Using Operators and Decision Constructs: reference and local variables

Question

N.B. What I don't understand about the answer to the Enthuware.com answer below is that sb.append(str) seems to result in a change to sb of main but sb = null does not.

What will be the output of the following class?

 public class TestClass{
  public void testRefs(String str, StringBuilder sb){
    str = str + sb.toString();
    sb.append(str);
    str = null;
    sb = null;
  }
  public static void main(String[] args){
    String s = "aaa";
    StringBuilder sb = new StringBuilder("bbb");
    new TestClass().testRefs(s, sb);
    System.out.println("s="+s+" sb="+sb);
  }
}

s=aaa sb=bbb

s=null sb=null

s=aaa sb=null

s=null sb=bbbaaa

ANSWER is: s=aaa sb=bbbaaabbb

Always remember that Strings are immutable, you cannot change them. In this case, s refers to "aaa", and no matter what testRefs() method does, the variable s of main() will keep pointing to the same string "aaa". StringBuilder on the other hand, is mutable. So, initially sb is pointing to a StringBuilder object containing "bbb". Its reference is passed to the testRefs() method. In that method, we change the local variable str to point to a new string "aaa"+"bbb" = "aaabbb". Then we append this to sb. Therefore sb now contains "bbbaaabbb". Setting the local reference str and sb (in method testRefs()) to null, does not affect the variables s and sb of the main() method.