object test {
  def main(args: Array[String]): Unit = {
    def listTestD = (0 to 20000000).toList
    var l1 = System.currentTimeMillis()
    var sum = 0
    listTestD.foreach{x=> if (x%2==0) sum = x}
    println(System.currentTimeMillis()-l1)
    sum = 0
    l1 = System.currentTimeMillis()
    listTestD.filter(_%2==0).foreach(sum = _)
    println(System.currentTimeMillis()-l1)
  }
}

执行结果是： 17585, 21059 效率差 20%左右

避免缓存，对掉两个 foreach

object test {
  def main(args: Array[String]): Unit = {
    def listTestD = (0 to 20000000).toList
    var l1 = System.currentTimeMillis()
    var sum = 0
    listTestD.filter(_%2==0).foreach(sum = _)
    println(System.currentTimeMillis()-l1)
    sum = 0
    l1 = System.currentTimeMillis()
    listTestD.foreach{x=> if (x%2==0) sum = x}
    println(System.currentTimeMillis()-l1)
  }
}

执行结果是： 35082, 16201 效率差近 50%。

如果 listTestD 存储一个对象的内容，效率是不是会差更大？