object Test {  def main(args: Array[String]): Unit = {    val a1 = new tt  //实例化对象    println(a1.a)    a1.f(2)    println(a1.a)    println(a1)  }}//定义一个类叫ttclass tt{  var a = 0 //成员变量a  def f(b:Int):Int = {    a=a+b    return a  }  override def toString = s"tt($a)" //toString方法}