can someone explain this behavior of StringBuffer?

Question

public class StrBuffer {

    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer();  //5
        sb.append("hello"); //6
        foo(sb); //7
        System.out.println(sb); //8
    }

    private static void foo(StringBuffer sb) {
        // TODO Auto-generated method stub
        sb.append("wow");    //1
        sb = new StringBuffer();  //2
        sb.append("foo"); //3
        System.out.println(sb); //4     
    }
}

in the above when i print in line8. output is "hellowow" .... can some one explain this please?

Answer 1

At first, you pass a reference to main's sb to foo. In foo, you append "wow" to the referenced object.

Then, in foo, you create a new StringBuffer object, and change the local sb to point to it.

It might be clearer if I rename the reference:

public static void main(String[] args) {
    StringBuffer sb = new StringBuffer();  //5
    sb.append("hello"); //6
    foo(sb); //7
    System.out.println(sb); //8
}

private static void foo(StringBuffer ref) {
    ref.append("wow");    //1
    ref = new StringBuffer();  //2
    ref.append("foo"); //3
    System.out.println(ref); //4     
}

'ref', as passed in to foo, is a just a reference to main's sb. You can operate on ref (ie., sb) in foo until you changed ref to refer to a different object.