Enhance docs, add tests in CircleLinkedList (#5991)

src/test/java/com/thealgorithms/datastructures/lists/CircleLinkedListTest.java

@@ -1,78 +1,115 @@
 package com.thealgorithms.datastructures.lists;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 public class CircleLinkedListTest {
 
+    private CircleLinkedList<Integer> list;
+
+    @BeforeEach
+    public void setUp() {
+        list = new CircleLinkedList<>();
+    }
+
+    @Test
+    public void testInitialSize() {
+        assertEquals(0, list.getSize(), "Initial size should be 0.");
+    }
+
     @Test
     public void testAppendAndSize() {
-        CircleLinkedList<Integer> list = new CircleLinkedList<>();
         list.append(1);
         list.append(2);
         list.append(3);
 
-        assertEquals(3, list.getSize());
-        assertEquals("[ 1, 2, 3 ]", list.toString());
+        assertEquals(3, list.getSize(), "Size after three appends should be 3.");
+        assertEquals("[ 1, 2, 3 ]", list.toString(), "List content should match appended values.");
     }
 
     @Test
     public void testRemove() {
-        CircleLinkedList<Integer> list = new CircleLinkedList<>();
         list.append(1);
         list.append(2);
         list.append(3);
         list.append(4);
 
-        assertEquals(2, list.remove(1));
-        assertEquals(3, list.remove(1));
-        assertEquals("[ 1, 4 ]", list.toString());
-        assertEquals(2, list.getSize());
+        assertEquals(2, list.remove(1), "Removed element at index 1 should be 2.");
+        assertEquals(3, list.remove(1), "Removed element at index 1 after update should be 3.");
+        assertEquals("[ 1, 4 ]", list.toString(), "List content should reflect removals.");
+        assertEquals(2, list.getSize(), "Size after two removals should be 2.");
     }
 
     @Test
     public void testRemoveInvalidIndex() {
-        CircleLinkedList<Integer> list = new CircleLinkedList<>();
         list.append(1);
         list.append(2);
 
-        assertThrows(IndexOutOfBoundsException.class, () -> list.remove(2));
-        assertThrows(IndexOutOfBoundsException.class, () -> list.remove(-1));
+        assertThrows(IndexOutOfBoundsException.class, () -> list.remove(2), "Should throw on out-of-bounds index.");
+        assertThrows(IndexOutOfBoundsException.class, () -> list.remove(-1), "Should throw on negative index.");
     }
 
     @Test
     public void testToStringEmpty() {
-        CircleLinkedList<Integer> list = new CircleLinkedList<>();
-        assertEquals("[]", list.toString());
+        assertEquals("[]", list.toString(), "Empty list should be represented by '[]'.");
     }
 
     @Test
     public void testToStringAfterRemoval() {
-        CircleLinkedList<Integer> list = new CircleLinkedList<>();
         list.append(1);
         list.append(2);
         list.append(3);
         list.remove(1);
 
-        assertEquals("[ 1, 3 ]", list.toString());
+        assertEquals("[ 1, 3 ]", list.toString(), "List content should match remaining elements after removal.");
     }
 
     @Test
     public void testSingleElement() {
-        CircleLinkedList<Integer> list = new CircleLinkedList<>();
         list.append(1);
 
-        assertEquals(1, list.getSize());
-        assertEquals("[ 1 ]", list.toString());
-        assertEquals(1, list.remove(0));
-        assertEquals("[]", list.toString());
+        assertEquals(1, list.getSize(), "Size after single append should be 1.");
+        assertEquals("[ 1 ]", list.toString(), "Single element list should display properly.");
+        assertEquals(1, list.remove(0), "Single element removed should match appended value.");
+        assertEquals("[]", list.toString(), "List should be empty after removing the single element.");
     }
 
     @Test
     public void testNullElement() {
-        CircleLinkedList<String> list = new CircleLinkedList<>();
-        assertThrows(NullPointerException.class, () -> list.append(null));
+        assertThrows(NullPointerException.class, () -> list.append(null), "Appending null should throw exception.");
+    }
+
+    @Test
+    public void testCircularReference() {
+        list.append(1);
+        list.append(2);
+        list.append(3);
+        CircleLinkedList.Node<Integer> current = list.head;
+
+        // Traverse one full cycle and verify the circular reference
+        for (int i = 0; i <= list.getSize(); i++) {
+            current = current.next;
+        }
+        assertEquals(list.head, current, "End of list should point back to the head (circular structure).");
+    }
+
+    @Test
+    public void testClear() {
+        list.append(1);
+        list.append(2);
+        list.append(3);
+
+        // Remove all elements to simulate clearing the list
+        for (int i = list.getSize() - 1; i >= 0; i--) {
+            list.remove(i);
+        }
+
+        assertEquals(0, list.getSize(), "Size after clearing should be 0.");
+        assertEquals("[]", list.toString(), "Empty list should be represented by '[]' after clear.");
+        assertSame(list.head.next, list.head, "Head's next should point to itself after clearing.");
     }
 }