实现自己的ArrayList

最近在学习数据结构和算法，书上有个ArrayList的简单实现，写的很不错。

 package cn.sp.test4;

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

 /**
  * Created by 2YSP on 2017/10/9.
  */
 public class MyArrayList<AnyType> implements Iterable<AnyType> {

     private static  final  int DEFAULT_CAPACITY = 10;

     private int theSize;
     private AnyType[] theItems;

     public MyArrayList(){
         doClear();
     }

     private void doClear() {
         theSize = 0;
         ensureCapacity(DEFAULT_CAPACITY);
     }

     public int size(){
         return theSize;
     }

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

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

     /**
      * 获取角标处的值
      * @param idx
      * @return
      */
     public AnyType get(int idx){
         if (idx < 0 || idx >= size()){
             //角标越界异常
             throw new ArrayIndexOutOfBoundsException();
         }

         return theItems[idx];
     }

     /**
      * 替换目标位置的值，并返回旧值
      * @param idx
      * @param newVal
      * @return
      */
     public AnyType set(int idx , AnyType newVal){
         if (idx < 0 || idx >= size()){
             //角标越界异常
             throw new ArrayIndexOutOfBoundsException();
         }

         AnyType old = theItems[idx];
         theItems[idx] = newVal;
         return old;
     }

     public void ensureCapacity(int newCapacity) {
         if (newCapacity < theSize){
             return;
         }

         AnyType[] old = theItems;
         theItems = (AnyType[]) new Object[newCapacity];
         for(int i = 0 ;i < old.length ; i++){
             theItems[i] = old[i];
         }
     }

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

     public void add(int idx,AnyType x){
         if (theItems.length == size()){
             //如果已满则扩容为之前的两倍
             ensureCapacity(size()*2 + 1);
         }

         for (int i = theSize ; i > idx ; i--){
             theItems[i] = theItems[i-1];//依次后挪
         }

         theItems[idx] = x;
         theSize++;
     }

     public AnyType remove(int idx){
         AnyType removedItem = theItems[idx];
         for(int i = idx ; i <size()-1;i++){//依次前移
             theItems[i] = theItems[i+1];
         }
         theSize--;
         return removedItem;
     }

     @Override
     public Iterator<AnyType> iterator() {
         return new ArrayListIterator();
     }

     private class ArrayListIterator<AnyType> implements java.util.Iterator<AnyType>{

         private int current = 0;

         @Override
         public boolean hasNext() {
             return current < size();
         }

         @Override
         public AnyType next() {
             if (!hasNext()){
                 throw new NoSuchElementException();
             }

             return (AnyType) theItems[ current++ ];
         }

         @Override
         public void remove() {
             MyArrayList.this.remove(current--);
         }
     }
 }

注意理解current++和current--