What is the algorithm represented in Java to convert an 12 hour am/pm meridiem format hour to 24 hour format?

Question

I needed to convert only the hour part of 12 hour time string to 24 hour format. I'm not interested in other than hour parts of a date string. SimpleDateFormatter is notoriously buggy and I in fact I'm more interested in the conversion algorithm itself. So what I want is to find the algorithm to convert the hour part. This is example of an uncleaned string containing hour part: "12:15PM". Sometimes there is spaces between minutes and meridiem sometimes not. Sometimes it has also character 160. So before conversion method I check and clean the string parts and then feed then meridiem(am or pm) and hour to the method which returns hour as 24 hour format int.

Answer 1

This is my naive style code for beginners to convert an hour in 12 hour format to an hour in 24 format.

public static int convert12to24(String meridiem, String hour) {
    //meridiem is that am or pm,
    meridiem = meridiem.toLowerCase();
    int h_12 = 0;
    int h_24 = 0;
    try {
        h_12 = Integer.parseInt(hour);
    } catch (NumberFormatException e) {
        e.printStackTrace();
    }
    if (h_12 == 12) {
        //this is the midnight hour
        if (meridiem.contains("am")) {//generally before noon
            h_24 = 0;
        } else {//this is the hour starting at noon
            h_24 = 12;
        }
    } else if (h_12 >= 1)//all the rest
    {
        if (meridiem.contains("am")) {
            //hour starting after first hour at midnight to 11 facing noon
            h_24 = h_12;
        } else {//pm hours starting right after first hour after noon
            h_24 = h_12 + 12;
        }
    }

    return h_24;
}

Answer 2

java.time

I recommend using java.time, the modern Java date and time API, rather than your own calendar implementation. It can be used in all locales.

    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("h:mma", Locale.ENGLISH);
    
    String timeString12Hours = "12:15PM";
    
    LocalTime time = LocalTime.parse(timeString12Hours, formatter);
    System.out.println("Time: " + time);
    int hour = time.getHour();
    System.out.println("Hour of day in 24 hour format: " + hour);

Output is:

Time: 12:15
Hour of day in 24 hour format: 12

What we get for free from java.time is validation. This code will detect if meridiem is not either AM or PM, or the hour is outside the range 1 through 12 or not a number at all.

Under no circumstances use the SimpleDateFormat class mentioned in the question. It’s a notorious troublemaker of a class and long outdated.

Edit: If for compatibility with old code that I don’t know and that you haven’t got time to clean up at the moment you need the method signature from your own answer:

private static DateTimeFormatter meridiemHourFormatter
        = DateTimeFormatter.ofPattern("ahh", Locale.ENGLISH);

public static int convert12to24(String meridiem, String hour) {
    String meridiemHour = meridiem + hour;
    return LocalTime.parse(meridiemHour, meridiemHourFormatter).getHour();
}

Trying it out:

    System.out.println(convert12to24("PM", "12"));

12

Every reader can decide for himself or herself: Which method implementation takes less energy to understand if in a hurry, yours or mine?

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