You may only use a data structure to store the list of random words / phrases to choose from.  After that once you get the word from the word pool, you need to store it as a string or stringbuilder and work with it that way.  You may not convert the String to an array using toCharArray().  I would like to see what you can come up with just using the String / StringBuilder classes.  HINT: Remember strings are indexed.....so you really don't need to use an Array or ArrayList....