通用键/值与相关类型的通用映射 [英] Generic Map of Generic key/values with related types

问题描述

我正在尝试创建一个通用类型，保留自己的版本的地图，这些版本是为以后使用而创建的。实际上，它是一个单例模式，其中每个类型有一个实例。我到目前为止的代码是：

I'm trying to create a generic type that keeps a map of the versions of itself that have been created for later use. Effectively, it's an singleton pattern where there's one instance per type. The code I have so far is:

public class FieldBinder<T> {
    static final Map<Class<? extends Object>,FieldBinder<? extends Object>> instanceMap = 
        new HashMap<Class<? extends Object>,FieldBinder<? extends Object>>();

    private FieldBinder() {}

    synchronized public static <V extends Object> FieldBinder<V> getInstance(Class<V> klass) {
        if(!instanceMap.containsKey(klass)) {
            instanceMap.put(klass, new FieldBinder<V>());
        }
        return (FieldBinder<V>)instanceMap.get(klass);
    }
}

但我还是不确定做正确。感觉像我应该能够指定集合是（Class - > FieldBinder）。事实上，IDE正在警告返回语句只会加强这个想法。

However, I'm still unsure that I'm "doing it right". It feels like I should be able to specify that the collection is (Class -> FieldBinder). The fact that the IDE is warning about the return statement only reinforces this thought.

有更好的方法来处理这个问题吗？

Is there a better way to handle this?

注意：这个问题似乎非常密切相关，但只是远远不够，我不知道如何应用其中的信息到我自己的问题。

Note: This question seems very closely related, but just far enough away that I can't figure out how to apply the information in it to my own problem.

推荐答案

您的实现是正确的。没有更好的做法（如果在代码中有这样的事情是更好的，这是另一个问题..）

Your implementation is correct. There's no "better" way of doing it (if there is such a thing is "better" in code, which is another issue..)

小修订：

• < V extends Object> 等效于 / code>这是较不冗长的


• 类< ;? extends Object> 等效于类<？> ，这是较少冗长的


• @SuppressWarnings（unchecked）注释告诉编译器该转换是安全的

• <V extends Object> is equivalent to V which is less verbose
• Class<? extends Object> is equivalent to Class<?> which is less verbose
• You can use the @SuppressWarnings("unchecked") annotation to tell your compiler that the cast is safe

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