Differences between methods in Calendar before and after using Android 5.0

my demand: get the Monday and Sunday dates of the specified number of weeks according to the year and the number of weeks.

public static String getWeek(String year, String num) {
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, Integer.parseInt(year));
        int value = Integer.parseInt(num);
        cal.set(Calendar.WEEK_OF_YEAR, value);
        // 127
        cal.set(Calendar.DAY_OF_WEEK, 2);
        Date date = cal.getTime();
       // 201610
        value += 1;
        cal.set(Calendar.WEEK_OF_YEAR, value);
       // 127
        cal.set(Calendar.DAY_OF_WEEK, 1);
        Date date1 = cal.getTime();
        return date2Month(date) + "/" + date2Date(date) + "~" + date2Month(date1) + "/" + date2Date(date1);
    }

when I input two values of getWeek (2018 Android 24) for phones above getWeek 5.0, the result is 6.11-6.17, which is the result I want.
but when the phone is less than Android 5.0, pass in the same value, and the result is 6.11-6.10.
that is,

in the code

 value += 1;
  cal.set(Calendar.WEEK_OF_YEAR, value);

these two lines of code didn"t work. I wanted to increase the number of weeks by 1, but I didn"t succeed below Android 5.0.