StringBuffer question

[Serena Zhou]

Could anyone explain why this code from Dan Chisholm mock exam have the output of "ABABCABC"?

  
class MWC204 {
  static void m1(StringBuffer s1) {
    s1 = s1.append("B"); System.out.print(s1);
  }
  static void m2(StringBuffer s1) {
    s1.append("C"); System.out.print(s1);
  }
  public static void main(String[] s) {
    StringBuffer s1 = new StringBuffer("A");
    m1(s1); m2(s1);
    System.out.print(s1);
}}

When s1 was passed to m1, shouldn't another copy of s1 be created and changed which does not affect s1 itself?

[Nathaniel Stoddard]

Serena,
Java does always pass by value, making a copy and then passing it. But also remember that in Java, references to Java is what is being copied. So, the reference is copied and pass. The copy still refers to the same StringBuffer though, so both m1 and m2 alter the same StringBuffer.

[Serena Zhou]

Yeah, Thanks!
String is immutable and StringBuffer is not.