How to handle ClickEvent for handset buttons in Android

My question is how can I use the button on my headphones in my app?

I just want a very simple example that displays a Toast when button is clicked. Also I would appreciate a detailed explanation. Thanks in advance. (I use lollipop 5.1)

There's a simpler way which doesn't require any BroadcastReceiver to be registered if you only want to listen for headset button callbacks while your app (particular activity) is running, simply override Activity onKeyDown method:

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if(keyCode == KeyEvent.KEYCODE_HEADSETHOOK){
        //handle click
        return true;
    }
    return super.onKeyDown(keyCode, event);
}

For more extended implementation use ACTION_MEDIA_BUTTON intent.