Android获取上周日期

Question

I have some code that gets the current week and loads it into an array:

DateFormat format = new SimpleDateFormat("M-dd");
calendar = Calendar.getInstance();
calendar.setFirstDayOfWeek(Calendar.SUNDAY);
calendar.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);

days = new String[7];
for (int i = 0; i < 7; i++)
{
calendar.add(Calendar.DATE, 1);
days[i] = format.format(calendar.getTime());
}

I then have some code that runs on the selection of a button that will retrieve the previous week dates and loads them into an array:

DateFormat format = new SimpleDateFormat("M-dd");

days = new String[7];

for (int i = 0; i < 7; i++)
{
calendar.add(Calendar.DATE, -1);
days[i]=format.format(calendar.getTime());
}

My issue is that it will load for example: 5-22-17, 5-23-17, 5-24-17, 5-25-17, 5-26-17, 5-27-17, 5-28-17 into the initial load for the first week and then when I click on my Previous button to run the previous button code it only goes back 1 day: 5-21-17 - 5-27-17. If I click it again it then goes back a whole week: 5-14-17 - 5-20-17.

How can I fix my code to retrieve the previous week days correctly on the first click?

Answer 1

I believe the issue is the fact the calendar is advancing and retreating, rather than being used as an anchor. This code shows one approach to handling the situation. It keeps the current start of the week, and obtains days from that point.

/**
 * Returns 7 days starting from the specified starting date
 */
private static String[] getDays(Calendar starting)
{
    DateFormat format = new SimpleDateFormat("M-dd");
    String[] days = new String[7];
    Calendar mod = Calendar.getInstance();
    mod.setTime(starting.getTime());

    for (int i = 0; i < days.length; ++i) {
        mod.add(Calendar.DATE, 1);
        days[i] = format.format(mod.getTime());
    }

    return days;
}

public static void main(String[] args)
{
    // this sets to the beginning of the current week
    Calendar cal = Calendar.getInstance();
    cal.setFirstDayOfWeek(Calendar.SUNDAY);
    cal.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);

    // need this to back up
    Calendar beginningOfWeek = cal;

    // generate the array
    String days[] = getDays(beginningOfWeek);

    System.out.println(Arrays.toString(getDays(beginningOfWeek)));

    // go forward one week
    beginningOfWeek.add(Calendar.DATE, 7);
    System.out.println(Arrays.toString(getDays(beginningOfWeek)));


    // go back one week; same as starting
    beginningOfWeek.add(Calendar.DATE, -7);
    System.out.println(Arrays.toString(getDays(beginningOfWeek)));

    // go back one week; one week before we began
    beginningOfWeek.add(Calendar.DATE, -7);
    System.out.println(Arrays.toString(getDays(beginningOfWeek)));

}

Example output:

[5-22, 5-23, 5-24, 5-25, 5-26, 5-27, 5-28]
[5-29, 5-30, 5-31, 6-01, 6-02, 6-03, 6-04]
[5-22, 5-23, 5-24, 5-25, 5-26, 5-27, 5-28]
[5-15, 5-16, 5-17, 5-18, 5-19, 5-20, 5-21]

Note there are several optimizations that could be applied, such as not creating the format every time the getDays(...) method is called. Also, the variable names are essentially from the OP, but are perhaps not optimal.

Answer 2

First, you set first day of week to Sunday, but you then add 1 before getting first value, so result is Mon 5/22 to Sun 5/28 , not Sun 5/21 to Sat 5/27 .

Second, when your first loop is done, the Calendar object is sitting at 5/28 . You then go backwards, subtracting 1 before getting last value, so result is 5/27 down to 5/21 , not 5/21 to 5/27 like you said.

That leaves Calendar object sitting at 5/21 . Repeating the above, you get 5/20 down to 5/14 , not 5/14 to 5/20 .

So, to get your results as a week of Sunday to Saturday, like you told the Calendar that you wanted, add 1 after the getting the value. And to get a week in the right order, always get them incrementally. To go back to previous week, since Calendar is sitting after the week, you go back 2 weeks to start building the previous week.

public class Test {
    private Calendar calendar;
    public String[] getCurrentWeek() {
        this.calendar = Calendar.getInstance();
        this.calendar.setFirstDayOfWeek(Calendar.SUNDAY);
        this.calendar.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
        return getNextWeek();
    }
    public String[] getNextWeek() {
        DateFormat format = new SimpleDateFormat("M-dd");
        String[] days = new String[7];
        for (int i = 0; i < 7; i++) {
            days[i] = format.format(this.calendar.getTime());
            this.calendar.add(Calendar.DATE, 1);
        }
        return days;
    }
    public String[] getPreviousWeek() {
        this.calendar.add(Calendar.DATE, -14);
        return getNextWeek();
    }
    public static void main(String[] args) {
        Test t = new Test();
        System.out.println("Current : " + Arrays.toString(t.getCurrentWeek()));
        System.out.println("Previous: " + Arrays.toString(t.getPreviousWeek()));
        System.out.println("Previous: " + Arrays.toString(t.getPreviousWeek()));
        System.out.println("Next    : " + Arrays.toString(t.getNextWeek()));
        System.out.println("Next    : " + Arrays.toString(t.getNextWeek()));
    }
}

Output

Current : [5-21, 5-22, 5-23, 5-24, 5-25, 5-26, 5-27]
Previous: [5-14, 5-15, 5-16, 5-17, 5-18, 5-19, 5-20]
Previous: [5-07, 5-08, 5-09, 5-10, 5-11, 5-12, 5-13]
Next    : [5-14, 5-15, 5-16, 5-17, 5-18, 5-19, 5-20]
Next    : [5-21, 5-22, 5-23, 5-24, 5-25, 5-26, 5-27]