Question

Sometimes I want a simple generic Listener without defining my own class. Is there a predefined “something happened and here is a string or object or whatever” in Java or Android?

Answer 1

A project I’ve worked on had that. It lead to some frightfully unreadable and unmaintainable code once a class implemented the “MyListener” interface to handle two completely different kind of events. There was a lack of separation of concerns, and you had no idea when or how that method might be invoked.

public interface GenericListener {
    public void handleMyEvent(Object sourceObj, int eventCode);
}

//...later on there's some implementation
public void handleMyEvent(Object sourceObj, int eventCode) {
    if ( sourceObj == startDownloadButton && eventCode == MyButton.CLICKED ) {
        //... 20 lines of code to start download
    } else if ( sourceObj instanceOf DownloadStatus && eventCode == DownloadStatus.COMPLETE ) {
        //... 10 lines of code to display status
    } else //... and on and on...
}

This isn’t the kind of code duplication you need to avoid. Just because two methods/interfaces share the same basic signature doesn’t mean they should be combined into one. I suggest you create listener interfaces that are completely self-documenting with regards to when and how they are used.