pass by reference - swapping of objects in java

I have studied that java is pass by reference but when I execute following code the strings are not swapped in main method why?

static void swap(String s1, String s2){
    String temp = s1;
    s1=s2;
    s2=temp;
}

public static void main(String[] args) {
    String s1 = "Hello", s2 = "world";
    swap(s1, s2);

    System.out.println(s1 + s2);
}

Answer

You studied wrong sources. Java is pass by value. Here is one source more you can study. And from here you can find discussion about example similar to yours.