JDK源码阅读笔记--ByteArrayOutputStream

 private void ensureCapacity(int minCapacity) {
        // overflow-conscious code
        if (minCapacity - buf.length > 0)
            grow(minCapacity);
    }
private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = buf.length;
        int newCapacity = oldCapacity << 1;
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        buf = Arrays.copyOf(buf, newCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

扩容判断大小的时候，先左移1位再判断。如果原容量比较小，左移后比minCapacity小，则直接等于minCapacity。如果比较大，大到左移后符号位变为1，这样比较结果为任为负数，仍然赋值为minCapacity。再跟最大值比较，这样可以处理溢出的问题。