Improving some plugin user-input code
There is absolutely no need to put parentheses around the first statement (small improvement, but it improves readability). "parameter" is also misspelled (improving readability but in a different way).

String x = JOptionPane.showInputDialog(". Each parameter should be separated with a ',' and without spaces. (eg. 1,2,3)") + ",";

There is no need to use Double[] when you can use double[]. The difference is, simply put, that a Double can be null while a double can not.
In the rest of your code it seems like you are reading all the doubles from the String (separated by commas), which can be done much easier like this:

String[] splitted = x.split(",");
double[] result = new double[splitted.length];
for (int i = 0; i &lt; splitted.length; i++) {
    result[i] = Double.parseDouble(splitted[i]);
}


The key here is to use the split-method on the String, which does most of the work for you. For example, it splits the string "0.37,42,21.4" into a string array with the three elements "0.37", "42" and "21.4". The rest of the code loops through that array and converts from String to double.