StringUtil use of Druid's subString

Today, when looking at Druid's source code, accidentally saw StringUtil
inside subString method quite interesting thought that says this method can mimic algorithm when the game
first on the code:

 /**
     * Example: subString("abcdc","a","c",true)="bcd"
     * 第四个参数表示是否到最后的一个c 这例子中 若为false则 结果为 b
     * @param src
     * @param start null while start from index=0
     * @param to null while to index=src.length
     * @param toLast true while to index=src.lastIndexOf(to)
     * @return
     */
    public static String subString(String src, String start, String to, boolean toLast) {
        if(!toLast) {
            return subString(src, start, to);
        }
        int indexFrom = start == null ? 0 : src.indexOf(start);
        int indexTo = to == null ? src.length() : src.lastIndexOf(to);
        if (indexFrom < 0 || indexTo < 0 || indexFrom > indexTo) {
            return null;
        }

        if (null != start) {
            indexFrom += start.length();
        }

        return src.substring(indexFrom, indexTo);

    }

As examples
Example: subString ( "abcdc",
"a", "c", true) = "bcd" fourth parameter to the last one indicates whether c is false this example, if the result is b
then
Example: subString ( "abcdc", "ab",
"c", true) = "cd" then
Example: subString ( "abcdc", "bc", "c", true) = "d"

Algorithm obvious first determine whether the fourth argument is false (false representation than the last "to")
If this argument is false jump following overloaded methods 2
and then give indexFrom indexTo were assigned to this well understood
in the following judgment null ! = start if you let indexFrom become established plus the length of the start position
meaning that is: remove the "start" position and before the letter
if not the only plus this judgment from "start" to start taking subString and contains the start letters
such StringUtils.subString ( "abcdc", "b ", "c", true) = "bcd"
only from the beginning and comprising b b
StringUtils.subString ( "dbcdc", "DB", "C", to true) = "dbcd"
will start from the db and contains db

In addition:
found end i.e. if the start and given to the same and there is still a problem at the beginning of the string is the same and the fourth parameter is not found to false
as StringUtils.subString ( "efcfee", "e ", "e", false);
will be reported java.lang.StringIndexOutOfBoundsException: begin 1, end 0,

length 6 ponder found that
since the beginning of the end of the same and does not explain the "to" e was the last he would think "to" is the first e i.e. end = 0, start = 1 ( end = 0, start = 1 because the above said indexFrom + = start.length ();)
attached: overloaded method 2

 /**
     * Example: subString("abcd","a","c")="b"
     * 
     * @param src
     * @param start null while start from index=0
     * @param to null while to index=src.length
     * @return
     */
    public static String subString(String src, String start, String to) {
        int indexFrom = start == null ? 0 : src.indexOf(start);
        int indexTo = to == null ? src.length() : src.indexOf(to);
        if (indexFrom < 0 || indexTo < 0 || indexFrom > indexTo) {
            return null;
        }

        if (null != start) {
            indexFrom += start.length();
        }

        return src.substring(indexFrom, indexTo);

    }