Convert 12 hr time to 24 hr format in java

public static String convertToTwentyFour(String tareekh) {

    SimpleDateFormat displayFormat = new SimpleDateFormat("HH:mm");
    SimpleDateFormat parseFormat = new SimpleDateFormat("hh:mm a");
    Date date = null;
    try {
        date = parseFormat.parse(tareekh);
        //System.out.println(parseFormat.format(date) + " = " + displayFormat.format(date));        return displayFormat.format(date);
    } catch (ParseException e) {
        e.printStackTrace();
    }
    return  null;
}