I'm trying to quickly learn GWT as part of a new project. I found out that GWT doesn't implement Java's String.format() function, so there's no printf()-like functionality. I knew that some printf() implementations exist for Javascript, so I figured I could paste one of those in as a GWT Javascript Native Interface function. I ran into problems, and decided I'd better make sure that the varargs values were being passed in correctly. That's where things got ugly. First, some example code:

// From Java, call the JSNI function:
test("sourceString", "params1", "params2", "params3");

....

public static native void test(Object... params) /*-{   
    // PROBLEM: this kills GWT!
    // alert(params.length);  

    // returns "function"
    alert(typeof(params));      

    // returns "[Ljava.lang.Object;@b97ff1"
    alert(params);
}-*/;

The GWT docs state that "calling a varargs JavaScript method from Java will result in the callee receiving the arguments in an array". I figured that meant I could at least check params.length, but accessing that throws a JavascriptException wrapped in an UmbrellaException, with no real information. When I do "typeof(params)", it returns "function". As if that weren't odd enough, if I check the string value of params, it returns what appears to be a string version of a Java reference.

So, I guess I'm asking a few different questions here:

1) How do GWT/JSNI varargs actually work, and do I need to do something special to pass in values?
2) What is actually going on here?
3) Is there any easier way to get printf()-style formatting in a GWT application?