ArrayList扩容机制

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在这之前首先看一下ArrayList的主要参数

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;

ArrayList常用的构造方法包括两个，一个是无参构造一个是无参构造，其中有参构造传值为ArrayList中Object数据的初始化长度

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;
}

从源码中我们可以看出，无参构造其实就是创建了一个空数组，而有参构造则是创建一个指定长度的数据，分为三种情况

– 当initialCapacity>0时候创建一个长度为initialCapacity的Object数组

– 当initialCapacity=0时创建一个空数组

– 当initialCapacity<0时就将会跑出一个参数异常

实际的ArrayList是在添加元素中进行的，包括add和addall方法，这里以add方法为例

/**
 * 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;
}
/**
 * Appends all of the elements in the specified collection to the end of
 * this list, in the order that they are returned by the
 * specified collection's Iterator.  The behavior of this operation is
 * undefined if the specified collection is modified while the operation
 * is in progress.  (This implies that the behavior of this call is
 * undefined if the specified collection is this list, and this
 * list is nonempty.)
 *
 * @param c collection containing elements to be added to this list
 * @return <tt>true</tt> if this list changed as a result of the call
 * @throws NullPointerException if the specified collection is null
 */
public boolean addAll(Collection<? extends E> c) {
    Object[] a = c.toArray();
    int numNew = a.length;
    ensureCapacityInternal(size + numNew);  // Increments modCount
    System.arraycopy(a, 0, elementData, size, numNew);
    size += numNew;
    return numNew != 0;
}

当我们去添加一个数据的时候ArrayList首先会对数组大小进行一个校验，add方法中将期望的最小长度设定为size+1，而addAll方法中则设定为size+numNew

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
}
private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

在这里首先会对当前数组进行一个判断，如果不是默认的空数组那么就直接返回期望的最小容量，否则就会对数组长度设置一个初始值，这里还有两种情况，如果是调用的add方法那么期望的最小长度肯定是1，这时候就会将数组minCapacity初始化为10，如果是addAll方法那么这个初始值可能就会超过10，这时候机会使用minCapacity作为数组的初始长度。

之后再去判断初始化的最小期望容量是否大于当前数组的长度，如果是那么就进行扩容

/**
 * Increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
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);
}

这里就是进行扩容的核心代码。

这里首先会定义一个新数组大小，约定于原大小的1.5倍，其实就是原大小除以2，然后判断新容量是否小于最小期望容量，然后判断新容量是否大于MAX_ARRAY_SIZE (2^31 -8)，如果是那么就是取值为Integer.MAx(2^31),最后将一个旧数组复制的内容复制到新的数组中