102.自己实现ArrayList

 package collection;

 import java.util.ArrayList;
 import java.util.List;

 /**
  * 自己实现一个ArrayList，帮助理解底层结构
  * @author Nicholas
  *
  */
 public class SxArrayList{
     private Object[] elementData;
     private int size;

     public int size(){
         return size;
     }
     public SxArrayList(){
         this(10);//默认长度为10

     }

     public SxArrayList(int initialCapacity){
         if(initialCapacity<0){
             try {
                 throw new Exception();
             } catch (Exception e) {
                 e.printStackTrace();
             }
         }
         elementData = new Object[initialCapacity];
     }

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

     public void add(Object obj){
         //数组扩容和数据的拷贝
         ensureCapacity();
         elementData[size++] = obj;
     }

     public Object get(int index){
         rangeCheck(index);
         return elementData[index];
     }

     public void remove(int index){
         rangeCheck(index);
         //删除指定位置的对象
         int numMoved = size-index-1;
         if(numMoved>0){
             System.arraycopy(elementData, index+1, elementData, size, numMoved);;
         }
         elementData[--size] = null;
     }

     public void remove(Object obj){
         for(int i=0;i<size;i++){
             if(get(i).equals(obj)){//调用的是equals不是==
                 remove(obj);
             }
         }
     }

     private void rangeCheck(int index){
         if(size<0||index>=size){
             try {
                 throw new Exception();
             } catch (Exception e) {
                 e.printStackTrace();
             }
         }
     }

     public Object set(int index,Object obj){
         rangeCheck(index);
         Object oldValue = elementData[index];
         elementData[index] = obj;
         return oldValue;
     }

     public void add(int index,Object obj){
         rangeCheck(index);
         ensureCapacity();
         System.arraycopy(elementData, index, elementData, index+1, size-index);
         elementData[index] = obj;
         size++;
     }

     private void ensureCapacity(){
         if(size == elementData.length){
             Object[] newArray = new Object[size*2+1];
             System.arraycopy(elementData, 0, newArray, 0, elementData.length);
             elementData = newArray;
         }
     }

     public static void main(String[] args) {
         SxArrayList list = new  SxArrayList(3);
         list.add("3333");
         list.add("4444");
         list.add("1111");
         list.add("2222");
         System.err.println(list.size);
         System.err.println(list.get(3));
     }

 }