您如何比较 Java 中的时间？

Question

I'm wondering how I would compare times in a Java program. For example: If I choose a time block from 09:00-12:00 and set that. If i choose a second time and choose 11:00-13:00 time period, I would want to be able to have an error pop up, but I'm not exactly sure how to set that up.

For example if you are reserving a time at a massage therapy place from 9-12, you can't reserve another one for yourself at 11-1, it wouldn't make sense. I'm trying to figure out how to put it into code. My noob level is having me think to have a string as a selection method, then parse the string to integer and compare the integer for overlapping? I want to use the Date class though because it seems very useful and I'd like to learn how to use it.

Answer 1

Let's say you have limiting times in which you must create a time block for whatever reason. Let's convert those limiting times into actual Date objects in Java. I am using

Date today = new Date();

to get today's date, you can use whatever day you want.
I then merge today's date (day of month of year) with the times I defined in two strings, but which you could get from an object, etc.

private static Date combineDates(Date date, Date time) {
    Calendar cal = Calendar.getInstance();

    cal.setTime(time);
    int hour = cal.get(Calendar.HOUR_OF_DAY);
    int min = cal.get(Calendar.MINUTE);

    cal.setTime(date);
    cal.set(Calendar.HOUR_OF_DAY, hour);
    cal.set(Calendar.MINUTE, min);

    return cal.getTime();
}


Date today = new Date();
SimpleDateFormat isoTime = new SimpleDateFormat("HH:mm", Locale.US);

String startTimeString =  "09:00";
String endTimeString =  "12:00";

Date startTime = new Date();
Date endTime = new Date();

try {
    startTime = isoTime.parse(startTimeString);
    endTime = isoTime.parse(endTimeString);
} catch (ParseException e){
    Log.e(TAG, e.getMessage());
}

Date startDate = combineDates(today, startTime);
Date endDate = combineDates(today, endTime);

From then on the only thing left to do is to get the second chosen time period and comparing it to the limiting times.

Something like:

if (newDateStart.after(startDate) && newDateEnd.before(endDate)) {
// Allow user to choose time
} else {
// Do not allow user to choose time
}

I am sure it is a bit convoluted but it's the best I've got right now. Also note that there are newer Java objects you can use but this will work with most Android APIs so...

Answer 2

LocalTime from java.time

    LocalTime blockStart = LocalTime.of(9, 0);
    LocalTime blockEnd = LocalTime.of(12, 0);
    assert blockEnd.isAfter(blockStart);

    LocalTime newBlockStart = LocalTime.of(11, 0);
    LocalTime newBlockEnd = LocalTime.of(13, 0);
    assert newBlockEnd.isAfter(newBlockStart);

    if (newBlockStart.isBefore(blockEnd) && newBlockEnd.isAfter(blockStart)) {
        System.out.println("" + newBlockStart + '–' + newBlockEnd
                + " overlaps with " + blockStart + '–' + blockEnd);
    }

Output from the snippet is:

11:00–13:00 overlaps with 09:00–12:00

A LocalTime is a time of day without a date. A potential liability is that it cannot take into account if the clock is turned forward or backward, for example when summer time (DST) begins or ends. But if you want nothing that fancy, it's the correct class to use.

The formula for detecting an overlap is standard. I include a link to a question about it below.

I want to use the Date class though…

Don't, don't, please don't. The Date is poorly designed and long outdated. You are seriously better off not learning how to use that class. Learn to use java.time, the modern Java date and time API. A possible place to start is the first link below.

Links

• Oracle tutorial: Date Time explaining how to use java.time.
• Algorithm to detect overlapping periods [duplicate] (in C#, but the formula used is the same)
• Determine Whether Two Date Ranges Overlap