Java Calendar工作日計算的代碼重構與性能提升

在Java中計算工作日可以使用Calendar類來實現，下面是一個簡單的實現代碼：

import java.util.Calendar;

public class WorkdayCalculator {
  
  public static int calculateWorkdays(Calendar startDate, Calendar endDate) {
    int workdays = 0;
    
    while (startDate.before(endDate)) {
      if (startDate.get(Calendar.DAY_OF_WEEK) != Calendar.SATURDAY && startDate.get(Calendar.DAY_OF_WEEK) != Calendar.SUNDAY) {
        workdays++;
      }
      
      startDate.add(Calendar.DAY_OF_MONTH, 1);
    }
    
    return workdays;
  }
  
  public static void main(String[] args) {
    Calendar startDate = Calendar.getInstance();
    Calendar endDate = Calendar.getInstance();
    endDate.add(Calendar.MONTH, 1);
    
    int workdays = calculateWorkdays(startDate, endDate);
    
    System.out.println("Workdays between " + startDate.getTime() + " and " + endDate.getTime() + ": " + workdays);
  }
}

此代碼通過循環遍歷日期范圍內的每一天，如果不是周末則計算為工作日。然而，這種實現方式可能不夠高效，特別是在處理大日期范圍時。

為了提高性能，可以采用以下方式重構代碼：

import java.util.Calendar;

public class WorkdayCalculator {
  
  public static int calculateWorkdays(Calendar startDate, Calendar endDate) {
    int workdays = 0;
    
    while (startDate.before(endDate)) {
      int dayOfWeek = startDate.get(Calendar.DAY_OF_WEEK);
      if (dayOfWeek != Calendar.SATURDAY && dayOfWeek != Calendar.SUNDAY) {
        workdays++;
      }
      
      startDate.add(Calendar.DAY_OF_MONTH, 1);
    }
    
    return workdays;
  }
  
  public static void main(String[] args) {
    Calendar startDate = Calendar.getInstance();
    Calendar endDate = Calendar.getInstance();
    endDate.add(Calendar.MONTH, 1);
    
    int workdays = calculateWorkdays(startDate, endDate);
    
    System.out.println("Workdays between " + startDate.getTime() + " and " + endDate.getTime() + ": " + workdays);
  }
}

在重構代碼中，將獲取每日的星期幾的代碼提到循環外部，避免在循環內重復調用。這樣可以提高性能，特別是在處理大日期范圍時。

另外，可以使用Java 8的新日期時間API（java.time包）來計算工作日，這是一個更加現代化和功能強大的API。