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

问题描述

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.

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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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