque<String> deque = new Deque<>();
+        deque.addFirst("first");
+        deque.addLast(null);
+        deque.addFirst(null);
+        deque.addLast("last");
+
+        assertEquals(4, deque.size(), "Should handle mixed null and non-null values");
+        assertNull(deque.pollFirst(), "Should poll null from front");
+        assertEquals("first", deque.pollFirst(), "Should poll non-null value");
+        assertNull(deque.pollLast().equals("last") ? null : deque.peekLast(), "Should handle null correctly");
+    }
+
+    @Test
+    void testSymmetricOperations() {
+        Deque<Integer> deque = new Deque<>();
+
+        // Test that addFirst/pollFirst and addLast/pollLast are symmetric
+        deque.addFirst(1);
+        deque.addLast(2);
+
+        assertEquals(1, deque.pollFirst(), "addFirst/pollFirst should be symmetric");
+        assertEquals(2, deque.pollLast(), "addLast/pollLast should be symmetric");
+        org.junit.jupiter.api.Assertions.assertTrue(deque.isEmpty(), "Deque should be empty after symmetric operations");
+    }
 }