Random replacement of letter

[afaq05]

Hi,

can somebody tell me why this doesn't work:

public static void main(String[] args)
{
String s = "0101";
int i = (int)(Math.random()*3);
if(s.charAt(i) == 'a')
{
s.replace(s.charAt(i), 'x');

}
else
{
s.replace(s.charAt(i), 'y');
}

System.out.println(s);
}

I am trying to replace randomly one of the letters in the string. Is there alternative way?

Note: I do not want to change every '0' or '1', but only that one whose index matches random integer (i)

Thanks...