尝试一下（

try {
    String startDateString = "08-12-2017";
    SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy");
    SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd");
    System.out.println(sdf2.format(sdf.parse(startDateString)));
} catch (ParseException e) {
    e.printStackTrace();
}

更新-Java 8

    String startDateString = "08-12-2017";
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
    DateTimeFormatter formatter2 = DateTimeFormatter.ofPattern("yyyy-MM-dd");
    System.out.println(LocalDate.parse(startDateString, formatter).format(formatter2));