as.java

 @RequestMapping("/authorize")
    public String authorize(@RequestParam(value="username", defaultValue="username") String username,
                              @RequestParam(value="password", defaultValue="password") String password) {


        FirebaseHandler firebaseHandler = new FirebaseHandler(mFirebaseDatabaseReference, new FirebaseHandler.FirebaseServiceListener() {
            @Override
            public void onSuccessResponseReceived(Object response) {
                return ; // << error
            }

            @Override
            public void onErrorResponseReceived(Object error) {

            }

        },  username, password);
        firebaseHandler.submitRequest();

        return "response";
    }