Iterator模式

解释

多态的一个应用，各自的实现类实现自己的遍历方法，实现自己的遍历逻辑。

类图

在这里插入图片描述

举例学习

建立一个容器，可以存放数据，多种数据结构。

遍历时怎么遍历？

各自的数据结构实现自己的遍历方法。

代码

顶级接口

public interface Collection_<E> {
    void add(E o);
    int size();

    Iterator_ iterator();
}

ArrayList实现

class ArrayList_<E> implements Collection_<E> {
    E[] objects = (E[]) new Object[10];
    //objects中下一个空的位置在哪儿,或者说，目前容器中有多少个元素
    private int index = 0;

    public void add(E o) {
        if (index == objects.length) {
            E[] newObjects = (E[]) new Object[objects.length * 2];
            System.arraycopy(objects, 0, newObjects, 0, objects.length);
            objects = newObjects;
        }

        objects[index] = o;
        index++;
    }

    public int size() {
        return index;
    }

    @Override
    public Iterator_<E> iterator() {
        return new ArrayListIterator();
    }

    private class ArrayListIterator<E> implements Iterator_<E> {

        private int currentIndex = 0;

        @Override
        public boolean hasNext() {
            if (currentIndex >= index) return false;
            return true;
        }

        @Override
        public E next() {
            E o = (E) objects[currentIndex];
            currentIndex++;
            return o;
        }
    }
    
}

LinkedList实现

class LinkedList_<E> implements Collection_<E> {
    Node head = null;
    Node tail = null;
    //目前容器中有多少个元素
    private int size = 0;

    @Override
    public void add(E o) {
        Node n = new Node(o);
        n.next = null;

        if (head == null) {
            head = n;
            tail = n;
        }

        tail.next = n;
        tail = n;
        size++;
    }

    private static class Node<E> {
        private E o;
        Node next;

        public Node(E o) {
            this.o = o;
        }
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public Iterator_<E> iterator() {
        return new LinkedListIterator();
    }

    private class LinkedListIterator<E> implements Iterator_<E> {

        private int currentIndex = 0;

        @Override
        public boolean hasNext() {
            if (currentIndex < size) {
                return true;
            }
            return false;
        }

        @Override
        public E next() {
            if (currentIndex == 0) {
                currentIndex++;
                return (E) head.o;
            }

            if (currentIndex == size) {
                currentIndex++;
                return (E) tail.o;
            }

            currentIndex++;
            return getNext();
        }

        private E getNext() {
            Node<E> result = head;
            for (int i = 0; i < this.currentIndex - 1; i++) {
                result = result.next;
            }

            return result.o;
        }
    }
}

测试方法

public class Main {
    public static void main(String[] args) {
        Collection_<String> list = new LinkedList_<>();
        for(int i=0; i<15; i++) {
            list.add(new String("s" + i));
        }
        System.out.println(list.size());

        //这个接口的调用方式：
        Iterator_<String> it = list.iterator();
        while(it.hasNext()) {
            String o = it.next();
            System.out.println(o);
        }

    }
}