Насколько я понял (не до конца, видимо), массив Format[] - это, типа, маска для массива Object[] testArgs. А в массиве testArgs семь объектов. 
public static void printStocks(List<Stock> stocks, Date actualDate) {
        SimpleDateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy");

        double[] filelimits = {0d, actualDate.getTime()};
        String[] filepart = {"change {4}", "open {2} and last {3}"};

        ChoiceFormat fileform = new ChoiceFormat(filelimits, filepart);
        Format[] testFormats = {null, null, dateFormat, fileform};
        MessageFormat pattform = new MessageFormat("{0} {1} | {5} {6}");
        pattform.setFormats(testFormats);

        for (Stock stock : stocks) {
            String name = ((String) stock.get("name"));
            String symbol = (String) stock.get("symbol");
            double open = !stock.containsKey("open") ? 0 : ((double) stock.get("open"));
            double last = !stock.containsKey("last") ? 0 : ((double) stock.get("last"));
            double change = !stock.containsKey("change") ? 0 : ((double) stock.get("change"));
            Date date = (Date) stock.get("date");
            Object[] testArgs = {name, symbol, open, last, change, date, date.getTime()};
            System.out.println(pattform.format(testArgs));
        }
    }