集合中的接口列表

Question

我有一个hashmap，其中键是UpgradePartType，而Value是一个    UpgradeParts列表。当EnginePart实现UpgradePart时，为什么我不能在Enginemap中放入EnginePart列表？

如果我把一个值为IUpgradePart的散列图当我放入EnginePart

时，它会起作用

      private HashMap<UpgradePartType, ArrayList<IUpgradePart>> purchasedUpgrades;
       ...
      purchasedUpgrades  = new HashMap<UpgradePartType,ArrayList<IUpgradePart>>();
      purchasedUpgrades.put(UpgradePartType.Engine,new ArrayList<EnginePart>());

      ...        
      public enum EnginePart implements IUpgradePart{
      Stock,PortPolish,Bore,Tune
      }

      ...

拥有空白界面也是正常的吗？我觉得这里应该有什么东西

      public interface IUpgradePart{

      }