问为什么要在jdk中使用"offset+length<0“来确定参数是否非法？

EN

/*..........
 *
 * @throws IllegalArgumentException
 *         If <tt>offset</tt> is negative or greater than
 *         <tt>buf.length</tt>, or if <tt>length</tt> is negative, or if
 *         the sum of these two values is negative.
 *
 * @param buf   Input buffer (not copied)
 * @param offset    Offset of the first char to read
 * @param length    Number of chars to read
 */
public CharArrayReader(char buf[], int offset, int length) {
    if ((offset < 0) || (offset > buf.length) || (length < 0) ||
            //$ offset+length
        (**(offset + length) < 0)**) {
        throw new IllegalArgumentException();
    }
    this.buf = buf;
    this.pos = offset;
    this.count = Math.min(offset + length, buf.length);
    this.markedPos = offset;
}