This week's giveaways are in the MongoDB and Jobs Discussion forums. We're giving away four copies of Mongo DB Applied Patterns and 4 resume reviews from Five Year Itch and have the authors/reps on-line! See this thread and this one for details.
That's right. Because Integer and String are in different parts of the inheritance tree - neither is a parent of the other - the compiler doesn't consider one to be more specific than the other. So it doesn't know which version to call, and so you get the error.
But Object is a parent of String, so the String version is more specific than the Object version.
You can get round the ambiguity, if necessary, by using an explicit cast: