Designing a flexible API with support for Callbacks

I am writing a Java library that needs to make http GET && POST requests. There will be two types of users of this library:

1. Those with understanding of callbacks and know how to use them.
2. Those that....don't.

So please consider the following two methods:

public void methodWithCallback(ServiceCallback callback) {
    try {
        // GET && POST code that cannot really be abstracted to a separate method
    } catch (IOException e) {
        callback.onError(e);
        callback = null;
    }
    callback.onSuccess(response);
    callback = null;
}

public Response methodWithoutCallback() throws IOException {

    try {
        // again, GET && POST code that cannot really be abstracted to a separate method
    } catch (IOException e) {
        logger.log(e);
        throw e;
    }
    return response;
}

Above code seems dirty to me, mainly because it has duplication of code to send the request. Also if I were writing a lot of such kind of methods, my code will very soon double up to hardly maintainable spaghetti.

I considered allowing the client to set a callback by providing a method like setCallback(Callback callback), and then have one general method that notifies the callback if it is NOT null. But this also seems dirty because I would be then writing a lot of if null checks!

What are your thoughts on it? What would be the desirable design pattern here that you would recommend?