手写ArrayList

　　ArrayList是Java中最为简单也是最为常用的容器之一，那么它的内部构造是什么呢？其实ArrayList的底层实现就是数组，它的各种方法就是对数组进行的一系列操作，例如对数组元素的修改，删除，位移，对数组长度的获取等。我们可以手写一个ArrayList容器，以助我们更好的理解这个容器：

package ArraryList;

import java.util.Arrays;
import java.util.Iterator;

/**
 * 深入迭代器的原理-->一个容器可以创建多个迭代器对象
 * 1.使用了内部类,实现迭代器
 * 2.使用Iterable实现foreach迭代
 * @author XieHuazhen
 *
 */
public class MyArrayList<E> implements Iterable<E>{  //迭代器接口
    
    //数组存储值
    private Object[] elementData;
    //大小
    private int size;
    
    public MyArrayList(int initialCapacity) {
        if(initialCapacity>=0) {
            elementData = new Object[initialCapacity];
        }else {
            try {
                throw new MyException();
            }catch(MyException e) {
                e.printInit();
            }
        }
    }
    
    public MyArrayList() {
        this(0);
    }
    
    @SuppressWarnings("unchecked")
    private E elementData(int index) {
        return (E)elementData[index];
    }
    
    private void ensureCapacity() {
        if(size>=elementData.length) {
            elementData = Arrays.copyOf(elementData,size*2+1);
        }
    }
    
    private void rangeCheck(int index) {
        if(index<0||index>=size) {
            try {
                throw new MyException();
            } catch (MyException e) {
                e.printIndex();
            }
        }
    }
    
    public int size() {
        return size;
    }
    
    public boolean isEmpty() {
        return size==0;
    }
    
    public boolean contains(Object o) {
        return indexOf(o)>=0;
    }
    
    public int indexOf(Object o) {
        for(int i=0;i<size;++i) {
            if(elementData[i].equals(o))
                return i;
        }
        return -1;
    }
    
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

    public E set(int index,E element) {
        rangeCheck(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    
    public void add(E element) {
        ensureCapacity();
        elementData[size++] = element;
    }
    
    public void add(int index,E element) {
        rangeCheck(index);
        ensureCapacity();
        System.arraycopy(elementData,index,elementData,index+1,size-index);
        elementData[index] = element;
        ++size;
    }
    
    public E remove(int index) {
        rangeCheck(index);
        E oldValue = elementData(index);
        System.arraycopy(elementData,index+1,elementData,index,size-index-1);
        elementData[--size] = null;
        return oldValue;
    }
    
    public boolean remove(E element) {
        for(int i=0;i<size;++i) {
            if(elementData[i].equals(element)) {
                remove(i);
                return true;
            }
        }
        return false;
    }
    
    //实现Iterable<E>接口的方法
    @Override
    public Iterator<E> iterator() {
        return new MyIter();
    }
    
    //内部类
    private class MyIter implements Iterator<E>{
        //计数器-->指针 游标
        private int cursor;
        private int lastRet = -1;
        
        //判断是否存在下一个
        @Override
        public boolean hasNext() {
            return cursor!=size;
        }
        
        //返回游标当前位置，并把游标移到下一位置
        @SuppressWarnings("unchecked")
        @Override
        public E next() {
            if(!hasNext()) {
                try {
                    throw new MyException();
                } catch (MyException e) {
                    e.printnext();
                }
            }
            lastRet = cursor;
            return (E)elementData[cursor++];
        }    
        
        //删除游标左面元素，执行完next后只能执行一次
        @Override
        public void remove() {
            if(lastRet<0) {
                try {
                    throw new MyException();
                } catch (MyException e) {
                    e.printremove();
                }
            }
            System.arraycopy(elementData,lastRet+1,elementData,lastRet,size-lastRet-1);
            cursor = lastRet;
            lastRet = -1;
            elementData[--size] = null;
        }    
    }
    
    public static void main(String[] args) {
        MyArrayList<String> list = new MyArrayList<String>();
        list.add("aaa");
        list.add("bbb");
        list.add("ccc");
        list.add(2,"123");
        list.remove("ccc");
        //使用迭代器遍历
        for(Iterator<String> itr=list.iterator();itr.hasNext();) {
            System.out.println(itr.next());
            //itr.remove();
        }
        
        System.out.println("---------------------");
        
        //使用增强for循环遍历
        for(String str:list){
            System.out.println(str);
        }
    }
}

package ArraryList;

public class MyException extends Exception{
    
    private static final long serialVersionUID = 1L;

    public MyException() {
        System.out.print("出现异常:");
    }
    
    public void printIndex() {
        System.out.println("索引范围越界!");
        System.exit(1);
    }
    
    public void printInit(){
        System.out.println("请重新初始化");
    }
    
    public void printnext() {
        System.out.println("游标越界!");
        System.exit(1);
    }
    
    public void printremove() {
        System.out.println("remove()只有在执行完next()后才能且只能执行一次!");
        System.exit(1);
    }
}