Stream对象的java平均方法。

Question

我收到一个编译错误，说在java中编译以下代码时找不到符号＆＃39; OptionalDouble＆＃39;（我在这里没有提到Pants类的代码）：

import java.util.ArrayList;

public class Application
{
public static void main(String[] args)
{
    ArrayList<Pants> HudsonBayPants = new ArrayList<Pants>();

    HudsonBayPants.add(new Pants("DKNY", Pants.pantType.RegularFit, 120));
    HudsonBayPants.add(new Pants("DKNY", Pants.pantType.SlimFit, 100));
    HudsonBayPants.add(new Pants("CelvinClien", Pants.pantType.StraightFit, 160));
    HudsonBayPants.add(new Pants("CelvinClien", Pants.pantType.SlimFit, 175));
    HudsonBayPants.add(new Pants("Buffalo", Pants.pantType.SlimFit, 80));
    HudsonBayPants.add(new Pants("Buffalo", Pants.pantType.RegularFit, 90));
    HudsonBayPants.add(new Pants("JeanMachine", Pants.pantType.SlimFit, 158));
    HudsonBayPants.add(new Pants("Bulgari", Pants.pantType.RegularFit, 450));

   OptionalDouble avg = HudsonBayPants.stream()
                                      .mapToDouble(s -> s.price)
                                      .average();
   if (avg.isPresent())
   {
       System.out.println("Average pants price is: " + avg.getAsDouble());
   }

    System.out.println("\n");
}
}

任何帮助将不胜感激。