Java implementation to determine the day of the week based on date

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class DateTools {
    public static String dateToWeek(String datetime) {
        SimpleDateFormat f = new SimpleDateFormat("yyyy-MM-dd");
        String[] weekDays = { "星期日", "星期一", "星期二", "星期三", 
        "星期四", "星期五", "星期六" };
        Calendar cal = Calendar.getInstance(); // 获得一个日历
        Date datet = null;
        try {
            datet = f.parse(datetime);
            cal.setTime(datet);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        int w = cal.get(Calendar.DAY_OF_WEEK) - 1; // 指示一个星期中的某天。
        if (w < 0)
            w = 0;
        return weekDays[w];
    }

    public static void main(String[] args) {
        System.out.println(DateTools.dateToWeek("2017-01-01"));
    }
}