docs: adding subject and main

subjects/java/checkpoints/circular-linkedlist/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        LinkedList list = new CircularLinkedList();
+
+        // Add elements to the list
+        list.add(1);
+        list.add(2);
+        list.add(3);
+
+        // Access elements by index
+        System.out.println("Element at index 0: " + list.at(0)); // Expected output: 1
+        System.out.println("Element at index 1: " + list.at(1)); // Expected output: 2
+        System.out.println("Element at index 2: " + list.at(2)); // Expected output: 3
+
+        // Remove an element by index
+        list.remove(1);
+        System.out.println("Element at index 1 after removal: " + list.at(1)); // Expected output: 3
+    }
+}

subjects/java/checkpoints/circular-linkedlist/LinkedList.java

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+public interface LinkedList {
+    int at(int index);
+    void add(int value);
+    void remove(int index);
+    int size();
+}

subjects/java/checkpoints/circular-linkedlist/README.md

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+## Circular Linked List
+
+### Instructions
+
+Create a circular single linked list data structure that implements the following methods:
+
+- `at(int index)`: to access an element by its index.
+- `add(int value)`: to add an element at the end of the list.
+- `remove(int index)`: to remove an element by its index.
+- `size()`: to return the size of the list.
+
+Define these methods in an interface called `LinkedList`, and implement this interface in a class that you will create. Additionally, add a private method `next(Node node)` in your class that will be used in the other methods. This method should print the message "Go to next node" each time it is called.
+
+### Explanation
+
+A circular linked list is a linear data structure where each element is a separate object called a node. Each node contains two fields:
+
+- `value`: stores the data.
+- `next`: stores a reference to the next node in the list.
+
+The first node is called the head of the list and the last node in a circular linked list points back to the first node, forming a circle. The list allows for efficient insertion and deletion of elements. However, accessing an element by its index requires traversing the list from the head to the desired position.
+
+### Expected Interface
+
+````java
+public interface LinkedList {
+    int at(int index);
+    void add(int value);
+    void remove(int index);
+    int size();
+}
+
+### Expected Class
+
+```java
+public class CircularLinkedList implements LinkedList {
+    private Node head;
+
+    private class Node {
+        int value;
+        Node next;
+
+        Node(int value) {
+            this.value = value;
+            this.next = null;
+        }
+    }
+
+    @Override
+    public int at(int index) {
+        // Implementation for accessing an element by its index
+    }
+
+    @Override
+    public void add(int value) {
+        // Implementation for adding an element at the end of the list
+    }
+
+    @Override
+    public void remove(int index) {
+        // Implementation for removing an element by its index
+    }
+
+    @Override
+    public int size() {
+        // Implementation getting the size of the list
+    }
+
+    private Node next(Node node) {
+        // Print the message "Go to next node"
+    }
+}
+````
+
+### Usage
+
+Here is a possible ExerciseRunner.java to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        LinkedList list = new CircularLinkedList();
+
+        // Add elements to the list
+        list.add(1);
+        list.add(2);
+        list.add(3);
+
+        // Access elements by index
+        System.out.println("Element at index 0: " + list.at(0)); // Expected output: 1
+        System.out.println("Element at index 1: " + list.at(1)); // Expected output: 2
+        System.out.println("Element at index 2: " + list.at(2)); // Expected output: 3
+
+        // Remove an element by index
+        list.remove(1);
+        System.out.println("Element at index 1 after removal: " + list.at(1)); // Expected output: 3
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Go to next node
+Element at index 0: 1
+Go to next node
+Element at index 1: 2
+Go to next node
+Go to next node
+Element at index 2: 3
+Go to next node
+Go to next node
+Element at index 1 after removal: 3
+$
+```