ArrayList扩容机制分析

    public boolean add(E e) {
        modCount++;
        add(e, elementData, size);
        return true;
    }

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

    private Object[] grow() {
        return grow(size + 1);
    }


    private Object[] grow(int minCapacity) {
        int oldCapacity = elementData.length;
        if (oldCapacity > 0 || elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            int newCapacity = ArraysSupport.newLength(oldCapacity,
                    minCapacity - oldCapacity, /* minimum growth */
                    oldCapacity >> 1           /* preferred growth */);
            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);
    }

    private static int hugeLength(int oldLength, int minGrowth) {
        int minLength = oldLength + minGrowth;
        if (minLength < 0) { // overflow
            throw new OutOfMemoryError("Required array length too large");
        }
        if (minLength <= MAX_ARRAY_LENGTH) {
            return MAX_ARRAY_LENGTH;
        }
        return Integer.MAX_VALUE;
    }