如何将 Class<?> 转换为泛型类型 [关闭]

Question

我有一种方法返回Class<? extends Animal>

public Class<? extends Animal> getAnimal(Animal animal)
{
    //....
}

在参数passGenericAnimal(D animal); 中接受泛型的另一种方法

我想从 getAnimal() 方法中获取 Animal 类类型，我只想通过将其转换为泛型类型来传递给 passGenericAnimal() 方法。

像这样：

public Class<? extends Animal> getAnimal()
{
    return Lion.class;
}

public <T extends Animal> void convertToGenericsAndAssign()
{
    T animal = getAnimal();
    //getting error here
    //Type mismatch: cannot convert from Class<capture#15-of ? extends Animal> to T
    passString(animal);
}

public void passGenericAnimal(D animal)
{
    //....;
}

有什么方法可以将 Class 类型转换为 Generics 类型？

Answer 1

我可以给你一些关于如何编译的提示，但我怀疑你在任何情况下都走错了路。即使您的代码编译（并运行）它也可能不会做您认为它会做的事情。但是，目前，只是编译更改：

public Class<? extends Something> getSomeClass() {
    return SomethingSpecific.class;
}

public void convertToGenericsAndAssign() {
    Class<? extends Something> str = getSomeClass();
    // you are getting a class object. You need to actually store a class to make it work.
    passString(str);
}

public <D extends Something> void passString(Class<D> strType) {
    // ....;
}

public static class Something {}

public static class SomethingSpecific extends Something {}

我将类型从String（不可扩展）更改为Something。

正如您在示例中看到的那样，问题在于，您实际上从您的方法中获得了 Class 对象。我不完全确定您为什么期望实际对象的实例。 如果你想实例化该类的一个对象，那么下面的代码可能会有所帮助：

public Class<? extends Something> getSomeClass() {
    return SomethingSpecific.class;
}

public void convertToGenericsAndAssign() {
    Class<? extends Something> someClass = getSomeClass();
    // try to get an parameter free constructor:
    
    try {
        Constructor<? extends Something> constructor = someClass.getConstructor();
        Something instance = constructor.newInstance();
        
        // Be aware, that the generic type `D` i smore or less just `Something`,
        // because the compiler cannot know at compile time, what subclass of `Something`
        // D should actually be. So it can just induce `Something`.
        passSomething(instance);
        
    } catch (NoSuchMethodException e) {
        // Handle , if the class does not have an empty constructor.
        e.printStackTrace();
        
    } catch (SecurityException e) {
        // Handle, if the constructor is not accessible, or if the
        // VM does not allow reflection.
        e.printStackTrace();

    } catch (InstantiationException e) {
        // Handle, if the instance failed to be created.
        e.printStackTrace();
        
    } catch (IllegalAccessException e) {
        // Handle, if the metohd is e.g. private or something.
        e.printStackTrace();
        
    } catch (IllegalArgumentException e) {
        // This should not be thrown, as you pass no arguments, but
        // you still have to catch it.
        e.printStackTrace();
        
    } catch (InvocationTargetException e) {
        // If the constructor threw an internal exception.
        e.printStackTrace();
    }
}

public <D extends Something> void passSomething(D strType) {
    // ....;
}

public static class Something {}

public static class SomethingSpecific extends Something {}