docs: adding subject and main

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

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        LinkedList list = new SingleLinkedList();
+
+        // 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/single-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/single-linkedlist/README.md

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+## Single Linked List
+
+### Instructions
+
+Create a single linked list data structure that implements the following methods:
+
+- `at(int index)`: to access an element by its index, if the index is out of bound return -1.
+- `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 get theh 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 to traversing the list in the other methods. This method should print the message "Go to next node" each time it is called.
+
+### Explanation
+
+A 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 is always null. 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.
+
+### Given Interface
+
+```java
+public interface LinkedList {
+    int at(int index);
+    void add(int value);
+    void remove(int index);
+    int size();
+}
+```
+
+### Expected Class
+
+```java
+public class SingleLinkedList implements LinkedList {
+    private Node head;
+    private int size;
+
+    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 for accessing an element by its index
+    }
+
+    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 SingleLinkedList();
+
+        // 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
+$
+```