为什么某个方法不能使用Collection< subClass>当方法的签名被定义为Collection< class> [英] Why can't a method take a Collection<subClass> when the method's signature is defined as Collection<class>

问题描述

我有一个方法，它接受一个SResource对象的列表。

I have a method that takes a list of SResource objects

public static List<STriple> listTriples(List<SResource> subjects){
//... do stuff
}

$ b b

为什么我不能这样做

Why can't I do this

List<IndexResource> resultsAsList = new ArrayList<IndexResource>();
    resultsAsList.addAll(allResults.keySet()); // I could possible not use lists and just use sets and therefore get rid of this line, but that is a different issue
List<STriple> triples = new ArrayList<STriple>();
    triples = TriplesDao.listTriples(resultsAsList);

（编译器告诉我我必须使三元组使用SResource对象。）

(The compiler tells me I have to make triples use SResource objects.)

当IndexResource是SResource的子类时

When IndexResource is a subclass of SResource

public class IndexResource extends SResource{ 
// .... class code here
}

我本以为这是可能的，所以也许我做错别的东西。我可以发布更多的代码，如果你建议它。

I would have thought this has to be possible, so maybe I am doing something else wrong. I can post more code if you suggest it.

推荐答案

您可以使用：

You can do it, using wildcards:

public static List<STriple> listTriples(List<? extends SResource> subjects){
    //... do stuff
}

b $ b

新声明使用一个有界通配符，其中通用参数将是 SResource

The new declaration uses a bounded wildcard, which says that the generic parameter will be either an SResource, or a type that extends it.

以这种方式接受列表<> 包括插入主题。如果你只是从方法中的 subject 中读取，那么这个更改应该会得到你想要的结果。

In exchange for accepting the List<> this way, "do stuff" can't include inserting into subjects. If you're just reading from the subjects in the method, then this change should get you the results you want.

EDIT ：要查看为什么需要通配符，请考虑这是（Java中的非法）代码：

EDIT: To see why wildcards are needed, consider this (illegal in Java) code:

List<String> strings = new ArrayList<String>();
List<Object> objList = string; // Not actually legal, even though string "is an" object
objList.add(new Integer(3)); // Oh no! We've put an Integer into an ArrayList<String>!

这显然不是类型安全。但是，使用wilcards，您可以执行此操作：

That's obviously not typesafe. With wilcards, though, you can do this:

List<String> strings = new ArrayList<String>();
string.add("Hello");
List<? extends Object> objList = strings; // Works!
objList.add(new Integer(3)); // Compile-time error due to the wildcard restriction

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