java - 需要帮助我的代码有错误

Question

创建一个名为Date的类。 Make Date具有一个名为getMonthName的静态方法，该方法采用一个整数，然后返回相应月份名称的String表示形式。数字1-12将分别映射到1月至12月。如果传递的值超出1-12，则返回“无效月份”。
查看所需的输出。在Date上添加两个构造函数:一个构造函数使用月，日和年的整数；另一个构造函数使用字符串月，日和年的整数。添加一个toString方法，该方法返回日期的格式如下:月日，年

我的代码如下

public class Date
{
private static String[] monName;
private int month, day, year;

public static String getMonthName(int num)
{
    if(num < 1 || num > 12)
    {
        return "invalid month";
    }
    else
    {
        String[] name = {" ", "January", "February", "March", "April", "May"            
    , "June", "July", "August", "September", "October","November",   "December"};
        return name[num];
    }
}     

public static int getMonthNumber(String monthName)
{
    for(int n = 1; n <= 12; n++)
    {
        if (monthName.equalsIgnoreCase(monName[n]))
            return n;
    }
    return -1;
}

public Date(int month, int day, int year)
{
    this.month = month;
    this.day = day;
    this.year = year;
}     

public Date (String month, int day, int year)
{
    this(day, getMonthNumber(month), year);
}   

public String toString()
{

    String m = month + day + ", " + year;
    return m;
}
}

运行我的代码的测试代码是

import java.util.Random;
public class DateDemo
{
public static void main(String[] args)
{
    System.out.println("-- Number Tests --");
    Random r = new Random(0);
    for (int x = 0; x < 50; x++)
    {
        Date d = new Date(Math.abs(r.nextInt() % 12) + 1, //month
                          Math.abs(r.nextInt()) % 28, //day
                          1900 + r.nextInt() % 100); //year
        System.out.println(d);
    }

    System.out.println("-- Month Name Tests --");
    System.out.println(new Date("January", 1, 1970));
    System.out.println(new Date("February", 1, 1970));
    System.out.println(new Date("March", 1, 1970));
    System.out.println(new Date("April", 1, 1970));
    System.out.println(new Date("May", 1, 1970));
    System.out.println(new Date("June", 1, 1970));
    System.out.println(new Date("July", 1, 1970));
    System.out.println(new Date("August", 1, 1970));
    System.out.println(new Date("September", 1, 1970));
    System.out.println(new Date("October", 1, 1970));
    System.out.println(new Date("November", 1, 1970));
    System.out.println(new Date("December", 1, 1970));

}
}

我得到的错误是

Exception in thread "main" java.lang.NullPointerException at Date.getMonthNumber(Date.java:32) at Date.(Date.java:47) at DateDemo.main(DateDemo.java:19)

并且“月份”不显示，仅显示日期和年份。请帮忙!!

Answer 1

private static String[] monName;从未分配值，因此它是null
我“认为”您想从String[] name = {" ", "January", "February", "March", "April", "May", "June", "July", "August", "September", "October","November", "December"};方法中获取getMonthName，然后分配给monName ...

private static final String[] MONTH_NAMES = String[] name = {
            " ", 
            "January", 
            "February", 
            "March", 
            "April", 
            "May", 
            "June", 
            "July", 
            "August", 
            "September", 
            "October",
            "November",
            "December"};

是的，您必须重命名 monName，因为人们会认识到 MONTH_NAMES是一个常量