Java中ArrayList的自我实现

   对于ArrayList相比大家都很熟悉，它是java中最常用的集合之一。下面就给出它的自我实现的java代码。

   需要说明的一点是，它是基于数组创建的。所以它在内存中是顺序存储，对于查找十分的方便。

   

package com.pinjia.shop.common.collection;

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

/**
 * Created by wangwei on 2017/1/3.
 * ArrayList的自己封装，实现
 */
public class MyArrayList<E> implements Iterable<E> {

    private static final int DEFAULT_SIZE = 10;//容器默认大小
    private int theSize;
    private E[] theItems;

    public MyArrayList(){
        clear();
    }

    public void clear(){
        theSize = 0;
        ensureCapacity(DEFAULT_SIZE);
    }

    public int size(){
        return theSize;
    }

    public boolean isEmpty(){
        return size() == 0;
    }

    public void trimToSize(){
        ensureCapacity(size());
    }

    public E get(int idx){
        if(idx<0||idx>=size())
            throw new ArrayIndexOutOfBoundsException();
        return theItems[idx];
    }

    public E set(int idx,E newVal){
        if(idx<0||idx>size())
            throw new ArrayIndexOutOfBoundsException();
        E old = theItems[idx];
        theItems[idx] = newVal;
        return old;
    }

    public  void ensureCapacity(int newCapacity) { //数组的扩展
        if(newCapacity<theSize)
            return;
        E[] old = theItems;
        theItems = (E[])new Object[newCapacity];
        for(int i=0;i<size();i++){
            theItems[i] = old[i];
        }
    }

    public boolean add(E x){
        add(size(),x);
        return true;
    }

    public void add(int idx,E x){
        if(theItems.length == size())
            ensureCapacity(size()*2+1);
        for(int i=theSize;i>idx;i--){
            theItems[i] = theItems[i-1];
        }
        theItems[idx] = x;
        theSize++;
    }

    public E remove(int idx){
        E removeItem = theItems[idx];
        for(int i=idx;i<size()-1;i++){
            theItems[i] = theItems[i+1];
        }
        theSize--;
        return removeItem;
    }


    public Iterator<E> iterator() {
        return new ArrayListIterator();
    }

    private class ArrayListIterator implements Iterator<E>{ //内部类
        private int current = 0;
        public boolean hasNext(){
            return current<size();
        }
        public E next(){
            if(!hasNext())
                throw new NoSuchElementException();
            return theItems[current++];
        }
        public void remove(){
            MyArrayList.this.remove(--current);
        }
    }
}