ArrayList扩容机制

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

如上所示，初始化时，如果不带初始容量大小会默认使用空数组；

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);
    }
}

当设置初始值大小时，则会初始化默认容器数组elementData的大小；

private void add(E e, Object[] elementData, int s) {
    if (s == elementData.length)
        elementData = grow();
    elementData[s] = e;
    size = s + 1;
}

add方法最终都会指向`add(E e, Object[] elementData, int s)`方法，该方法中会通过`grow()`方法进行数组的扩容；

private static final int DEFAULT_CAPACITY = 10;

private Object[] grow(int minCapacity) {
    int oldCapacity = elementData.length;
    if (oldCapacity > 0 || elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        int newCapacity = ArraysSupport.newLength(oldCapacity,
                minCapacity - oldCapacity, 
                oldCapacity >> 1           );
        return elementData = Arrays.copyOf(elementData, newCapacity);
    } else {
        return elementData = new Object[Math.max(DEFAULT_CAPACITY, minCapacity)];
    }
}

public static int newLength(int oldLength, int minGrowth, int prefGrowth) {
    // assert oldLength >= 0
    // assert minGrowth > 0

    int newLength = Math.max(minGrowth, prefGrowth) + oldLength;
    if (newLength - MAX_ARRAY_LENGTH <= 0) {
        return newLength;
    }
    return hugeLength(oldLength, minGrowth);
}

由上述的代码可见，当数组扩容时，如果是默认空数组，且所需扩大容量小于默认容量10，则直接创建为大小为10的数组，如果不是默认空数组，则扩容1.5倍或所需扩容的大小；