Calendar类中的roll(int calndr_field，boolean up_down)方法用于通过上下移动传递的字段单个时间单位来对传递的日历字段进行操作。这涉及在不更改较大字段的情况下对时间字段进行加法或减法。

用法:

public abstract void roll(int calndr_field, boolean up_down)

参数：该方法有两个参数：

calndr_field：这是日历类型，是指要对其进行操作的日历字段。

up_down：这是布尔类型，用于指示是向上或向下移动calndr_field还是增大或减小。 true表示添加时间单位，false表示减去时间单位。

返回值：该方法不返回任何值。

以下示例程序旨在说明Calendar类的roll()方法的用法：

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

// false indicates subtraction

calndr.roll(Calendar.YEAR, false);

// Displaying the result after operation

System.out.println("The New Year is: "

+ calndr.get(

Calendar.YEAR));

// Incrementing the year

// true indicates addition

calndr.roll(Calendar.YEAR, true);

// 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: 2018

The new year is: 2019

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

// true indicates addition

calndr.roll(Calendar.MONTH, true);

// Displaying the result after operation

System.out.println("The New Month is: "

+ calndr.get(

Calendar.MONTH));

// Decrementing the month

// false indicates subtraction

calndr.roll(Calendar.MONTH, false);

// Displaying the result after operation

System.out.println("The new month is: "

+ calndr.get(

Calendar.MONTH));

}

}

输出：

The Current Month is: 1

The New Month is: 2

The new month is: 1