public static String replaceString(String mainString,String oldString,String newString){
  if (mainString == null) {
    return null;
  }
  if (UtilValidate.isEmpty(oldString)) {
    return mainString;
  }
  if (newString == null) {
    newString="";
  }
  int i=mainString.lastIndexOf(oldString);
  if (i < 0)   return mainString;
  StringBuilder mainSb=new StringBuilder(mainString);
  while (i >= 0) {
    mainSb.replace(i,i + oldString.length(),newString);
    i=mainString.lastIndexOf(oldString,i - 1);
  }
  return mainSb.toString();
}

public static String replaceString ( String mainString , String oldString , String newString ) { if ( mainString == null ) { return null ; } if ( UtilValidate . isEmpty ( oldString ) ) { return mainString ; } if ( newString == null ) { newString = "" ; } int i = mainString . lastIndexOf ( oldString ) ; if ( i < 0 ) return mainString ; StringBuilder mainSb = new StringBuilder ( mainString ) ; while ( i >= 0 ) { mainSb . replace ( i , i + oldString . length ( ) , newString ) ; i = mainString . lastIndexOf ( oldString , i - 1 ) ; } return mainSb . toString ( ) ; }
replaces all occurrences of oldstring in mainstring with newstring
