pop()

/* 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
}