How can I select random words from an external file in java processing?

I have this assignment that I have to make a word cloud of the most common words used in an external text file which all words will start from the center and expand/animate to a word cloud... I already made a word counter to determine which words are most often used in the text file, now that I know which words are most often used, how can I select at least 15 random words from the top 50 most used words in the text file?

if that question can be easily answered how can I overlap all the selected words on the canvas?

note: I am very noob at java and processing so a code would help...

here is my code:

    String [] words;
    IntDict concordance;
    int index = 0;

    void setup () {
      size (500,500);
      background (0);
      String [] lines = loadStrings ("alice_just_text.txt"); 
      String entireplay = join(lines, " ");
      words = splitTokens (entireplay, ",.?!:-;:()03 ");
      concordance = new IntDict () ;
      frameRate (5);

      for (int i = 0; i < words.length;i++) {
        concordance.increment(words[i].toLowerCase());
       }
      concordance.sortValuesReverse();

      String [] keys = concordance.keyArray();
      for (int i = 0; i < keys.length; i++) {
      int count = concordance.get(keys[i]); //word counts
      println(keys[i], count);

       }

      }

      void draw () {

      background (0);
      textSize (64);
      textAlign (CENTER);
      text (words [index], width/2, height/2);
      index++;

      }