ArrayList数据结构的实现

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

public class MyArrayList<T> implements Iterable<T> {
    //默认数组大小
     private static final int DEFAULT_CAPACITY=10;
     //表大小
     private int theSize;
     //数组存储
     private T[] theItems;
     //初始化表
     public MyArrayList(){
         doClear();
     }
    private void doClear() {
        // TODO Auto-generated method stub
        theSize=0;
        ensureCapacity(DEFAULT_CAPACITY);
    }
    //清空表
    public void Clear(){
        doClear();
    }
    //判断是否为空
    public boolean isEmpty(){
        return size()==0;
    }
    //将此 ArrayList 实例的容量调整为列表的当前大小
    public void trimToSize(){
        ensureCapacity(size());
    }

    public T get(int index){
        if(index<0||index>size()){
            throw new ArrayIndexOutOfBoundsException();
        }
        return theItems[index];
    }
    public T set(int index,T val){
        if(index<0||index>size()){
            throw new ArrayIndexOutOfBoundsException();
        }
        T old=theItems[index];
        theItems[index]=val;
        return old;
    }
    public boolean add(T val){
        add(size(),val);
        return true;
    }
    public  void add(int index, T val) {
        // TODO Auto-generated method stub
        if(theItems.length==size()){
            ensureCapacity(size()*2+1);
        }
        for(int i=theSize;i>index;i--){
            theItems[i]=theItems[i-1];
        }
        theItems[index]=val;
        theSize++;
    }
    public T remove(int index){
        T removedItem=theItems[index];
        for(int i=index;i<size()-1;i++){
            theItems[i]=theItems[i+1];
        }
        theSize--;
        return removedItem;
    }
    public java.util.Iterator<T> iterator(){
        return  new ArrayListIterator();
    }
    private class ArrayListIterator implements java.util.Iterator<T>{
        private int current=0;
        public boolean hasNext(){
            return current<size();
        }
        public T next(){
            if(!hasNext()){
                throw new NoSuchElementException();
            }
            return theItems[current++];
        }
        public void remove(){
            MyArrayList.this.remove(--current);
        }
    }     
    private void ensureCapacity(int newCapacity) {
        // TODO Auto-generated method stub
        if(newCapacity<theSize){
            return;
        }
        T[] old=theItems;
        theItems=(T[]) new Object[newCapacity];
        for(int i=0;i<size();i++){
            theItems[i]=old[i];
        }
    }
    private int size() {
        // TODO Auto-generated method stub
        return theSize;
    }
    

}