Java中的 Calendar roll(int calndr_field, int amt) 方法及示例

Calendar 类中的roll(int calndr_field , int amt )方法用于通过将传递的金额添加到特定的日历字段来对传递的日历字段进行操作，而无需更改大字段值。
注：正数必须加，负数必须减去。

句法：

public void roll(int calndr_field, int amt)

参数：该方法有两个参数：

1. calndr_field：这是Calendar类型，是指要操作的日历字段。
2. amt：这是整数类型，根据值的符号进行加减运算。

返回值：该方法不返回任何值。

下面的程序说明了 Calendar 类的 roll() 方法的工作：
示例 1：

// Java code to illustrate
// isSet() method
  
import java.util.*;
public class Calendar_Demo {
    public static void main(String args[])
    {
  
        // Creating a calendar
        Calendar calndr = Calendar.getInstance();
  
        // Displaying the year
        System.out.println("The Current Year"
                           + " is: "
                           + calndr.get(
                                 Calendar.YEAR));
  
        // Decrementing the year
        // by 5
        calndr.roll(Calendar.YEAR, -5);
  
        // Displaying the result after the operation
        System.out.println("The New Year is: "
                           + calndr.get(
                                 Calendar.YEAR));
  
        // Incrementing the year
        // by 2
        calndr.roll(Calendar.YEAR, 2);
  
        // Displaying the result after operation
        System.out.println("The new year is: "
                           + calndr.get(
                                 Calendar.YEAR));
    }
}

输出：

The Current Year is: 2019
The New Year is: 2014
The new year is: 2016

示例 2：

// Java code to illustrate
// isSet() method
  
import java.util.*;
public class Calendar_Demo {
    public static void main(String args[])
    {
  
        // Creating a calendar
        Calendar calndr = Calendar.getInstance();
  
        // Displaying the month
        System.out.println("The Current Month"
                           + " is: "
                           + calndr.get(
                                 Calendar.MONTH));
  
        // Incrementing the month
        // by 3
        calndr.roll(Calendar.MONTH, 3);
  
        // Displaying the result after operation
        System.out.println("The New Month is: "
                           + calndr.get(
                                 Calendar.MONTH));
  
        // Decrementing the month
        // by 1
        calndr.roll(Calendar.MONTH, -1);
  
        // Displaying the result after operation
        System.out.println("The new month is: "
                           + calndr.get(
                                 Calendar.MONTH));
    }
}

输出：

The Current Month is: 2
The New Month is: 5
The new month is: 4

参考： https: Java/util/Calendar.html#roll(int, %20int)