CODEWITHSUNDEEP
javadatecalendarincrement
user1765804
user1765804

Reputation: 181

Java get the date X days later

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)

Upvotes: 1

Views: 244

Answers (2)

Basil Bourque
Basil Bourque

Reputation: 338171

The Answer by Jon Skeet is good, of course. But I would do it more simply by using text, assuming we always have 4-digit years in contemporary times (no leading zeros in the year).

For clarity, let's change your example data to 20140123 for January 23, 2014.

int ➡️ LocalDate

To go from int to LocalDate, convert the number to a string. Then parse by LocalDate.

We can use the predefined formatter DateTimeFormatter.BASIC_ISO_DATE because your int-as-text complies with the ISO 8601 standard’s “basic” format that omits the hyphen delimiters.

LocalDate myLocalDate = LocalDate.parse ( Integer.toString( 20140123 ) , DateTimeFormatter.BASIC_ISO_DATE ) ;

The code in broken-lines style:

LocalDate myLocalDate = 
    LocalDate
    .parse 
    ( 
        Integer.toString( 20140123 ) , 
        DateTimeFormatter.BASIC_ISO_DATE 
    ) 
;

LocalDate ➡️ int

Going the other direction, from LocalDate to int number, generate text to represent the value of the LocalDate object. Then parse that text as an int number.

int num = Integer.parseInt ( myLocalDate.format( DateTimeFormatter.BASIC_ISO_DATE ) ) ;

The code in broken-lines style:

int num = 
    Integer
    .parseInt 
    ( 
        myLocalDate.format( DateTimeFormatter.BASIC_ISO_DATE ) 
    ) 
;

Upvotes: 2

Jon Skeet
Jon Skeet

Reputation: 1499760

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
}

Upvotes: 5

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