需要帮助我的代码有错误

Question

创建一个名为Date的类。 使日期具有一个名为 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。 如果传递的值等于 1-12，则返回"月份无效"。查看所需的输出。将两个构造函数添加到 Date：一个采用整数月、日和年 另一个采用字符串月和整数日和年。添加一个 toString 方法，该方法返回的日期格式如下：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

线程"main"中的异常 java.lang.NullPointerException at Date.getMonthNumber(Date.java：32( 在日期。(日期.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;永远不会被赋值，因此，它是null

我"认为"您想从getMonthName方法中获取String[] name = {" ", "January", "February", "March", "April", "May", "June", "July", "August", "September", "October","November", "December"};并分配给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MONTH_NAMES识别为常量