Convert 12-hour time formats to 24-hour formats

Question

I am trying to create a time conversion function that converts 12-hour time formats to 24-hour formats. My code works for all inputs except the following: 12:05:39AM.

I am having trouble understanding what I am doing wrong

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.function.*;
import java.util.regex.*;
import java.util.stream.*;
import static java.util.stream.Collectors.joining;
import static java.util.stream.Collectors.toList;

class Result {

    /*
     * Complete the 'timeConversion' function below.
     *
     * The function is expected to return a STRING.
     * The function accepts STRING s as parameter.
     */

    public static String timeConversion(String s) {
    // Write your code here
    
    if (s.substring(8).equals("PM")){
        String hour = s.substring(0,2);
        if (Integer.parseInt(hour) != 12){
            int newHour = (Integer.parseInt(hour) + 12) % 24;
            return (newHour + s.substring(2,8));
        }
        return (hour + s.substring(2,8));    
    }
    else if (s.substring(8).equals("AM")){
        String AMhour = s.substring(0,2);
        if (Integer.parseInt(AMhour) == 12){
            int newHour = ((Integer.parseInt(AMhour) + 12) % 24);
            return (newHour + s.substring(2,8));  
        }
        return  (AMhour + s.substring(2,8));
    }
    else {
        return("Not found"); 
    }
    }
}

Please explain what "does not work" exactly means. Does the code produce a result you don't expect? Does the code throw an exception/error? When I'm testing your code with the value "12:05:39AM" it returns "0:05:39", which seems pretty correct to me. — OH GOD SPIDERS, Aug 09 '21 at 10:52
[The answer by Cloud here](https://stackoverflow.com/a/37388524/5772882), doesn’t it give you most of what you want? You will want to add support for seconds yourself. Use format pattern letter `ss` for second of minute. — Ole V.V., Aug 09 '21 at 17:18

devReddit · Answer 1 · 2021-08-09T11:33:55.697

The problem is in this segment:

if (Integer.parseInt(AMhour) == 12){
    int newHour = ((Integer.parseInt(AMhour) + 12) % 24);
    return (newHour + s.substring(2,8));
}

You're getting 0 in newHour, and when you're converting it into string, you're getting 0:mm:ss.

To solve this, type of problem you can add an extra 0 in the front as this is a fixed case. Or, just do:

return String.format("%02d%s", newHour,  s.substring(2,8));

NOTE: USE java.time

Please use actual java libraries to deal with these type of conversions. You could do the conversion with java.time by doing:

return LocalTime.parse(s, DateTimeFormatter.ofPattern("hh:mm:ssa", Locale.US))
                .format(DateTimeFormatter.ofPattern("HH:mm:ss"));

score 1 · Answer 2 · answered Aug 09 '21 at 11:06

java.time

I recommend you do it using java.time, the modern Date-Time API.

Demo:

import java.time.LocalTime;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        // Test
        System.out.println(timeConversion("12:05:39AM"));
    }

    public static String timeConversion(String s) {
        DateTimeFormatter dtfInput = DateTimeFormatter.ofPattern("h:m:sa", Locale.ENGLISH);
        LocalTime time = LocalTime.parse(s, dtfInput);

        DateTimeFormatter dtfOutput = DateTimeFormatter.ofPattern("HH:mm:ss", Locale.ENGLISH);
        return time.format(dtfOutput);
    }
}

Output:

00:05:39

ONLINE DEMO

Learn more about the modern Date-Time API^* from Trail: Date Time.

^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. 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.}

score 0 · Answer 3 · answered Aug 09 '21 at 11:18

A simple example of Date and Time formatting:

public static void main(String[] args) {
    String timePattern = "H:mm:ss SSS";
    DateTimeFormatter timeFormatter = DateTimeFormatter.ofPattern(timePattern);
    
    LocalDateTime ldt = LocalDateTime.now(ZoneId.systemDefault());

    System.out.println("The date is: " +DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL).format(ldt));
    System.out.println("The time is: " + timeFormatter.format(ldt));
}

Official JavaDocs

Convert 12-hour time formats to 24-hour formats

3 Answers3

java.time