Java Iterable interface[Algorithm]

Q：什么是Iterable接口？

A：只有一个方法，放回Iterator，如下：

public interface Iterable<Item>
{
     Iterator<Item> iterator();
}

Q：什么是Iterator接口？

A：有两个方法：hasNext()和next()。第三个方法: remove()可选

Q：为什么要用数据结构Iterable?

A：Java支持优雅的客户代码，应用场景如下：

for (String s : stack)
    System.out.println(s);

Iterator<String> i = stack.iterator(); 
while (i.hasNext())
{
     String s = i.next();
     System.out.println(s);
}

下面，以Stack类实现Iterable为例，介绍Iterable的实现方法：

/*************************************************************************
 *  Compilation:  javac Stack.java
 *  Execution:    java Stack < input.txt
 *
 *  A generic stack, implemented using a singly-linked list.
 *  Each stack element is of type Item.
 *
 *  This version uses a static nested class Node (to save 8 bytes per
 *  Node), whereas the version in the textbook uses a non-static nested
 *  class (for simplicity).
 *  
 *  % more tobe.txt 
 *  to be or not to - be - - that - - - is
 *
 *  % java Stack < tobe.txt
 *  to be not that or be (2 left on stack)
 *
 *************************************************************************/

import java.util.Iterator;
import java.util.NoSuchElementException;


/**
 *  The <tt>Stack</tt> class represents a last-in-first-out (LIFO) stack of generic items.
 *  It supports the usual <em>push</em> and <em>pop</em> operations, along with methods
 *  for peeking at the top item, testing if the stack is empty, and iterating through
 *  the items in LIFO order.
 *  <p>
 *  This implementation uses a singly-linked list with a static nested class for
 *  linked-list nodes. See {@link LinkedStack} for the version from the
 *  textbook that uses a non-static nested class.
 *  The <em>push</em>, <em>pop</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em>
 *  operations all take constant time in the worst case.
 *  <p>
 *  For additional documentation, see <a href="/algs4/13stacks">Section 1.3</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class Stack<Item> implements Iterable<Item> {<span style="color:#ff0000;">//声明，实现Iterable接口</span>
    private int N;                // size of the stack
    private Node<Item> first;     // top of stack

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

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

    /**
     * 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<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        N++;
    }

    /**
     * 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--;
        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() {<span style="color:#ff0000;">//实现iterator方法，返回Iterator</span>
        return new ListIterator<Item>(first);
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator<Item> implements Iterator<Item> {<span style="color:#ff0000;">//实现Iterator</span>
        private Node<Item> current;

        public ListIterator(Node<Item> first) {
            current = first;
        }
        public boolean hasNext()  { return current != null;                     }<span style="color:#ff0000;">//实现hasNext方法</span>
        public void remove()      { throw new UnsupportedOperationException();  }//remove方法可选

        public Item next() {<span style="color:#ff0000;">//实现next方法</span>
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }


    /**
     * Unit tests the <tt>Stack</tt> data type.
     */
    public static void main(String[] args) {
        Stack<String> s = new Stack<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)");
    }
}