Calendar類中的roll(int calndr_field，boolean up_down)方法用於通過上下移動傳遞的字段單個時間單位來對傳遞的日曆字段進行操作。這涉及在不更改較大字段的情況下對時間字段進行加法或減法。

用法:

public abstract void roll(int calndr_field, boolean up_down)

參數：該方法有兩個參數：

calndr_field：這是日曆類型，是指要對其進行操作的日曆字段。

up_down：這是布爾類型，用於指示是向上或向下移動calndr_field還是增大或減小。 true表示添加時間單位，false表示減去時間單位。

返回值：該方法不返回任何值。

以下示例程序旨在說明Calendar類的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));

}

}

輸出：

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));

}

}

輸出：

The Current Month is: 1

The New Month is: 2

The new month is: 1