使用可选时的Java通用类型擦除

Question

Definition in class:

class XXX {
    Map<String, Class> someMap;

    public <K, V>
    Optional<V> get(K key) {
        return Optional.ofNullable(getNullable(key));
    }

    public <K, V>
    V getNullable(K key) {
        //someMap already holds key -> class of V
        String value = DB.fetch(key);

        return gson.fromJson(value, someMap.get(key.toString()));
    }
}

Usage:

//xxx is an instance of XXX
Optional<ClassA> valueOpt = xxx.get(key); //this is fine
ClassA value = valueOpt.get(); //this is fine

//The intermediate valueOpt seems redundant, try to bypass it.
ClassA value = xxx.get(key).get(); //this does not work

My question: in the last line, seems the type info is deducible, why cannot it work? any workaround to make it work in one line?

Workarounds summary:
1) xxx.get(key, ClassA.class)
2) (ClassA)xxx.get(key)
3) xxx.<K,ClassA>get(key)

I still feel those are all workarounds because ClassA value = xxx.getNullable(key); can work.

Answer 1

您可以使用方法类型参数来明确表示：

ClassA value2 = xxx.<String, ClassA>get(key).get();