GWT's Activities & Places API and the visitor pattern (http://klimer.eu/2014/09/22/activities-places-and-the-visitor-pattern/)

AppActivityMapper1.java

public class AppActivityMapper implements ActivityMapper {
    @Override
    public Activity getActivity(Place place) {
        activity = null;
        if (place instanceof StartPlace) {
            return new StartActivity();
        } else if (place instanceof SearchPlace) {
            return new SearchActivity();
        } else if (place instanceof CustomerPlace) {
            return new CustomerActivity();
        }
        return null;
    }
}

AppActivityMapper2.java

public class AppActivityMapper implements ActivityMapper, PlaceVisitor {
    private Activity activity;

    @Override
    public Activity getActivity(Place place) {
        activity = null;
        if (place instanceof AppPlace) {
            ((AppPlace) place).accept(this);
        }
        return activity;
    }

    @Override
    public void visitStartPlace(StartPlace place) {
        activity = injector.getStartActivity();
    }
    
    @Override
    public void visitSearchPlace(SearchPlace place) {
        activity = injector.getSearchActivity();
    }
    
    @Override
    public void visitCustomerPlace(CustomerPlace place) {
        activity = injector.getCustomerActivity();
    }
}

AppPlace.java

public abstract class AppPlace extends Place {
    public abstract void accept(PlaceVisitor visitor);
}

PlaceChangeHandler.java

public class RootPresenter ... implements PlaceVisitor {

    public RootPresenter(...) {
        eventBus.addHandler(PlaceChangeEvent.TYPE, new PlaceChangeEvent.Handler() {
            @Override
            public void onPlaceChange(PlaceChangeEvent event) {
                Place place = event.getNewPlace();
                if (place instanceof AppPlace) {
                    ((AppPlace) place).accept(RootPresenter.this);
                }
            }
        });
    }
    
    @Override
    public void visitStartPlace(StartPlace place) {
        view.getStartButton.setActive(true);
        // Or whatever else
    }
    
    // etc...
}

Places.java

public class StartPlace extends AppPlace {
    @Override
    public void accept(PlaceVisitor visitor) {
        visitor.visitStartPlace(this);
    }
}

public class SearchPlace extends AppPlace {
    @Override
    public void accept(PlaceVisitor visitor) {
        visitor.visitSearchPlace(this);
    }
}

public class CustomerPlace extends AppPlace {
    @Override
    public void accept(PlaceVisitor visitor) {
        visitor.visitCustomerPlace(this);
    }
}

PlaceVisitor.java

public interface PlaceVisitor {
    void visitStartPlace(StartPlace place);
    void visitSearchPlace(SearchPlace place);
    void visitCustomerPlace(CustomerPlace place);
}

I implemented this design and it was a great improvement. The only problem is that you had to cast to AppPlace. Casting should be avoided but this is pretty insignificant. Anyway, the way I see it, the whole reason that it's the AppActivityMappers 1 and 2 have to go through those acrobatics and downcast is because the Place was up-casted. When the place was originally created(Places are usually created in the Tokenizer) it was in it's downcast form. So a simple way of implementing this without any down casting is to go get the place in the Tokenizer.