各种类的扩容

public Object[] toArray() {
    // Estimate size of array; be prepared to see more or fewer elements
    Object[] r = new Object[size()];
    Iterator it = iterator();
    for (int i = 0; i < r.length; i++) {
        if (! it.hasNext()) // fewer elements than expected
            return Arrays.copyOf(r, i);
        r[i] = it.next();
    }
    return it.hasNext() ? finishToArray(r, it) : r;
}

private static  T[] finishToArray(T[] r, Iterator it) {
    int i = r.length;
    while (it.hasNext()) {
        int cap = r.length;
        if (i == cap) {
            int newCap = cap + (cap >> 1) + 1;
            // overflow-conscious code
            if (newCap - MAX_ARRAY_SIZE > 0)
                newCap = hugeCapacity(cap + 1);
            r = Arrays.copyOf(r, newCap);
        }
        r[i++] = (T)it.next();
    }
    // trim if overallocated
    return (i == r.length) ? r : Arrays.copyOf(r, i);
}

AbstractCollection抽象类遍历时集合扩大，发生多线程 *** 作的时候扩容为原来的1.5倍+1。这里最大值不是Integer.MAX_VALUE而是MAX_ARRAY_SIZE，因为不同的虚拟机厂商会添加头文件。

// 初始容积为10
private static final int DEFAULT_CAPACITY = 10;

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);
    }
}

ArrayList初始传initialCapacity值多大就创建多大空间。默认空间是10。

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);
}

ArrayList扩容为原来的1.5倍。尽量不要超过MAX_ARRAY_SIZE，每个虚拟机厂商不一样。