格式化整数并将其转换为分钟和秒

Question

In my Java application we have a text field where users enter numbers and we need to parse that number properly to format it to minutes and seconds. Here is a table of possible values and what needs to be produced:

2     -> 02:00 OR 00:02
21    -> 00:21
215   -> 02:15
2157  -> 21:57

2:7   -> 02:07 OR 03:10
2:1   -> 02:01
2:71  -> 03:11
2:07  -> 02:07
20:7  -> 20:07 OR 21:10
20:61 -> 21:01

The OR parts I'm still thinking what should be the best way to do.

I'm sure this problem before has been resolved before, I'm just looking for an already existing library that can handle this.

Answer 1

Depending on what option you use you could do it by using standard java.time functionality

LocalTime time = LocalTime.of(value / 100, value % 100);
String timeString = time.format(DateTimeFormatter.ofPattern("HH:mm"));

Example

for (int value : values) {
    LocalTime time = LocalTime.of(value / 100, value % 100);
    String timeString = time.format(DateTimeFormatter.ofPattern("HH:mm"));
    System.out.println(timeString);
}

00:02
00:21
02:15
21:57

Answer 2

Consider forbidding typing more than 59 seconds. I doubt that users would have any reason to want to do that, so 2:71 is more likely a typo, and you can do the user a favour when you make them aware of that. At least only allow it when no minutes are present.

RegEx and Duration

In any case, for the question as asked, one suggestion is to use a regular expression and the Duration class like this:

    Pattern durationPattern = Pattern.compile("(?:(\\d{1,2}?):?)??(\\d{1,2})");

    String[] inputs = { "2", "21", "215", "2157", "2:7",
            "2:1", "2:71", "2:07", "20:7", "20:61" };

    for (String input : inputs) {
        Matcher m = durationPattern.matcher(input);
        if (m.matches()) {
            String minuteStringOrNull = m.group(1);
            String secondString = m.group(2);
            Duration dur = minuteStringOrNull == null ? Duration.ZERO : Duration.ofMinutes(Integer.parseInt(minuteStringOrNull));
            dur = dur.plusSeconds(Integer.parseInt(secondString));

            String durationString = String.format("%02d:%02d", dur.toMinutes(), dur.toSecondsPart());

            System.out.format("%-5s -> %s%n", input, durationString);
        } else {
            System.out.println("Invalid input: " + input);
        }
    }

Output from this piece of code is:

 2 -> 00:02 21 -> 00:21 215 -> 02:15 2157 -> 21:57 2:7 -> 02:07 2:1 -> 02:01 2:71 -> 03:11 2:07 -> 02:07 20:7 -> 20:07 20:61 -> 21:01

What I don't like about it myself is that the regular expression is so hard to read. (?:(\\\\d{1,2}?):?) is a non-capturing group, (?: … ) works the same as brackets in math and programming. And ?? after it says that it may or not be there and take it reluctantly, that is, try without it first, then with. In this way, when there are few digits, we use them for seconds rather than minutes. Inside the group (\\\\d{1,2}?) is a capturing group consisting of 1 or 2 digits (for minutes), again reluctant, prefer 1 over 2. :? means that the 1 or two digits may or may not be followed by a colon. Finally (\\\\d{1,2}) is another capturing group of 1 or 2 digits (for seconds). The capturing groups are what we take out using m.group() later in the code (they are numbered from 1).

If you end up deciding on other interpretations, you can probably modify the idea and/or the code accordingly.

More readable alternative

You may well prefer more easily readable code even when it's longer. In this case hand parsing may be better than using a regular expression. I believe you can build an algorithm along the following incomplete lines (not tested).

    String[] parts = input.split(":", 3);
    if (parts.length == 1) { // no colon
        if (input.length > 2) {
            // take last two digits as seconds, everything else as minutes
        } else {
            // take everything as seconds
        }
    } else if (parts.length == 2) {
        // take minutes from parts[0] and seconds from parts[1]
    } else {
        // invalid input, too many colons
    }

Still use the Duration class as before.

An existing library?

This is off-topic for Stack Overflow, but now we're at it: I don't know well enough to tell, but you may see whether Time4J or Joda-Time may help you here.

Link

Documentation of regular expressions in Java