Added Circular Doubly Linked List in the datastructures section

src/main/java/com/thealgorithms/datastructures/lists/CircularDoublyLinkedList.java

@@ -0,0 +1,118 @@
+package com.thealgorithms.datastructures.lists;
+
+/**
+ * This class is a circular doubly linked list implementation. In a circular
+ * doubly linked list,
+ * the last node points back to the first node and the first node points back to
+ * the last node,
+ * creating a circular chain in both directions.
+ *
+ * This implementation includes basic operations such as appending elements to
+ * the end,
+ * removing elements from a specified position, and converting the list to a
+ * string representation.
+ *
+ * @param <E> the type of elements held in this list
+ */
+public class CircularDoublyLinkedList<E> {
+    static final class Node<E> {
+        Node<E> next;
+        Node<E> prev;
+        E value;
+
+        private Node(E value, Node<E> next, Node<E> prev) {
+            this.value = value;
+            this.next = next;
+            this.prev = prev;
+        }
+    }
+
+    private int size;
+    Node<E> head = null;
+
+    /**
+     * Initializes a new circular doubly linked list. A dummy head node is used for
+     * simplicity,
+     * pointing initially to itself to ensure the list is never empty.
+     */
+    public CircularDoublyLinkedList() {
+        head = new Node<>(null, null, null);
+        head.next = head;
+        head.prev = head;
+        size = 0;
+    }
+
+    /**
+     * Returns the current size of the list.
+     *
+     * @return the number of elements in the list
+     */
+    public int getSize() {
+        return size;
+    }
+
+    /**
+     * Appends a new element to the end of the list. Throws a NullPointerException
+     * if
+     * a null value is provided.
+     *
+     * @param value the value to append to the list
+     * @throws NullPointerException if the value is null
+     */
+    public void append(E value) {
+        if (value == null) {
+            throw new NullPointerException("Cannot add null element to the list");
+        }
+        Node<E> newNode = new Node<>(value, head, head.prev);
+        head.prev.next = newNode;
+        head.prev = newNode;
+        size++;
+    }
+
+    /**
+     * Returns a string representation of the list in the format "[ element1,
+     * element2, ... ]".
+     * An empty list is represented as "[]".
+     *
+     * @return the string representation of the list
+     */
+    public String toString() {
+        if (size == 0) {
+            return "[]";
+        }
+        StringBuilder sb = new StringBuilder("[ ");
+        Node<E> current = head.next;
+        while (current != head) {
+            sb.append(current.value);
+            if (current.next != head) {
+                sb.append(", ");
+            }
+            current = current.next;
+        }
+        sb.append(" ]");
+        return sb.toString();
+    }
+
+    /**
+     * Removes and returns the element at the specified position in the list.
+     * Throws an IndexOutOfBoundsException if the position is invalid.
+     *
+     * @param pos the position of the element to remove
+     * @return the value of the removed element - pop operation
+     * @throws IndexOutOfBoundsException if the position is out of range
+     */
+    public E remove(int pos) {
+        if (pos >= size || pos < 0) {
+            throw new IndexOutOfBoundsException("Position out of bounds");
+        }
+        Node<E> current = head.next;
+        for (int i = 0; i < pos; i++) {
+            current = current.next;
+        }
+        current.prev.next = current.next;
+        current.next.prev = current.prev;
+        E removedValue = current.value;
+        size--;
+        return removedValue;
+    }
+}

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

@@ -0,0 +1,120 @@
+package com.thealgorithms.datastructures.lists;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class CircularDoublyLinkedListTest {
+
+    private CircularDoublyLinkedList<Integer> list;
+
+    @BeforeEach
+    public void setUp() {
+        list = new CircularDoublyLinkedList<>();
+    }
+
+    @Test
+    public void testInitialSize() {
+        assertEquals(0, list.getSize(), "Initial size should be 0.");
+    }
+
+    @Test
+    public void testAppendAndSize() {
+        list.append(10);
+        list.append(20);
+        list.append(30);
+
+        assertEquals(3, list.getSize(), "Size after appends should be 3.");
+        assertEquals("[ 10, 20, 30 ]", list.toString(), "List content should match appended values.");
+    }
+
+    @Test
+    public void testRemove() {
+        list.append(10);
+        list.append(20);
+        list.append(30);
+
+        int removed = list.remove(1);
+        assertEquals(20, removed, "Removed element at index 1 should be 20.");
+
+        assertEquals("[ 10, 30 ]", list.toString(), "List content should reflect removal.");
+        assertEquals(2, list.getSize(), "Size after removal should be 2.");
+
+        removed = list.remove(0);
+        assertEquals(10, removed, "Removed element at index 0 should be 10.");
+        assertEquals("[ 30 ]", list.toString(), "List content should reflect second removal.");
+        assertEquals(1, list.getSize(), "Size after second removal should be 1.");
+    }
+
+    @Test
+    public void testRemoveInvalidIndex() {
+        list.append(10);
+        list.append(20);
+
+        assertThrows(IndexOutOfBoundsException.class, () -> list.remove(2), "Removing at invalid index 2 should throw exception.");
+        assertThrows(IndexOutOfBoundsException.class, () -> list.remove(-1), "Removing at negative index should throw exception.");
+    }
+
+    @Test
+    public void testToStringEmpty() {
+        assertEquals("[]", list.toString(), "Empty list should display as [].");
+    }
+
+    @Test
+    public void testSingleElement() {
+        list.append(10);
+
+        assertEquals(1, list.getSize(), "Size after adding single element should be 1.");
+        assertEquals("[ 10 ]", list.toString(), "Single element list string should be formatted correctly.");
+        int removed = list.remove(0);
+        assertEquals(10, removed, "Removed element should be the one appended.");
+        assertEquals("[]", list.toString(), "List should be empty after removing last element.");
+        assertEquals(0, list.getSize(), "Size after removing last element should be 0.");
+    }
+
+    @Test
+    public void testNullAppend() {
+        assertThrows(NullPointerException.class, () -> list.append(null), "Appending null should throw NullPointerException.");
+    }
+
+    @Test
+    public void testRemoveLastPosition() {
+        list.append(10);
+        list.append(20);
+        list.append(30);
+        int removed = list.remove(list.getSize() - 1);
+        assertEquals(30, removed, "Last element removed should be 30.");
+        assertEquals(2, list.getSize(), "Size should decrease after removing last element.");
+    }
+
+    @Test
+    public void testRemoveFromEmptyThrows() {
+        assertThrows(IndexOutOfBoundsException.class, () -> list.remove(0), "Remove from empty list should throw.");
+    }
+
+    @Test
+    public void testRepeatedAppendAndRemove() {
+        for (int i = 0; i < 100; i++) {
+            list.append(i);
+        }
+        assertEquals(100, list.getSize());
+
+        for (int i = 99; i >= 0; i--) {
+            int removed = list.remove(i);
+            assertEquals(i, removed, "Removed element should match appended value.");
+        }
+        assertEquals(0, list.getSize(), "List should be empty after all removes.");
+    }
+
+    @Test
+    public void testToStringAfterMultipleRemoves() {
+        list.append(1);
+        list.append(2);
+        list.append(3);
+        list.remove(2);
+        list.remove(0);
+        assertEquals("[ 2 ]", list.toString(), "ToString should correctly represent remaining elements.");
+    }
+}