How to generate an array of Date objects in Kotlin?

I want to generate an array of date objects Kotlin style, from current day descending each 5 days programmatically:

> {now, 5_days_ago, 10_days_ago, etc..}

private fun dateArray() : ArrayList<Date> {
    val date = Date()
    val date2 = Date(date.time - 432000000)
    val date3 = Date(date2.time - 432000000)
    val date4 = Date(date3.time - 432000000)
    val date5 = Date(date4.time - 432000000)
    val date6 = Date(date5.time - 432000000)
    val date7 = Date(date6.time - 432000000)
    val date8 = Date(date7.time - 432000000)
    val date9 = Date(date8.time - 432000000)
    val dateA = Date(date9.time - 432000000)
    return arrayListOf(date, date2, date3, date4, date5, date6, date7, date8, date9, dateA)
}

This way is to much overheaded. I bet Kotlin offers an elegant way?

java.time

Here’s a simple pure Java solution. Like others I am recommending that you use java.time, the modern Java date and time API, for your date work.

	Period step = Period.ofDays(-5);
	int numberOfDates = 10;
	
	Period totalPeriod = step.multipliedBy(numberOfDates);
	
	LocalDate today = LocalDate.now(ZoneId.of("America/Rosario"));
	List<LocalDate> dates = today.datesUntil(today.plus(totalPeriod), step)
			.collect(Collectors.toList());
	
	System.out.println(dates);

Output:

> [2020-09-14, 2020-09-09, 2020-09-04, 2020-08-30, 2020-08-25,
> 2020-08-20, 2020-08-15, 2020-08-10, 2020-08-05, 2020-07-31]

I trust that you can translate to Kotlin and maybe even refine it further in Kotlin.

It’s not well documented that you may use the two-arg LocalDate.datesUntil() with a negative period, but at least it works on my Java 11.

Sorry to say, your solution in the question not only has more code than needed, it is also incorrect. Subtracting 432 000 000 milliseconds each time assumes that a day is always 24 hours. Because of summer time (DST) and other time anomalies a day may be 23 or 25 hours or something in between. By using LocalDate we are free of such issues. You may also use for example ZonedDateTime of LocalDateTime instead if you want to include time of day, they work fine across summer time transitions too.

Link: Oracle tutorial: Date Time explaining how to use java.time.

I recommend you switch from the outdated and error-prone java.util date-time API to the modern java.time date-time API. Learn more about the modern date-time API from Trail: Date Time. If your Android API level is still not compliant with Java8, check https://stackoverflow.com/questions/38922754/how-to-use-threetenabp-in-android-project and Java 8+ APIs available through desugaring.

Do it as follows using the Java modern date-time API:

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

public class Main {
	public static void main(String[] args) {
		// Tests
		System.out.println(getPastDaysOnIntervalOf(10, 5));// 10 days in past at an interval of 5 days
		System.out.println(getPastDaysOnIntervalOf(5, 10));// 5 days in past at an interval of 10 days
	}

	static List<LocalDate> getPastDaysOnIntervalOf(int times, int interval) {
		// The list to be populated with the desired dates
		List<LocalDate> list = new ArrayList<>();

		// Today
		LocalDate date = LocalDate.now();

		for (int i = 1; i <= times; i++) {
			list.add(date);
			date = date.minusDays(interval);
		}

		// Return the populated list
		return list;
	}
}

Output:

[2020-09-14, 2020-09-09, 2020-09-04, 2020-08-30, 2020-08-25, 2020-08-20, 2020-08-15, 2020-08-10, 2020-08-05, 2020-07-31]
[2020-09-14, 2020-09-04, 2020-08-25, 2020-08-15, 2020-08-05]

Using legacy API:

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;

public class Main {
	public static void main(String[] args) {
		// Tests
		System.out.println(getPastDaysOnIntervalOf(10, 5));// 10 days in past at an interval of 5 days
		System.out.println(getPastDaysOnIntervalOf(5, 10));// 5 days in past at an interval of 10 days
	}

	static List<Date> getPastDaysOnIntervalOf(int times, int interval) {
		// The list to be populated with the desired dates
		List<Date> list = new ArrayList<>();

		// Today
		Calendar cal = Calendar.getInstance();
		Date date = cal.getTime();

		for (int i = 1; i <= times; i++) {
			list.add(date);
			cal.add(Calendar.DATE, -interval);
			date = cal.getTime();
		}

		// Return the populated list
		return list;
	}
}