Passing a String by Reference in Java?

I am used to doing the following in C:

void main() {
    String zText = "";
    fillString(zText);
    printf(zText);
}


void fillString(String zText) {
    zText += "foo";
}

And the output is:

foo

However, in Java, this does not seem to work. I assume because the String object is copied instead of passed by referenced. I thought Strings were objects, which are always passed by reference.

What is going on here?