Why the output of the following code is still "Java" even though objects are passed by reference not value?

Question

String in Java is a non-primitive data type, because it refers to an object. Still answer is not "JavaJava".

class Java 
{ 
    public static void concat(String s1) 
    { 
        s1 = s1 + "Java"; 
    } 

    public static void main(String[] args) 
    { 
        String s1 = "Java"; 
        concat(s1);  
        System.out.println("String: " + s1); 
    } 
}