Java String.replace() - replaces more than just the substring I specify?

Question

As per this CodingBat problem I am trying to do the following:

Given a string, if the first or last chars are 'x', return the string without those 'x' chars, and otherwise return the string unchanged.

My code:

public String withoutX(String str) {
    if (str.startsWith("x")) {
        str = str.replace(str.substring(0, 1), "");
    }
    if (str.endsWith("x")) {
        str = str.replace(str.substring(str.length()-1), "");
    }
    return str;
}

This code replaces ALL the x characters in the string, rather than just the first and last. Why does this happen, and what would be a good way to solve it?

Answer 1

From the sdk for the replace method:

Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.

You can solve this without replace:

public String withoutX(String str) {   
    if (str == null) { 
        return null;
    }

    if (str.startsWith("x")) {
        str = str.substring(1);
    }
    if (str.endsWith("x")) {
        str = str.substring(0, str.length()-1);
    }

    return str;
}