Java 日历 roll()方法及实例

Calendar类中的 roll(int calndr_field , boolean up_down ) 方法用于对传递的日历字段进行操作，将传递的字段向上或向下移动一个单位的时间。这涉及到时间字段的加法或减法，而不改变更大的字段。

语法

public abstract void roll(int calndr_field, boolean up_down)

参数： 该方法需要两个参数。

• calndr_field : 这是日历类型的，指的是要操作的日历字段。
• up_down : 这是一个布尔类型的参数，用于指示是否向上或向下移动calndr_field，或增加或减少。true表示增加一个时间单位，false表示减去同样的时间单位。

返回值： 该方法不返回任何值。

下面的程序说明了日历类的roll()方法的工作原理：

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

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

参考资料: https://docs.oracle.com/javase/8/docs/api/java/util/Calendar.html#roll-int-boolean-