【转】ArrayList其实就那么一回事儿之源码浅析

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

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

    //空数组，当调用无参数构造函数的时候默认给个空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};

    //这才是真正保存数据的数组
    private transient Object[] elementData;

    //arrayList的实际元素数量
    private int size;

    //构造方法传入默认的capacity 设置默认数组大小
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    //无参数构造方法默认为空数组
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

    //构造方法传入一个Collection， 则将Collection里面的值copy到arrayList
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }
    
    //下面主要看看ArrayList 是如何将数组进行动态扩充实现add 和 remove
    

    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }
    
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        //超出了数组可容纳的长度，需要进行动态扩展
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    
    //这才是动态扩展的精髓，看到这个方法，ArrayList瞬间被打回原形
    private void grow(int minCapacity) {
        int oldCapacity = elementData.length;
        //设置新数组的容量扩展为原来数组的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //再判断一下新数组的容量够不够，够了就直接使用这个长度创建新数组， 
        //不够就将数组长度设置为需要的长度
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //判断有没超过最大限制
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //将原来数组的值copy新数组中去， ArrayList的引用指向新数组
        //这儿会新创建数组，如果数据量很大，重复的创建的数组，那么还是会影响效率，
        //因此鼓励在合适的时候通过构造方法指定默认的capaticy大小
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    

}