基于对象实例进行类型转换的正确方法

Question

I have a very basic question which is more of a design question. Listed below are the classes where User is an abstract class. Only PremiumUser and AdminUser can post messages whereas GuestUser can't post messages (hence there is no postedMessages; property in the GuestUser class).

PROBLEM: I have a main class from where I call the postMessage() ; method but before calling this method I get the User object using another method (whose return type is User ). So, to call the postMessage() method, I will have to typecast the User object into either AdminUser or PremiumUser object. I see three ways to do that:

1. Use the property userType in User class to make if-else comparison and typecast based on that.
2. Use java reflection to get method list for the object and call the postMessage method.
3. Consider using an interface: I considered using an interface which will be implemented by AdminUser and PremiumUser class. Even in that case, I can't type cast the object of type User into the type interface and use it seamlessly to call postMessage();

I was wondering if my design is deficient in some way or it is a common problem and there is a smarter solution for this.

Answer 1

To safe cast you basically use something like:

PremiumUser.class.isAssignableFrom(object);

which also works for objects that inherit from PremiumUser (for example). There are multiple options to avoid this check but it is entirely a matter of design. Some examples:

• Declare the postMessage() method in User class and implement it differently for each type of user.
• Implement Groups, Roles and Rights and check if the particular User has the right to post messages.

Answer 2

Use an interface with the postMessage method in it:

interface HasPostMessage {
    boolean postMessage(String message);
}

Make PremiumUser and AdminUser implement it. Then:

if (user instanceOf HasPostMessage) {
    ((HasPostMessage) user).postMessage("The message");
}