Stack.java

public class Stack<Data> {
    private Node<Data> top = null;
    private int size = 0;


    /* Push an item onto the stack */
    public void push(Data item) {
        Node<Data> newNode = new Node<>();      // 1
        newNode.data = item;                    // 1
        newNode.next = top;                     // 2
        top = newNode;                          // 3
        size++;                                 // 4
    }

    /* Check if the stack is empty */
    public boolean isEmpty() {
        return size == 0;
    }

    /* Remove the top item from the stack, and return its data */
    public Data pop() {
        if (isEmpty()) {                                                    // 1
            throw new NoSuchElementException("The stack is empty.");        // 1
        }                                                                   // 1
        Data item = top.data;                                               // 2
        top = top.next;                                                     // 3
        size--;                                                             // 4
        return item;                                                        // 5
    }

    /* Return the current size of the stack */
    public int size() {
        return size;
    }

    /* Reads an item at the top of the stack, without removing it */
    public Data peek() {
        if (isEmpty()) {                                                // 1
            throw new NoSuchElementException("Stack is empty!");        // 1
        }

        return top.data;                                                // 2
    }
}