gistfile1.txt

// ===== USAGE
    private void fetchevent() {
        eventService.getEvent(id, new MethodCallbackImpl<Event>() {
            @Override
            public String getErrorMessage() {
                return "Could not fetch event " + id;
            }

            @Override
            public void onSuccess(Event event) {
                displayEvent(event);
            }
        });
    }

// ===== THE ABSTRACT CLASS

package com.sceneverse.shozon.client.util;

import org.fusesource.restygwt.client.Method;
import org.fusesource.restygwt.client.MethodCallback;

import com.google.gwt.user.client.Window;

public abstract class MethodCallbackImpl<T> implements MethodCallback<T> {
    @Override
    public void onSuccess(Method method, T result) {
        onSuccess(result);
    }

    @Override
    public void onFailure(Method method, Throwable throwable) {
        String message = getErrorMessage();

        Window.alert(message);
    }

    public abstract String getErrorMessage();

    public abstract void onSuccess(T result);
}