(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.