// beta
// Lisence Under GPL2
// Author: S.Mahbub-Uz-Zaman

public static String a = "test,test,test";
public static String replaceWith = " and ";

// using StringBuffer's replace method
public static void replaceLastCommaSB () {
        StringBuffer sb = new StringBuffer(a);
        sb.replace(a.lastIndexOf(","), a.lastIndexOf(",") + 1, replaceWith);
        System.out.println(sb);
}

output: test,test and test

// using String's substring method
public static void replaceLastCommaSS () {
       String temp = a.substring(0, a.lastIndexOf(",")) + replaceWith + a.substring(a.lastIndexOf(",") + 1);
       System.out.println(temp);
}


output: test,test and test

// regular expression [work on progress]
 public static void replaceLastCommaRE () {
      String temp = a.replaceFirst(",", replaceWith);
      System.out.println(temp);
}

output: test and test,test