ArrayList扩容原理

转自：https://www.cnblogs.com/SunArmy/p/9844022.html，有修改

1)List扩容实现步骤

         总的来说就是分两步：1、扩容

                                              把原来的数组复制到另一个内存空间更大的数组中

                                     2、添加元素

                                               把新元素添加到扩容以后的数组中

2)源码分析

   先把ArrayList中定义的一些属性贴出来方便下面源码分析

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

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;
}

分析之前我们先看一下ArrayList的两个构造方法

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

在无参构造中，我们看到了在用无参构造来创建对象的时候其实就是创建了一个空数组，长度为0

在有参构造中，传入的参数是正整数就按照传入的参数来确定创建数组的大小，否则异常

接下来我们来看扩容，扩容的方法就是 add(E e)

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

看，其实add方法就两步，第一步：增加长度，第二步：添加元素到数组，第二步没什么说的，我们看看ensureCapacityInternal(int minCapacity)这个增加长度的方法

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

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

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

这个地方我们看到了，如果在添加的时候远数组是空的，就直接给一个10的长度，否则的话就加一

 if (minCapacity - elementData.length > 0)
            grow(minCapacity);

通过这个地方是真正的增加长度，当需要的长度大于原来数组长度的时候就需要扩容了，相反的则不需要扩容

    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

这个地方注意这一句 

     int newCapacity = oldCapacity + (oldCapacity >> 1);

    oldCapacity >> 1    右移运算符   原来长度的一半 再加上原长度也就是每次扩容是原来的1.5倍

之前的所有都是确定新数组的长度，确定之后就是把老数组copy到新数组中，这样数组的扩容就结束了

以上的一切都是ArrayList扩容的第一步，第二步就没啥说的了，就是把需要添加的元素添加到数组的最后一位