Well, you can write a loop:

public static void replaceAll(StringBuilder builder, String from, String to) {
    int index = builder.indexOf(from);
    while (index != -1) {
        builder.replace(index, index + from.length(), to);
        index += to.length(); // Move to the end of the replacement
        index = builder.indexOf(from, index);
    }
}

Note that in some cases it may be faster to use lastIndexOf, working from the back. I suspect that's the case if you're replacing a long string with a short one - so when you get to the start, any replacements have less to copy. Anyway, this should give you a starting point.

There is no such method on StringBuffer. Perhaps the following will help:

StringBuffer str1 = new StringBuffer("whatever");
// to get a String result:
String str2 = str1.toString().replaceAll(regex, substr);
// to get a StringBuffer result:
StringBuffer str3 = new StringBuffer(str2);

This is an off-by-one error. The first two parameters to replace(...) are the starting index of the substring to replace, inclusive, and the ending of the substring to replace, exclusive. By using the same value for both, you are in effect "replacing" the nothing between indexes 1 and 2. This inclusive/exclusive way of expressing a range is extremely common.

This might work for you:

StringBuffer s=...
for(char c: passcode.toCharArray()){
   int index=-1;
   while((index=s.indexOf(c))!=-1){
     s.deleteCharAt(index);
   }
}

Well, you can use a regular expression to find the cases where "Milan" isn't followed by "Vasic":

Milan(?! Vasic)

and replace that by the full name:

String.replaceAll("Milan(?! Vasic)", "Milan Vasic")

The (?!...) part is a negative lookahead which ensures that whatever matches isn't followed by the part in parentheses. It doesn't consume any characters in the match itself.

Alternatively, you can simply insert (well, technically replacing a zero-width match) the last name after the first name, unless it's followed by the last name already. This looks similar, but uses a positive lookbehind as well:

(?<=Milan)(?! Vasic)

You can replace this by just " Vasic" (note the space at the start of the string):

String.replaceAll("(?<=Milan)(?! Vasic)", " Vasic")

You can try those things out here for example.