获取通用类型或通用实例的类

Question

I have a method that

• has two type parameters T and S where S is superclass of T ,
• has an instance t of T as argument,
• creates a new instance s of S ,
• fills all fields of s with the content of t .

Here is the code:

public static <T extends S, S> S copyAs(T t, Class<S> sClass) {
    S s = sClass.newInstance();
    for (Field f : getAllFields(sClass)) {
        f.setAccessible(true);
        f.set(s, f.get(t));
    }
    return s;
}

private static Collection<Field> getAllFields(Class<?> cls) {
    Collection<Field> fields = new ArrayList<Field>();
    while (cls != Object.class)
    {
        fields.addAll(Arrays.asList(cls.getDeclaredFields()));
        cls = cls.getSuperclass();
    }
    return fields;
}

This works perfectly.

For convenience - in case I want to copy as the same class - I want a second method with just one argument. That method should internally just call copyAs(myInstance, MyClass.class) if myInstance is of type MyClass .

How do I do this? Is this possible at all?

I have tried two ways which both do not work:

public static <T> T copy(T t) {
    return copyAs(t, T.class);
    // error: .class cannot be used on type parameter
}

and

public static <T> copy(T t) {
    return copyAs(t, t.getClass());
    // error: t.getClass() gives Class<? extends Object>, not Class<T>
}

Answer 1

T.class will never work -> you need to do some more reading about how java generics work.

This will do what you want:

public static <T> T copy(T t) {
    Class<T> tc = (Class<T>) t.getClass();
    return copyAs(t, tc);
}

but it does raise a class cast warning -- there isn't really a way to not do that with the code that you want, feel free to suppress it.