15.2.2 一个神奇的堆栈类

public class LinkedStack<T> {

    private static class Node<U>{
        U item;
        Node<U> next;
        Node()   {
            item = null;
            next = null;
        }

        public Node(U item, Node<U> next) {
            this.item = item;
            this.next = next;
        }
        boolean end(){
            return item == null && next == null;
        }
    }

    private Node<T> top = new Node<>();

    public void push(T item){
        top = new Node<T>(item,top);
    }

    public T pop(){
        T result = top.item;
        if(!top.end())
            top = top.next;
        return result;
    }

    public static void main(String[] args) {
        LinkedStack<String> lss = new LinkedStack<>();
        List<String> list = Arrays.asList("Phasers on stun what what!".split(" "));

        list.forEach(s -> lss.push(s));

        String s;
        while((s = lss.pop()) != null){
            System.out.println(s);
        }

    }
}

what!
what
stun
on
Phasers

内部类Node也是一个泛型,它拥有自己的参数类型

这个例子使用的是一个末端哨兵的模式来判断堆栈何时为空。这个末端哨兵是在构建LinkedList构建的。然后，每调用一次push()方法，就会创建一个Node<T>对象，并将其连接到前一个Node<T>对象。当你调用pop()方法时，总是返回top.item,然后丢弃当前对象，将top转换成下一个Node对象，除非你碰见了末端哨兵，这个时候就不能再移动了，在继续调用pop()方法，只能得到null

当然也可以这么做

public class LinkedStack<T> {

    private class Node{
        T item;
        Node next;
        Node()   {
            item = null;
            next = null;
        }

        public Node(T item, Node next) {
            this.item = item;
            this.next = next;
        }
        boolean end(){
            return item == null && next == null;
        }
    }

    private Node top = new Node();

    public void push(T item){
        top = new Node(item,top);
    }

    public T pop(){
        T result = top.item;
        if(!top.end())
            top = top.next;
        return result;
    }

    public static void main(String[] args) {
        LinkedStack<String> lss = new LinkedStack<>();
        List<String> list = Arrays.asList("Phasers on stun what what!".split(" "));

        list.forEach(s -> lss.push(s));

        String s;
        while((s = lss.pop()) != null){
            System.out.println(s);
        }

    }
}