🚀 go-pugleaf

I just started to learn Java.  I learned that Strings are constant and
their values cannot be changed after they are created. And that
StringBuffers support mutable strings and should be used if the string
needs to be manipulated.

I created the following class which compiles and runs ok.  Therefore, it
seems like I can manipulate a String.  Pls help me understand this. 
Thanks.

class ChangeString {
      public static void main(String args[]) {
 	     String aStr = "string";
             aStr = aStr.toUpperCase();
             System.out.println(aStr);
       }
}