有人能给我解释一下下面在Java中使用链表实现堆栈的方法吗？

Question

有人能给我解释一下下面在Java中使用链表实现堆栈的方法吗？链接如下:http://algs4.cs.princeton.edu/13stacks/linkedstack.java.html，下面是代码：

       public class LinkedStack<Item> implements Iterable<Item> {
    private int N;          // size of the stack
    private Node first;     // top of stack

    // helper linked list class
    private class Node {
        private Item item;
        private Node next;
    }

    /**
     * Initializes an empty stack.
     */
    public LinkedStack() {
        first = null;
        N = 0;
        assert check();
    }

    /**
     * Is this stack empty?
     * @return true if this stack is empty; false otherwise
     */
    public boolean isEmpty() {
        return first == null;
    }

    /**
     * Returns the number of items in the stack.
     * @return the number of items in the stack
     */
    public int size() {
        return N;
    }

    /**
     * Adds the item to this stack.
     * @param item the item to add
     */
    public void push(Item item) {
        Node oldfirst = first;
        first = new Node();
        first.item = item;
        first.next = oldfirst;
        N++;
        assert check();
    }

    /**
     * Removes and returns the item most recently added to this stack.
     * @return the item most recently added
     * @throws java.util.NoSuchElementException if this stack is empty
     */
    public Item pop() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        Item item = first.item;        // save item to return
        first = first.next;            // delete first node
        N--;
        assert check();
        return item;                   // return the saved item
    }


    /**
     * Returns (but does not remove) the item most recently added to this stack.
     * @return the item most recently added to this stack
     * @throws java.util.NoSuchElementException if this stack is empty
     */
    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        return first.item;
    }

    /**
     * Returns a string representation of this stack.
     * @return the sequence of items in the stack in LIFO order, separated by spaces
     */
    public String toString() {
        StringBuilder s = new StringBuilder();
        for (Item item : this)
            s.append(item + " ");
        return s.toString();
    }

    /**
     * Returns an iterator to this stack that iterates through the items in LIFO order.
     * @return an iterator to this stack that iterates through the items in LIFO order.
     */
    public Iterator<Item> iterator()  { return new ListIterator();  }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator implements Iterator<Item> {
        private Node current = first;
        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }


    // check internal invariants
    private boolean check() {
        if (N == 0) {
            if (first != null) return false;
        }
        else if (N == 1) {
            if (first == null)      return false;
            if (first.next != null) return false;
        }
        else {
            if (first.next == null) return false;
        }

        // check internal consistency of instance variable N
        int numberOfNodes = 0;
        for (Node x = first; x != null; x = x.next) {
            numberOfNodes++;
        }
        if (numberOfNodes != N) return false;

        return true;
    }

    /**
     * Unit tests the <tt>LinkedStack</tt> data type.
     */
    public static void main(String[] args) {
        LinkedStack<String> s = new LinkedStack<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (!item.equals("-")) s.push(item);
            else if (!s.isEmpty()) StdOut.print(s.pop() + " ");
        }
        StdOut.println("(" + s.size() + " left on stack)");
    }
}

除了需要check()方法之外，所有内容都很清楚。我不明白的是，为什么在每次操作（例如pop、peek)之后，我们都需要检查堆栈中元素的数量和变量n（堆栈的大小）是否一致。我们不是一直保持这两个价值观一致吗？我真的不明白check()方法有什么用？

Answer 1

上面的check()方法仅在断言中使用。

断言可以提供对预期不变量的实时检查。它们在默认情况下是禁用的。

您可以启用断言来检测错误或调试，通常是在开发环境中。