在本文中，我们将介绍如何在Java中将24小时制转换为12小时制，将12小时制转换为24小时制。

对于转换，我们可以使用SimpleDateFormat或者Java 8或者更高版本，可以使用DateTimeFormatter类在24小时内转换为12小时，反之亦然。

使用SimpleDateFormat将24小时制转换为12小时制

请记住，对于24小时格式，我们必须使用HH，而对于12小时时间，则必须使用hh。基于此，我们可以通过传递适当的模式来创建两个SimpleDateFormat实例。

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class TimeConversion {
  public static void main(String[] args) {
    String fromPattern = "HH:mm:ss"; // 24 hour time pattern
    String toPattern = "hh:mm:ss a"; // 12 hour time pattern with AM/PM
    String time = "21:15:34";
    convertUsingPattern(time, fromPattern, toPattern);
  }
	
  public static void convertUsingPattern(String time, String convertFromPattern, String convertToPattern){
    DateFormat dfFrom = new SimpleDateFormat(convertFromPattern);
    DateFormat dfTo = new SimpleDateFormat(convertToPattern);
    try {
      Date date = dfFrom.parse(time);
      System.out.println("From Time- " + date);
      String changedTime = dfTo.format(date);
      System.out.println("To Time- " + changedTime);
    } catch (ParseException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }		
  }
}

输出量

From Time- Thu Jan 01 21:15:34 IST 1970
To Time- 09:15:34 PM

使用SimpleDateFormat将12小时制转换为24小时制

在上面的程序中，只需将fromPattern和toPattern互换。

public class TimeConversion {
  public static void main(String[] args) {
    String fromPattern = "hh:mm:ss a"; // 12 hour time pattern with AM/PM
    String toPattern = "HH:mm:ss"; // 24 hour time pattern
    String time = "09:15:34 AM";
    convertUsingPattern(time, fromPattern, toPattern);
  }
	
  public static void convertUsingPattern(String time, String convertFromPattern, String convertToPattern){
    DateFormat dfFrom = new SimpleDateFormat(convertFromPattern);
    DateFormat dfTo = new SimpleDateFormat(convertToPattern);
    try {
      Date date = dfFrom.parse(time);
      System.out.println("From Time- " + date);
      String changedTime = dfTo.format(date);
      System.out.println("To Time- " + changedTime);
    } catch (ParseException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }		
  }
}

输出量

From Time- Thu Jan 01 09:15:34 IST 1970
To Time- 09:15:34

If input is- 09:15:34 PM then output 

From Time- Thu Jan 01 21:15:34 IST 1970
To Time- 21:15:34

使用DateTimeFormatter将24小时格式转换为12小时格式

import java.time.LocalTime;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;

public class TimeConversion {
  public static void main(String[] args) {
    String fromPattern = "HH:mm:ss"; // 24 hour time pattern
    String toPattern = "hh:mm:ss a"; // 12 hour time pattern with AM/PM
    String time = "14:34:45";
    convertUsingPattern(time, fromPattern, toPattern);
  }
	
  public static void convertUsingPattern(String time, String convertFromPattern, String convertToPattern){
    try {
      LocalTime lt = LocalTime.parse(time, DateTimeFormatter.ofPattern(convertFromPattern));
      System.out.println("From Time- " + lt.toString());
      String changedTime = lt.format(DateTimeFormatter.ofPattern(convertToPattern));
      System.out.println("To Time- " + changedTime);
    } catch (DateTimeParseException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }		
  }
}

输出量

From Time- 14:34:45
To Time- 02:34:45 PM

使用DateTimeFormatter将12小时时间格式转换为24小时时间格式

在上面的程序中，只需将fromPattern和toPattern互换。

public class TimeConversion {
  public static void main(String[] args) {
    String fromPattern = "hh:mm:ss a"; // 12 hour time pattern with AM/PM
    String toPattern = "HH:mm:ss"; // 24 hour time pattern
    String time = "12:34:45 AM";
    convertUsingPattern(time, fromPattern, toPattern);
  }
	
  public static void convertUsingPattern(String time, String convertFromPattern, String convertToPattern){
    try {
      LocalTime lt = LocalTime.parse(time, DateTimeFormatter.ofPattern(convertFromPattern));
      System.out.println("From Time- " + lt.format(DateTimeFormatter.ofPattern(convertFromPattern)));
      String changedTime = lt.format(DateTimeFormatter.ofPattern(convertToPattern));
      System.out.println("To Time- " + changedTime);
    } catch (DateTimeParseException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }		
  }
}

输出量

From Time- 12:34:45 AM
To Time- 00:34:45