public class MyArrayList<AntType> implements Iterable<AntType> {

     @Override

     public Iterator<AntType> iterator() { //实现接口

         return new MyIterator();

     }

     private class MyIterator implements Iterator<AntType> {

         private int current = 0;

         @Override

         public boolean hasNext() {

             return current < size();

         }

         @Override

         public AntType next() {

             if(!hasNext())

                 throw new NoSuchElementException();

             return theItems[current ++];//迭代器从第一个元素开始迭代,即theItem[0]

         }

         public void remove(){

             MyArrayList.this.remove(--current);

         }

     }

     private static final int DEFAULT_CAPACITY = 10; //设置默认容量

     private int theSize;  //当前大小

     private AntType [] theItems;  //元素对象数组

     public MyArrayList(){ //构造函数，生成空表

         clear();

     }

     public void clear(){ //归为默认(清空)

         theSize = 0;

         ensureCapacity(DEFAULT_CAPACITY);

     }

     public void ensureCapacity(int newCapacity ){ //重置表的容量

         AntType [] oldItems = theItems;

         theItems = (AntType []) new Object[newCapacity];//****重新分配空间 注意使用强制转换的方式进行定义

         for (int i = 0 ; i < theSize; i++){

             theItems[i] = oldItems[i];

         }

     }

     public int size(){ //当前使用大小

         return theSize;

     }

     public boolean isEmpty(){  //判断是否为空

         return theSize == 0;

     }

     public void trimToSize(){  //将表的容量设为当前使用的大小

         ensureCapacity(size());

     }

     public AntType get(int idx){

         //判断是否越界的合法性

         if(idx < 0 || idx >= theSize)

             throw new ArrayIndexOutOfBoundsException();//越界异常

         else

             return theItems[idx];

     }

     //替换元素

     public AntType set(int idx, AntType newVal){

         if( idx < 0 || idx >= theSize)

             throw  new ArrayIndexOutOfBoundsException();

         AntType oldVal = theItems[idx];

         theItems[idx] = newVal;

         return oldVal;

     }

     public boolean add( AntType newVal){//末尾添加元素

         if(theItems.length == size())

             ensureCapacity(size() * 2 + 1);

         set(theSize++ , newVal);

         return true;

     }

     public void add( int idx, AntType newVal){//任意位置添加元素

         if(theItems.length == size())

             ensureCapacity(size() * 2 + 1);

         for(int i = size(); i > idx; i--)

             theItems[i] = theItems[i - 1];

         theItems[idx] = newVal;

         theSize ++;

     }

     public AntType remove(int idx) {

         AntType val = theItems[idx];

         for (int i = idx; i < theSize - 1; i++) {

             theItems[i] = theItems[i + 1];

         }

         theSize--;

         return val;

     }

 }