Replace all letters of a string minus the first and the last in Java

Question

I have a string, for example this one:

stackoverflow

And I would like to get the following output in Java (Android):

s***********w

So we keep the first and the last letters but the other ones are replaced by this sign "*". I want the length of the string to be the same from the input to the output.

Here is the code of the function I have now.

String transform_username(String username)
{
    // Get the length of the username
    int username_length = username.length();

    String first_letter = username.substring(0, 1);
    String last_letter = username.substring(username_length - 1);

    String new_string = "";

    return new_string;
}

I am able to get the first and the last letter, but I don't really know how to put the "*" in the middle of the word. I know I could loop over it, but it's obviously not a good solution.

Answer 1

In one line regex you can do:

String str = "stackoverflow";
String repl = str.replaceAll("\\B\\w\\B", "*");
//=> s***********w

RegEx Demo

\B is zero-width assertion that matches positions where \b doesn't match. That means it matches every letter except first and last.