设计模式-17-迭代器

简介：提供一种方法顺序访问一个聚合对象中各个元素, 而又不需暴露该对象的内部表示。

目的：

总结：使用Iterator避免二次迭代

组成：

1，举例

package com.design.q.iterator.standard;

import java.util.Iterator;

@SuppressWarnings("unchecked")
public class ArrayList<E> implements Iterable<E>{

    private E[] array = (E[]) new Object[10];  //集合
    private static final int INCREMENT = 10;   //集合扩展大小
    private int size = 0;                      //集合实时大小

    /**
     * 集合元素添加
     * @param e
     */
    public void add(E e){
        if(size < array.length){
            array[size ++] = e;
        }else{
            //集合大小不足时进行扩容
            E[] copy = (E[]) new Object[array.length + INCREMENT];
            System.arraycopy(array, 0, copy, 0, size);
            copy[size ++] = e;
            array = copy;
        }
    }

    /**
     * 集合数组化
     * @return
     */
    public Object[] toArray(){
        Object[] copy = new Object[size];
        System.arraycopy(array, 0, copy, 0, size);
        return copy;
    }

    /**
     * 返回集合大小
     * @return
     */
    public int size(){
        return size;
    }

    @Override
    public Iterator<E> iterator() {
        return new Ite();
    }

    private class Ite implements Iterator<E>{

        int index = 0;

        @Override
        public boolean hasNext() {
            return index != size();
        }

        @Override
        public E next() {
            return array[index ++];
        }

        public void remove(){

        }

    }

}

LinkedList

package com.design.q.iterator.standard;

import java.util.Iterator;

public class LinkedList<E> implements Iterable<E>{

    private Entry<E> header = new Entry<E>(null, null, null);
    private int size;

    public LinkedList() {
        header.next = header.previous = header;
    }

    /**
     * 集合新增元素
     * @param e
     */
    public void add(E e){
        Entry<E> newEntry = new Entry<E>(e, header, header.next);
        newEntry.previous.next = newEntry;
        newEntry.next.previous = newEntry;
        size ++;
    }

    /**
     * 集合数组化
     * @return
     */
    public Object[] toArray(){
        Object[] result = new Object[size];
        int i = size - 1;
        for(Entry<E> e = header.next; e != header; e = e.next){
            result[i --] = e.value;
        }
        return result;
    }

    /**
     * 返回集合大小
     * @return
     */
    public int size(){
        return size;
    }

    private static class Entry<E>{
        E value;
        Entry<E> previous;
        Entry<E> next;
        public Entry(E value, Entry<E> previous, Entry<E> next){
            super();
            this.value = value;
            this.previous = previous;
            this.next = next;
        }
    }

    @Override
    public Iterator<E> iterator() {
        return new Ite();
    }

    private class Ite implements Iterator<E>{

        Entry<E> index = header;

        @Override
        public boolean hasNext() {
            return index.previous != header;
        }

        @Override
        public E next() {
            E e = index.previous.value;
            index = index.previous;
            return e;
        }

        @Override
        public void remove() {
            // TODO Auto-generated method stub

        }

    }
}

HashSet

package com.design.q.iterator.standard;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class HashSet<E> implements Iterable<E>{

    private static final Object NULL = new Object();
    private Map<E, Object> map = new HashMap<>();

    /**
     * 集合元素新增
     * @param e
     */
    public void add(E e){
        map.put(e, NULL);
    }

    /**
     * 集合数组化
     * @return
     */
    public Object[] toArray(){
        return map.keySet().toArray();
    }

    /**
     * 集合大小
     * @return
     */
    public int size(){
        return map.size();
    }

    @Override
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }
}

2，Main

package com.design.q.iterator.standard;

import java.util.Iterator;

/**
 * 17-迭代器模式
 *
 * 使用Iterator避免二次迭代
 */
public class MainTest {

    public static void main(String[] args) {

        /**
         * ArrayList迭代
         */
        ArrayList<Integer> arrayList = new ArrayList<>();
        for(int i = 0; i < 10; i++){
            arrayList.add(i);
        }
        System.out.println("ArrayList 大小=" + arrayList.size());
        Iterator<Integer> arrayListIterator = arrayList.iterator();
        while(arrayListIterator.hasNext()){
            System.out.print(arrayListIterator.next() + " ");
        }
//        for(Integer i : arrayList){
//            System.out.print(i + " ");
//        }

        /**
         * LinkedList迭代
         */
        LinkedList<Integer> linkedList = new LinkedList<>();
        for(int i = 0; i < 10; i ++){
            linkedList.add(i);
        }
        System.out.println("\nLinkedList 大小=" + linkedList.size());
        Iterator<Integer> linkedListIterator = linkedList.iterator();
        while(linkedListIterator.hasNext()){
            System.out.print(linkedListIterator.next() + " ");
        }
//        for(Integer i : linkedList){
//            System.out.print(i + " ");
//        }

        /**
         * HashSet迭代
         */
        HashSet<Integer> hashSet = new HashSet<>();
        for(int i = 0; i < 10; i ++){
            hashSet.add(i);
        }
        System.out.println("\nHashSet 大小=" + hashSet.size());
        Iterator<Integer> hashSetIterator = hashSet.iterator();
        while(hashSetIterator.hasNext()){
            System.out.print(hashSetIterator.next() + " ");
        }
//        for(Integer i : hashSet){
//            System.out.print(i + " ");
//        }
    }
}

3，Result

ArrayList 大小=10
0 1 2 3 4 5 6 7 8 9
LinkedList 大小=10
0 1 2 3 4 5 6 7 8 9
HashSet 大小=10
0 1 2 3 4 5 6 7 8 9