java - 如何将泛型参数传递给方法？

Question

我正在尝试存储作为抽象类的子类的对象，但这里有一个问题:

abstract class Letter {
    public void store_object(Letter letter) {
        HashMap<Class<? extends Letter>, ? extends Letter> map = new HashMap<>();
        map.put(letter.getClass(), letter); <-- this line throws an error
    }
}
class A extends Letter {}
class B extends Letter {}

map.put 抛出错误的原因是 letter 不是 Letter 的子类(它是信件).

有没有办法将方法参数更改为 Letter 的通用子类？像这样的东西:

public void store_object(? extends Letter letter) {

或

public void store_object(Object<? extends Letter> letter) {

错误是:

no suitable method found for put(Class<CAP#1>,NewClass19.Letter)
    method Map.put(Class<? extends NewClass19.Letter>,CAP#2) is not applicable
      (argument mismatch; NewClass19.Letter cannot be converted to CAP#2)
    method AbstractMap.put(Class<? extends NewClass19.Letter>,CAP#2) is not applicable
      (argument mismatch; NewClass19.Letter cannot be converted to CAP#2)
    method HashMap.put(Class<? extends NewClass19.Letter>,CAP#2) is not applicable
      (argument mismatch; NewClass19.Letter cannot be converted to CAP#2)
  where CAP#1,CAP#2 are fresh type-variables:
    CAP#1 extends NewClass19.Letter from capture of ? extends NewClass19.Letter
    CAP#2 extends NewClass19.Letter from capture of ? extends NewClass19.Letter

Answer 1

我真的不确定你想在这里做什么，但这将使你的代码编译:

abstract class Letter {
    public <T extends Letter> void store_object(T letter) {
        HashMap<Class<? extends Letter>, T> map = new HashMap<>();
        map.put(letter.getClass(), letter); // <-- this line throws an error
    }
}
class A extends Letter {}
class B extends Letter {}

为什么要生成一个新的 HashMap每次 store_object方法被调用？你真正的问题是什么？请阅读What is the XY problem?

相关:Canonical Java Generics FAQ

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顺便说一下，Object<Anything>总是错的，因为 Object未参数化。

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所以我看到了您的编辑，仅供引用:

private static HashMap<Class<? extends Test>, Test> map = new HashMap<>();

static void addTest(Test test) {
    map.put(test.getClass(), test);
}

你的问题到底是什么？