Question

I am new to Java and would like to know a bit more. I have a current problem that I would like an answer to, but I would also like to know what the technique is referred to as so that I can do some further reading.

I currently have something like this:

public class BasicActivityExtension {
    public Boolean basicExtensionMethod() { }
    ...
}

public class MyExtension extends BasicActivityExtension {
    public Boolean myExtensionMethod() { }
    ...
}

Then in a service method I have (ignore the ( I couldnt work out how to get the < to stay in this):

public Class < ? extends BasicActivityExtension> getExtensionByActivity(
                                                     BasicActivity activity,
                                                     ExtensionTypes type)
                                                 throws Exception {
    ...
}

My Question is why does this give an error and what is the name for this technique (when done correctly!)

MyExtension members = (MyExtension) activityService.getExtensionByActivity(activity,
                                        ExtensionTypes.member);

The error is
Cannot cast from Class to MyExtension;

Also if I have an object Class< ? extends BasicActivityExtension> how can I just call the generic methods in BasicActivityExtension without caring what class it is?

Answer 1

It appears you want

<T extends BasicActivityExtension>
public T getExtensionByActivity(BasicActivity activity, ExtensionTypes type) {
....
}

When you specify Class it has to be a class object, not an instance of a class.

BTW: I would avoid blindly throwing Exception There are better ways of handling checked exceptions.