(from JavaDesignFlaws)
Adding MultipleDispatch to Java would necessitate massive changes to the core libraries and a completely new approach to the design of Java programs. I think you're asking for a whole new language! Perhaps you'd be better off programming in the DylanLanguage.
Why would it? It would require changes if MultipleDispatch was automatic, but it ain't necessarily so. In CommonLisp, for example, you only get MultipleDispatch if you ask for it, by making a generic function. In much the same way, you could apply the same rule in Java.
A compatible extension of Java with MultipleDispatch is the NiceLanguage.
See MultipleDispatch
MultipleDispatch can lead to ambiguity if the methods in question take more than one argument; this ambiguity could be resolved with a set of rules for which one would take precedence, but overall, I prefer NoMultipleDispatchInJava.