空指针异常

import java.io.PrintWriter
//演示一个经典错误：空指针异常
trait FileLogger {
//抽象属性,没有=
  val filename:String
  println("父类", filename)

  //lazy 不立刻求值，而是等到这个变量被使用的时候，去求值
  //lazy val fileout = new PrintWriter(filename)
  lazy val fileout = new PrintWriter(filename)

  //用来把 msg 写入到相应的文件中
  def log(msg: String): Unit = {
    fileout.println(msg)
    fileout.flush()
  }
}

class p4 extends FileLogger {
  val filename = "2024-10-28.txt"
  println("子类",filename)
}

object p5 {
  def main(args: Array[String]): Unit = {
    //val t1 = new p4
    val t1 = new{ val filename="2024-10-29.txt"} with FileLogger //匿名类，提前定义
    t1.log("test!"*10)
  }

}