In some real code, I changed an int that was used to index into an array of ViewerPanels, so that the ViewerPanel was passed around instead of the int. I missed some cases in the first pass, so I had a think about how to make sure I don't do that again.
The cases I missed were PropertyChangeListeners. If you're not familiar with that type, you can use it with a PropertyChangeSupport to listen for changes in values. It has Object in its API, rather than using generics. I don't particularly like the idea of listening for changes, particularly if the listeners then end up interfering with the objects they're listening to, but that's what I have in existing code, so I need to get on with it for now.
The tests didn't pick it up, I found it by coming across a firePropertyChange and wondering what the listening end now looked like. The tests could be better, sure.
Here follows four versions of a simpler code sample. The refactor we want to do in this case is just to go from double to float. Version 2 shows what happens when one forgets to check the PropertyChangeListeners. Version 3 steps back to Version 1 but alters it to use a type-safe version of PropertyChangeListener. Version 4 does the refactor again, but this time the compiler forces the ChangeListener to be altered, because we're no longer based on PropertyChangeListener.
The below code is executable.