如何用Java计算两个时间的总和？代码异常排查求助

Hey Michael, let's break down why your time sum is spitting out negative values and inaccurate results, then fix it properly.

The Root of the Problem #

Chances are you're making one of two common mistakes:

1. Confusing time points with durations: If you're adding the timestamp values (like Date.getTime() or LocalTime.toSecondOfDay()) directly, you're treating "23:00" as a point in time, not a length of time. This leads to overflow or wrapping around the 24-hour clock (e.g., 23h + 2h gets truncated to 1h instead of 25h).
2. Using integer overflow: If you're storing seconds/hours in int variables instead of long, large values can wrap into negative numbers (since int has a max value of ~2 billion, which is only about 23 days of seconds).

The Correct Approach: Use Durations #

Java 8+ gives us the Duration class, which is designed specifically to represent lengths of time. We'll convert each input time to a duration (relative to midnight), add them, then format the total into a readable format.

Here's a working implementation:

import java.time.Duration;
import java.time.LocalTime;

public class TimeSumCalculator {
    public static void main(String[] args) {
        // Example input times (replace these with your actual values)
        LocalTime time1 = LocalTime.of(23, 30, 15); // 23h 30m 15s
        LocalTime time2 = LocalTime.of(2, 45, 30);  // 2h 45m 30s

        // Convert each time to a duration from midnight
        Duration duration1 = Duration.between(LocalTime.MIDNIGHT, time1);
        Duration duration2 = Duration.between(LocalTime.MIDNIGHT, time2);

        // Safely add the two durations
        Duration totalDuration = duration1.plus(duration2);

        // Format the total duration into a human-readable format
        long totalHours = totalDuration.toHours();
        long remainingMinutes = totalDuration.toMinutes() % 60;
        long remainingSeconds = totalDuration.getSeconds() % 60;

        System.out.printf("Total time: %d:%02d:%02d%n", totalHours, remainingMinutes, remainingSeconds);
        // Or split into days + hours if needed:
        long days = totalDuration.toDays();
        long hours = totalDuration.toHours() % 24;
        System.out.printf("Total time (with days): %d days, %d:%02d:%02d%n", days, hours, remainingMinutes, remainingSeconds);
    }
}

What This Code Does #

• Duration.between(LocalTime.MIDNIGHT, timeX): Converts your input time (e.g., 23:30) into a duration representing how much time has passed since midnight (23.5 hours).
• duration1.plus(duration2): Adds the two durations without overflow (since Duration uses long internally to store values).
• The formatting steps let you display the total as a single "over-24-hour" time (like 26:15:45) or split into days + hours for clarity.

If You're Stuck With Legacy Java (Pre-8) #

If you can't use Java 8+, you can manually calculate the total seconds using long to avoid overflow:

public class LegacyTimeSum {
    public static void main(String[] args) {
        // Example times: hours, minutes, seconds
        int t1Hours = 23, t1Mins = 30, t1Secs = 15;
        int t2Hours = 2, t2Mins = 45, t2Secs = 30;

        // Calculate total seconds using long to prevent overflow
        long totalSeconds = (long)t1Hours * 3600 + t1Mins * 60 + t1Secs
                          + (long)t2Hours * 3600 + t2Mins * 60 + t2Secs;

        // Convert back to hours, minutes, seconds
        long totalHours = totalSeconds / 3600;
        long remainingMins = (totalSeconds % 3600) / 60;
        long remainingSecs = totalSeconds % 60;

        System.out.printf("Total time: %d:%02d:%02d%n", totalHours, remainingMins, remainingSecs);
    }
}

关键要记住：你要计算的是时长的总和，不是两个时间点相加得到的新时间点。这是很多人容易混淆的概念！

内容的提问来源于stack exchange，提问作者Michael1