Java get the date X days later

Question

I am setting up a db40 database, and it is filled with instances of my class Day() which has sales information for that day in history, and an integer for its ID. The id is formatted like so: YYYYMMDD, so that they are all unique. My db is set up with the correct data chronologically, but right now I am forced to go in and edit the id's manually for over a year's worth of data.

Question: assuming day 1 is January 1, 2014 which would be formatted: 20140101, how do I correctly increment the date by one day each time? I have 424 dates I need to calculate after that, but I can't think of a clever solution without brute forcing it.

So ideally, i would want something like this getDate(ThisDate+1 Day)

What would "brute forcing it" entail? With that fixed format there is only one solution. — Jongware, Mar 22 '15 at 16:34
Brute forcing it as in adding one, and having a bunch of if statements for months that have 30/31/28 days — user1765804, Mar 22 '15 at 16:37
This seems better though http://stackoverflow.com/a/23910924/3973077 — Paul Boddington, Mar 22 '15 at 16:45
@pbabcdefp: Given that the OP is using Java 8, I'd use java.time. Note that the O Phas an int, not a string. — Jon Skeet, Mar 22 '15 at 16:45
@JonSkeet I quickly realised I was suggesting the wrong class too. I like your solution although one of the *s in `idFromDate` should be a +. — Paul Boddington, Mar 22 '15 at 16:51

Jon Skeet · Accepted Answer · 2015-03-22T16:52:47.253

2

As you're using Java 8, I'd use java.time for this:

Convert the integer (ick) into a LocalDate
Add however many days you want
Convert back to an integer (ick)

So something like:

static LocalDate dateFromId(int id) {
    int year = id / 10000;
    int month = (id / 100) % 100;
    int day = id % 100;
    return LocalDate.of(year, month, day);
}

static int idFromDate(LocalDate date) {
    return date.getYear * 10000
        + date.getMonthValue() * 100 
        + date.getDayOfMonth();
}

static int advanceId(int id, int days) {
    LocalDate original = dateFromId(id);
    LocalDate advanced = original.plusDays(days);
    return idFromDate(advanced);
}

Or to compute a bunch of them:

LocalDate original = dateFromId(id);
for (int i = 1; i <= count; i++) {
    LocalDate advanced = original.plusDays(i);
    int advancedId = idFromDate(advanced);
    // Use advanced
}

edited Mar 22 '15 at 16:52

answered Mar 22 '15 at 16:45

Jon Skeet

1,421,763
867
9,128
9,194

Hmm I think I see what you're doing. Thanks for the help. It gives me an error saying that the * operator is undefined for Month and int in the idFromDate() method – user1765804 Mar 22 '15 at 16:54
@user1765804: Sorry, fixed that. – Jon Skeet Mar 22 '15 at 16:54
Thank you! You have been so helpful! Your solution is fantastic for the future as well since I can always be sure to have the correct dates. Thanks mate! – user1765804 Mar 22 '15 at 16:59

Java get the date X days later

1 Answers1