将String转成数字类型, 如果转成成功就返回数字, 转换失败则返回null

    函数 案例 结果
    int(): Int? val i: Int? = “111”.int() 111
    long(): Long? val i: Long? = “111”.long() 111
    double(): Double? val i: Double? = “111.23”.double() 111.23
    float(): Float? val i: Float? = “111.23”.float() 111.23