只需要日历实例的日期(而不是时间)，并在滞后时与Kotlin中的日期字符串进行比较 [英] Need Calendar Instance only date (not time) and compare with date String in Kotlin as it lags

问题描述

我只需要日历实例中的日期，而不需要时间.每当我使用日历对象时，它都会返回带有时间的日期.

I need only date from Calendar Instance not the time. Whenever i used calendar object it returns the date with time.

val calendar = Calendar.getInstance()
calendar.time. // Mon Nov 09 11:41:29 GMT 2020

我通过使用SimpleDateFormat对此进行了更改

I change this by using SimpleDateFormat

SimpleDateFormat("dd/MM/yyyy").format(date)

09/09/2020

我正在创建日历，因此列表中有大量数据.我要在特定日期添加数据.因此，我正在将日期与字符串日期进行比较.我的字符串日期格式如下所示:-

I am creating calendar so i have huge amount of data in list. I am adding data at specific date. So I am comparing dates with string date. My string date Format is look like this :-

20/05/2020

因此存在太多的性能问题，例如滞后于视图.因此，有什么我可以避免的事情.

So there is too much performance issue like lagging the view. So is there any thing which i can use to avoid all this thing.

val calendarModel = dataList?.find {
     SimpleDateFormat("dd/MM/yyyy").format(it.date) == item
}

推荐答案

Calendar#getTime returns a java.util.Date object representing this Calendar's time value which is a millisecond value that is an offset from the Epoch, January 1, 1970 00:00:00.000 GMT.

因此，java.util.Date不代表实际日期或时间或日期时间对象.当您打印此毫秒值时，JVM将在其时区中计算日期和时间，并且在打印其对象时，您将得到java.util.Date#toString返回的内容.从这个解释中，您必须已经了解到，该毫秒值与时区无关，因为它不是基于时区的值，所以它将是相同的.相反，它被java.util.Date#toString假表示为基于时区的值.只是为了演示我刚才所说的内容，请看以下程序的输出:

Thus, java.util.Date does not represent a real date or time or date-time object. When you print this millisecond value, your JVM calculates the date and time in its time-zone and when you print its object, you get what java.util.Date#toString returns. From this explanation, you must have already understood that this millisecond value will be the same irrespective of the timezone as it is not a timezone based value; rather, it is fakely represented by java.util.Date#toString as a timezone based value. Just to demonstrate what I have just said, look at the output of the following program:

import java.util.Date;
import java.util.TimeZone;

public class Main {
    public static void main(String[] args) {
        Date date = new Date();

        System.out.println("Asia/Calcutta:");
        TimeZone.setDefault(TimeZone.getTimeZone("Asia/Calcutta"));
        System.out.println(date.getTime());
        System.out.println(date);

        System.out.println("\nEurope/London:");
        TimeZone.setDefault(TimeZone.getTimeZone("Europe/London"));
        System.out.println(date.getTime());
        System.out.println(date);

        System.out.println("\nAfrica/Johannesburg:");
        TimeZone.setDefault(TimeZone.getTimeZone("Africa/Johannesburg"));
        System.out.println(date.getTime());
        System.out.println(date);

        System.out.println("\nAmerica/New_York:");
        TimeZone.setDefault(TimeZone.getTimeZone("America/New_York"));
        System.out.println(date.getTime());
        System.out.println(date);
    }
}

输出:

Asia/Calcutta:
1604747702688
Sat Nov 07 16:45:02 IST 2020

Europe/London:
1604747702688
Sat Nov 07 11:15:02 GMT 2020

Africa/Johannesburg:
1604747702688
Sat Nov 07 13:15:02 SAST 2020

America/New_York:
1604747702688
Sat Nov 07 06:15:02 EST 2020

现代日期时间API 具有真实的日期时间类.下面给出的是这些类的概述:

The modern date-time API has real date-time classes. Given below is an overview of these classes:

您可以在此表中找到一个类LocalDate，该类仅表示日期(由年，月和日组成).以下是现代java.time API的快速演示:

As you can find in this table, there is a class, LocalDate which represents just date (consisting of a year, month, and day). Given below is a quick demo of the modern java.time API:

import java.time.LocalDate;
import java.time.Month;
import java.time.ZoneId;
import java.time.ZoneOffset;

public class Main {
    public static void main(String[] args) {
        // A date with the given year, month and day-of-month
        LocalDate date = LocalDate.of(2010, Month.NOVEMBER, 7);
        System.out.println(date);

        // Today (in the JVM's timezone)
        LocalDate today = LocalDate.now(); // Same as LocalDate.now(ZoneId.systemDefault())
        System.out.println(today);

        // Today at UTC
        LocalDate todayAtUTC = LocalDate.now(ZoneOffset.UTC);
        System.out.println(todayAtUTC);

        // Today in India
        LocalDate todayInIndia = LocalDate.now(ZoneId.of("Asia/Calcutta"));
        System.out.println(todayAtUTC);
    }
}

输出:

2010-11-07
2020-11-07
2020-11-07
2020-11-07

在 跟踪中了解有关现代日期时间API的更多信息:日期时间 .

Learn more about the modern date-time API at Trail: Date Time.

建议:java.util的日期时间API及其格式API SimpleDateFormat已过时且容易出错.我建议您应该完全停止使用它们，并切换到现代日期时间API .

Recommendation: The date-time API of java.util and their formatting API, SimpleDateFormat are outdated and error-prone. I suggest you should stop using them completely and switch to the modern date-time API.

如果您正在为Android项目工作，并且您的Android API级别仍不符合Java-8，请检查如何在Android Project中使用ThreeTenABP .

If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.

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