Java反射/泛型

Question

public static <T> T inCache(T obj) throws ClassNotFoundException {
        String[] token = validateCookie(); //gives me to strings
        if (token == null)
            return null;
        if (Cache.get(token[0]) != null) {
            if (Cache.get(token[0]).getClass() == Class.forName(token[1])
                    && obj.getClass() == Cache.get(token[0]).getClass()) {
                T test = (T) Cache.get(token[0]);
                return test;
            }
        }
        return null;
    }

The code above is completely wrong.

Basicly I want to do something like this:

• I want to set the class in my function. for example inCache<User>();

• check if the object that i get out of my cache has the same class that i have specified before. (obj.getClass == User.class)

• If the classes matches , cast the object to the class and return it. return (User)obj

I want to use it like this.

User user = inCache<User>();

Answer 1

As for the class signature, why don't you use something like this:

public static <T> T inCache(Class<T> clazz) throws ClassNotFoundException {
  ...
}

And then call it like this:

User user = inCache(User.class);

Generics can't be used the way you described ( User user = inCache<User>(); ) due to type erasure at runtime, ie the type of T is unknown at runtime in that case.

Also note that it might be better to test using Class#isAssignableFrom(...) to be able to check for subclasses as well, eg clazz.isAssignableFrom(Cache.get(token[0]).getClass()) . That way you could pass an interface or super class and still get a match if the object is of a subtype.