StringBuffer parameter passing.

[Suresh Rajadurai]

Hi Gurus,

I have a problem in the following code.
Why, the output produced in "line 3" and "line 4" are not same.
Even though the S1 is getting assigned the value of S2 in "line 2", why , this is not getting passed to "line 4".

I always get confused with this parameter passing. Could you please explain me.

Thanks in advance.

Suresh.

class TestBuffer 
     {
 
        public void myBuf( StringBuffer s1, StringBuffer s2) 
            {
               s1.append(" how are you") ;  // 1
               s1 = s2;  // 2
               System.out.println("Inside myBuf method: " + s1);  //3
            }
 
     public static void main ( String args[] ) 
         {
             TestBuffer tb = new TestBuffer();
              StringBuffer s1 = new StringBuffer("Hello");
              StringBuffer s2 = new StringBuffer("doing");
              tb.myBuf(s1, s2);
              System.out.print(s1); //4
        }
}

[James Tharakan]

First thing to remember is that ,stringBuffer's methods dont create a new object(where as string create a new object when used with "+" operator.).

before the call to the mybuff(),
s1--> hello
s2--> doing
after the call,in the method,
s1 and(s1) --> hello // (s1) is local variable
s2 and (s2)--> doing // (s2) is local varaiable

After append
s1 and(s1) --> hello how are you
s2 and (s2)--> doing


After s1=s2
s1 --> hello how are you
s2 and (s1),(s2)--> doing

When method returns,
s1 --> hello how are you
s2 --> doing


Hope you got it.
And s1 of line 3 is local variable
s1 at line 4 is not same as line 3 s1
[ December 02, 2008: Message edited by: James Tharakan ]

[Suresh Rajadurai]

Hi James

please explain me the following quote.

Thanks you so much. Much appreciated.

After s1=s2
s1 --> hello how are you
s2 and (s1),(s2)--> doing

Suresh

[James Tharakan]

After s1=s2
s1 --> hello how are you
s2 and (s1),(s2)--> doing

Before the assignment s1=s2(these two are local i.e scope is only within the method)local variable (s1) and s1 is refereing "Hello how are you".

After assignment local varible(s1) is refering to doing(s2).
But still s1 of the main is pointing to "Hello how are you".

Hope you got it by now...
If not change the local variable name .It would be clear to understand.

[Bill Shirley]

The object references are passed by-value not by-reference.
Even if they are named the same, they are different variables in different scopes.

The StringBuffer object the local variable is pointing to is mutable, and is changed in the method. So, it is still changed when the method returns, though the reference has gone out of scope.