How do I change this so the average has a decimal and zero?

Question

I'm trying to get this so that instead of like 90 I'll get 90.0
oneString = JOptionPane.showInputDialog("Choose a test score 0-100");
          one = Integer.parseInt(oneString);
          twoString = JOptionPane.showInputDialog("Choose another test score");
          two = Integer.parseInt(twoString);
          threeString = JOptionPane.showInputDialog("Choose another test score");
          three = Integer.parseInt(threeString);
          fourString = JOptionPane.showInputDialog("Choose another test score");
          four = Integer.parseInt(fourString);
          fiveString = JOptionPane.showInputDialog("Choose another test score");
          five = Integer.parseInt(fiveString);
          sixString = JOptionPane.showInputDialog("Choose another test score");
          six = Integer.parseInt(sixString);
          sevenString = JOptionPane.showInputDialog("Choose another test score");
          seven = Integer.parseInt(sevenString);
          eightString = JOptionPane.showInputDialog("Choose another test score");
          eight= Integer.parseInt(eightString);
          nineString = JOptionPane.showInputDialog("Choose another test score");
          nine = Integer.parseInt(nineString);
          tenString = JOptionPane.showInputDialog("Choose another test score");
          ten = Integer.parseInt(tenString);
          elevenString = JOptionPane.showInputDialog("Choose another test score");
          eleven = Integer.parseInt(elevenString);
          twelveString = JOptionPane.showInputDialog("Choose another test score");
          twelve = Integer.parseInt(twelveString);
          thirteenString = JOptionPane.showInputDialog("Choose another test score");
          thirteen = Integer.parseInt(thirteenString);
          sum = (one+two+three+four+five+six+seven+eight+nine+ten+eleven+twelve+thirteen);
          mean = (sum / 13);
          System.out.println("The average of the scores is "+mean);

Answer 1

I advise you to look at your code and ask yourself if it's efficient. What is you wan to add a few numbers to your program? You'll have to add a lot of lines of code.
Ah well, here's the answer to your question (partially):
    double sum = (one+two+three+four+five+six+seven+eight+nine+ten+eleven+twelve+thirteen);
    double mean = (sum / 13);
//...