I am reading the following code from a tutorial but I don't really get it.
Basically it tries to use operator overloading so that the following code works:
return today + YEAR * 2 + WEEK * 3 + DAY * 5
What I understand:
This part:
operator fun MyDate.plus(timeInterval: TimeInterval): MyDate {
return addTimeIntervals(timeInterval, 1)
}
Enhances the class MyDate to support the + with a timeInterval so this would work myDate + YEAR for example.
This part:
operator fun MyDate.plus(timeIntervals: RepeatedTimeInterval)
= addTimeIntervals(timeIntervals.timeInterval, timeIntervals.number)
Enhances the class MyDate to support * with a RepeatedInterval
This part just declares an empty class with 2 member variables timeInterval and number
class RepeatedTimeInterval(val timeInterval: TimeInterval, val number: Int)
What I don't understand is how the multiplication is actually happening
since RepeatedInterval is just an empty class.
Could someone please help my understand this?
import TimeInterval.*
data class MyDate(val year: Int, val month: Int, val dayOfMonth: Int)
enum class TimeInterval { DAY, WEEK, YEAR }
operator fun MyDate.plus(timeInterval: TimeInterval): MyDate {
return addTimeIntervals(timeInterval, 1)
}
class RepeatedTimeInterval(val timeInterval: TimeInterval, val number: Int)
operator fun TimeInterval.times(number: Int) = RepeatedTimeInterval(this, number)
operator fun MyDate.plus(timeIntervals: RepeatedTimeInterval)
= addTimeIntervals(timeIntervals.timeInterval, timeIntervals.number)
fun task1(today: MyDate): MyDate {
return today + YEAR + WEEK
}
fun task2(today: MyDate): MyDate {
return today + YEAR * 2 + WEEK * 3 + DAY * 5
}
Also this is part of the tutorial:
import java.util.Calendar
fun MyDate.addTimeIntervals(timeInterval: TimeInterval, number: Int): MyDate {
val c = Calendar.getInstance()
c.set(year, month, dayOfMonth)
when (timeInterval) {
TimeInterval.DAY -> c.add(Calendar.DAY_OF_MONTH, number)
TimeInterval.WEEK -> c.add(Calendar.WEEK_OF_MONTH, number)
TimeInterval.YEAR -> c.add(Calendar.YEAR, number)
}
return MyDate(c.get(Calendar.YEAR), c.get(Calendar.MONTH), c.get(Calendar.DATE))
}
YEAR * 2 is TimeInterval * Int. The compiler sees it isn't a built-in combination, so it looks for a method times marked as operator on TimeInterval which accepts Int (so e.g. TimeInterval.times(Int) or TimeInterval.times(Any)). This method can be a member of TimeInterval or an extension; there's absolutely no reason for it to be a member of RepeatedTimeInterval.
In fact, RepeatedTimeInterval doesn't have any part in resolving YEAR * 2 at all, it just happens to be the return type. Then today + YEAR * 2 is MyDate + RepeatedTimeInterval and the same rule is applied to pick operator fun MyDate.plus(timeIntervals: RepeatedTimeInterval) (and not operator fun MyDate.plus(timeInterval: TimeInterval) which is used for today + YEAR).
Note that with this code it isn't legal to have e.g. YEAR * 2 * 2; that would require RepeatedTimeInterval.times(Int) which again could be a member or an extension.
What I don't understand is how the multiplication is actually happening since RepeatedInterval is just an empty class.Could someone please help my understand this?
Well, the RepeatedTimeInterval class is indeed "an empty class", but notice that it has an extension function TimeInterval.times(number: Int) which makes it support the operation TimeInterval * Int, for example: YEAR * 2
I came accross this program while doing the Kotlin Koans: Operators overloading task, here's my solution:
import TimeInterval.*
data class MyDate(val year: Int, val month: Int, val dayOfMonth: Int)
// Supported intervals that might be added to dates:
enum class TimeInterval { DAY, WEEK, YEAR }
class MultipleTimeInterval(val timeInterval: TimeInterval, val amont : Int)
operator fun TimeInterval.times(amont: Int) = MultipleTimeInterval (this, amont)
operator fun MyDate.plus(timeInterval: TimeInterval): MyDate = addTimeIntervals(timeInterval, 1)
operator fun MyDate.plus(multi: MultipleTimeInterval): MyDate = addTimeIntervals(multi.timeInterval, multi.amont)
fun task1(today: MyDate): MyDate {
return today + YEAR + WEEK
}
fun task2(today: MyDate): MyDate {
return today + YEAR * 2 + WEEK * 3 + DAY * 5
}
The utility function MyDate.addTimeIntervals() is:
import java.util.Calendar
fun MyDate.addTimeIntervals(timeInterval: TimeInterval, amount: Int): MyDate {
val c = Calendar.getInstance()
c.set(year + if (timeInterval == TimeInterval.YEAR) amount else 0, month, dayOfMonth)
var timeInMillis = c.timeInMillis
val millisecondsInADay = 24 * 60 * 60 * 1000L
timeInMillis += amount * when (timeInterval) {
TimeInterval.DAY -> millisecondsInADay
TimeInterval.WEEK -> 7 * millisecondsInADay
TimeInterval.YEAR -> 0L
}
val result = Calendar.getInstance()
result.timeInMillis = timeInMillis
return MyDate(result.get(Calendar.YEAR), result.get(Calendar.MONTH), result.get(Calendar.DATE))
}
Related
so I'm having a little problem with kotlin LocalDateTime
val now = CurrentDateTime
val someDate = someService.someLocalDateTime
I have two dates as you can see and I want to know if now is bigger than someDate or not.
and also I need to know if it's bigger, how much is bigger.
i can do it by checking year, month, day, minute and second like this:
if (now.year().toString() == someDate.year.toString())
but it's not a good way
any suggesstions would be welcome.
You can simply convert both dates in seconds and:
compare them to know which one is bigger
subtract them to know how much one is bigger than the other
an example would be
val now = CurrentDateTime.toEpochSeconds()
val someDate = someService.someLocalDateTime.toEpochSeconds();
if(now > someDate)
//someDate is in the past
else
//someDate is in the future or both dates are equal
val distance = now-someDate
hope this helps
The standard solution to compare two Date objects is by using the compareTo() function. It returns a value
= 0, if both dates are equal.
< 0, if date is before the specified date.
> 0, if date is after the specified date.
The following program demonstrates it:
import java.text.SimpleDateFormat
import java.util.*
fun main() {
val now = CurrentDateTime // "01/21/2023"
val someDate = someService.someLocalDateTime // "01/21/2020"
val sdf = SimpleDateFormat("MM/dd/yyyy")
val firstDate: Date = sdf.parse(now)
val secondDate: Date = sdf.parse(someDate)
val cmp = firstDate.compareTo(secondDate)
when {
cmp > 0 -> {
System.out.printf("%s is after %s", d1, d2)
}
cmp < 0 -> {
System.out.printf("%s is before %s", d1, d2)
}
else -> {
print("Both dates are equal")
}
}
}
Convert Kotlin LocalDateTime to Java LocalDateTime
To convert Kotlin LocalDateTime to Java LocalDateTime, you can make use of this function:
fun LocalDateTime.toJavaLocalDateTime(): LocalDateTime
Converts this kotlinx.datetime.LocalDateTime value to a java.time.LocalDateTime value.
And then you can choose to use the following method or other suggested method to compare the converted java.time.LocalDateTime.
Compare Java LocalDateTime
To compare LocalDateTime, you can use LocalDateTime's isAfter(), isBefore(), isEqual().
import java.time.LocalDateTime
fun main() {
val currentTime = LocalDateTime.now()
val ytdTime = LocalDateTime.now().minusDays(1)
println(currentTime.isAfter(ytdTime))
println(currentTime.isBefore(ytdTime))
println(currentTime.isEqual(ytdTime))
}
Output
true
false
false
To find the difference between LocalDateTime, you can use ChronoUnit:
import java.time.temporal.ChronoUnit
fun main() {
val currentTime = LocalDateTime.now()
val ytdTime = LocalDateTime.now().minusDays(1)
val secondDifference = ChronoUnit.SECONDS.between(ytdTime, currentTime)
val minutesDifference = ChronoUnit.MINUTES.between(ytdTime, currentTime)
val hourDifference = ChronoUnit.HOURS.between(ytdTime, currentTime)
println(secondDifference)
println(minutesDifference)
println(hourDifference)
}
Output
86399
1439
23
I have a compose view that is given a month and year. I want to find the first weekday of the given month/year combo in the idiomatic Kotlin way. This is what I have so far:
fun CalendarMonthView(
month: Month = LocalDate.now().month,
year: Int = LocalDate.now().year
) {
// ...
val weekDayNames = DateFormatSymbols.getInstance().weekdays
val firstWeekdayOfMonth = //type annotation in IntelliJ: (Calendar .. Calendar?)
Calendar.getInstance().also {
it.set(Calendar.DATE, 1)
it.set(Calendar.MONTH, month.value)
it.set(Calendar.YEAR, year)
it.set(Calendar.DAY_OF_MONTH, 1)
// does not evaluate to a string?
weekDayNames[it.get(Calendar.DAY_OF_WEEK)]
}
// ...
}
Is there an idiomatic way to do this instead of something like:
val cal = Calendar.getInstance()
cal.set(Calendar.DATE, 1)
cal.set(Calendar.MONTH, month.value)
cal.set(Calendar.YEAR, year)
cal.set(Calendar.DAY_OF_MONTH, 1)
val firstDay = weekDayNames[cal.get(Calendar.DAY_OF_WEEK)] // type annotation: String
You can do something like:
val month = LocalDate.now().month
val year = LocalDate.now().year
val dayOfWeek = LocalDate.of(year, month, 1).dayOfWeek
println(dayOfWeek.name) // prints "FRIDAY" for April 2022
Try it yourself
How do I write code to return a range of 10 numbers from a given number.
i.e if I am given 5, code should return 0..9
358 should return 350..359
33 should return 30..39 etc
Do you mean something like this?
fun range10(contained: Int): IntRange {
val start = contained - contained % 10
val end = start + 9
return start..end
}
If the given number is integer type, you can simply write
val x = 358
(x / 10 * 10)..(x / 10 * 10 + 9)
You can use the following code:
fun answer(givenNum: Int) : IntRange {
val startOfRange = givenNum - (givenNum % 10)
return startOfRange until (startOfRange + 10)
}
fun main() {
val givenNum = 33
println(answer(givenNum)) // 30..39
}
I'm doing Kotlin Koans's Operator Overloading exercise and am wondering how the compiler chooses which MyDate.plus() function to use:
import TimeInterval.*
import java.util.Calendar
data class MyDate(val year: Int, val month: Int, val dayOfMonth: Int)
enum class TimeInterval { DAY, WEEK, YEAR }
operator fun MyDate.plus(timeInterval: TimeInterval) = addTimeIntervals(timeInterval, 1)
class FullTimeInterval(val timeInterval: TimeInterval, val number: Int)
operator fun TimeInterval.times(number: Int) = FullTimeInterval(this, number)
operator fun MyDate.plus(timeIntervals: FullTimeInterval)
= addTimeIntervals(timeIntervals.timeInterval, timeIntervals.number)
fun task1(today: MyDate): MyDate {
return today + YEAR + WEEK
}
fun task2(today: MyDate): MyDate {
return today + YEAR * 2 + WEEK * 3 + DAY * 5
}
You have two classes: TimeInterval and FullTimeInterval and two overloaded functions:
MyDate.plus(timeIntervals: TimeInterval) and MyDate.plus(timeIntervals: FullTimeInterval)
The compiler knows the type of the argument and it selects the closest function by the signature. The decision is made at the compilation time and depends on the computed types of the arguments.
You may find more information on that at
https://jetbrains.github.io/kotlin-spec/#overload-resolution
Just learning to define a DateRange type
val wholeYear2017 = Date(2017,1,1)..Date(2017,12,31)
So I created the type as below
class DateRange<Date: Comparable<Date>>(override val start: Date, override val endInclusive: Date)
: ClosedRange<Date>
class Date (val year: Int, val month: Int, val day: Int) {
operator fun compareTo(other: Date): Int {
if (this.year > other.year) return 1
if (this.year < other.year) return -1
if (this.month > other.month) return 1
if (this.month < other.month) return -1
if (this.day > other.day) return 1
if (this.day < other.day) return -1
return 0
}
operator fun rangeTo(that: Date): DateRange = DateRange(this, that)
}
But I got a compile error
One type of argument expected for class DateRange<Date: Comparable<Date>> : ClosedRange<Date>
What did I missed? Did I do it correctly?
You need to implement Comparable interface. You can use compareValuesBy helper function:
data class Data(
val a: Int,
val b: Int
) : Comparable<Data> {
override fun compareTo(other: Data) = compareValuesBy(this, other,
{ it.a },
{ it.b }
)
}
Is your question really about how to create a Comparable type? Then just have your type implement the Comparable interface (override compareTo).
class Date(val year: Int, val month: Int, val day: Int) : Comparable<Date> {
override operator fun compareTo(other: Date): Int {
if (this.year > other.year) return 1
if (this.year < other.year) return -1
if (this.month > other.month) return 1
if (this.month < other.month) return -1
if (this.day > other.day) return 1
if (this.day < other.day) return -1
return 0
}
}
You don't need a rangeTo method because all Comparable<T> types have a rangeTo extension already defined. See Ranges and rangeTo. But, if you still want your own DateRange type (for other purposes), the simpler form of the DateRange class would be...
class DateRange(override val start: Date, override val endInclusive: Date)
: ClosedRange<Date>
In other words, there is no need for the generic parameter on DateRange.
Then you would write your own rangeTo operator. Either, add operator fun rangeTo to your Date class, or provide a root level extension function (my preference which is consistent with the Kotlin library approach). Both will shadow/hide the Comparable<T>.rangeTo extension function for your Date type.
// class level rangeTo operator
operator fun rangeTo(that: Date) = DateRange(this,that)
// root level extension
operator fun Date.rangeTo(that: Date) = DateRange(this,that)
You are almost there:
Firstly, you must make your class Comparable:
class Date (val year: Int, val month: Int, val day: Int): Comparable<Date> {
Secondly, you must specify the generic of the return type or just omit it (let the compiler infer it)
operator fun rangeTo(that: Date): DateRange<Date> = DateRange(this, that)
operator fun rangeTo(that: Date) = DateRange(this, that)
Straight from Koans. I really encourage you to get to know the documentation first.
override fun compareTo(other: Date) = when {
year != other.year -> year - other.year
month != other.month -> month - other.month
else -> day - other.day
}
You can translate date to the Integer representation using such formula, for example: year * 10000 + month * 100 + day.
class Date (val year: Int, val month: Int, val day: Int) {
operator fun compareTo(other: Date): Int {
return (this.year * 10000 + this.month * 100 + this.day) - (other.year * 10000 + other.month * 100 + other.day)
}
}