Java集合 - 对元素的多态访问 [英] Java collections -- polymorphic access to elements

问题描述

我有 ThisType 的 LinkedHashSet 值。
ThisType 正在实现 ThatType 界面。

我需要这个集合的多态使用 - 能够指向 LinkedHashSet< ThatType>。

这是怎么做的？

我知道这是一个天真的问题，但我尝试的几件事情没有工作，我想做正确的方式。

提前感谢。

// ==========================

UPDATE：更多详情：

在下面的代码中， ThisType - ThatType 是一个界面。

  // LinkedHashMap< Integer，？ extends ThatType> theMap = new LinkedHashMap<>（）; 
 LinkedHashMap< Integer，ThisType> theMap = new LinkedHashMap<>（）; 
 for（Integer key：intArray）{
 ThisType a = new ThisType（）; 
 if（theMap.containsKey（key））{
 a = theMap.get（key）; 
 a.doStuff（）; 
} else {
 a.doOtherStuff（）; 
} 
 theMap.put（key，a）; 
} 
  

最后，我要返回 theMap 作为 ThatType **，而不是** ThisType 的集合。

这是链断开的地方。使用注释行（第一行）声明是在 put（）& get（）方法的哈希。

不确定这是否重要 - 但是，
我将返回结果 LinkedHashSet< ThatType> 。我正在从 LinkedHashMap 到 LinkedHashSet 的转换。然而，对地图或集合中的集合值的多态引用没有任何作用。目前为止，所有这些操作中使用的所有且唯一的类型是 ThisType 。

解决方案

我认为你想要的是以使用通配符。

  LinkedHashSet< ;? extends ThatType> someFunction（LinkedHashSet< ;? extends ThatType> set）{
 return set; 
} 
  

如其他地方所述 LinkedHashSet< ThisType> / code>不是 LinkedHashSet< ThatType> 的子类，因为 LinkedHashSet< ThisType> 接受对象。



LinkedHashSet中的通配符<扩展ThatType> 意味着 ThatType 的一些类（不确定是什么）的 LinkedHastSet

虽然您可能需要考虑使用：

  ;？ extends ThatType> someFunction（Set< ;? extends ThatType> set）{
 return set; 
} 
  

I have a LinkedHashSet of values of ThisType. ThisType is implementing the interface ThatType.

I need the polymorphic use of this collection-- be able to point to it as LinkedHashSet< ThatType >.

How is this done?

i know this is a naive question, but several things i tried didn't work, and i wanna do it the right way.

Thanks in advance.

//==============================

UPDATE: more details:

In the following code, ThisType implements ThatType-- ThatType is an interface.

//LinkedHashMap<Integer, ? extends ThatType> theMap = new LinkedHashMap<>();
LinkedHashMap<Integer, ThisType> theMap = new LinkedHashMap<>();
for (Integer key:intArray)  {
    ThisType a = new ThisType();
    if (theMap.containsKey(key))  {
        a = theMap.get(key);    
        a.doStuff();
    } else {
        a.doOtherStuff();
    }
    theMap.put(key, a);
}

In the end, i wanna return the theMap as a collection of ThatType **, not **ThisType.

this is where the chain is broken. using the commented line (first line) for declaration is giving type mismatch error on put() & get() methods of the hash.

not sure whether this is significant-- but, i'm returning the result as LinkedHashSet< ThatType >. i'm doing the conversion from LinkedHashMap to LinkedHashSet. However, nothing worked nowhere for the polymorphic reference to collection values in the map or set. All and only type i used in all these operations so far is ThisType. ThatType gave me some error just about anywhere i tried it.

解决方案

I think what you want is to use wildcards.

 LinkedHashSet<? extends ThatType> someFunction(LinkedHashSet<? extends ThatType> set) {
      return set;
 }

As has been explained elsewhere LinkedHashSet<ThisType> is not a subclass of LinkedHashSet<ThatType>, since LinkedHashSet<ThisType> can't accept ThatType objects.

The wildcard in LinkedHashSet<? extends ThatType> means a LinkedHastSet of some class (not sure what) that extends ThatType.

Though you may want to consider using this:

 Set<? extends ThatType> someFunction(Set<? extends ThatType> set) {
      return set;
 }

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