Question

我有课程日期，只是简单地创建一周中几天的数组列表。

import java.util.*;

public class Date {

    public static final String[] daysofTheWeek = {"Mon" , "Tue" , "Wed" , "Thurs" , "Fri" , "Sat" , "Sun"};
    private String dayofweek;
    private int day;
    private int month;

    public int getDate() {
        return this.day;
    }

    public int getMonth() {
        return this.month;
    }

    public Date(String dayofweek, int day, int month) {
        this.day = 1;
        if (day >= 1 && day <= 31) {
            this.day = day;
        }

        this.month = 12;
        if (month >= 1 && month <= 12) {
            this.month = month;
        }

        this.dayofweek = "Mon";
        if (isValidDOW(dayofweek)) { // why is this in the parameters?
            this.dayofweek = dayofweek;
        }
    }

    // must validate is DOW 
    //  TRUE/FALSE is the day of the week matching?
    public boolean isValidDOW(String d) {
    for (int i = 0; i < dayofweek.length(); i++)
        if (d.equals(daysofTheWeek[i])) { // why i?
            return true;
        }
        return false;
    }

    // to string
    public static String monthToString(int i) {
        switch(i) {
            case 1: return "January";
            case 2: return "Feb";
            case 3: return "March";
            case 4: return "April";
            case 5: return "May";
            case 6: return "June";
            case 7: return "July";
            case 8: return "August";
            case 9: return "September";
            case 10: return "October";
            case 11: return "November";
            case 12: return "December";
            default: return "Invalid Month!!";
        }
    }

    public String toString() {
        return this.dayofweek + "," + Date.monthToString(this.month) + " " + this.day;
    }

    public String getDayOfWeek() {
        return this.dayofweek;
    }
}

现在我正在努力争取所有的日子（比如，返回弹出的次数）。我尝试使用循环来计算所有内容，但我得到一个空指针异常。我一直在看这个问题，并且无法弄清楚它有什么问题。任何人都可以指导我朝正确的方向发展吗？

import java.util.*;
/**
 * ArrayList example
 */
public class Calendar {

    private ArrayList<Date> Schedule;

    public ArrayList<Date> sundays(String dayofweek) {
        ArrayList<Date> app = new ArrayList<Date>();
        for (Date entry : Schedule) {
            String d = entry.getDayOfWeek();
            if (entry.getDayOfWeek() == dayofweek) {
                app.add(entry);
            }
            else if (app.size() == 0) {
                return null;
            }
        }
        return app;
    }
}

Answer 1

您永远不会初始化字段Schedule，因此默认初始化为null。因此，行for (Date entry:Schedule)（用于调用Schedule.iterator()的语法糖并迭代生成的迭代器）会抛出NullPointerException。

此外 - Java中的非静态字段名称应以小写字母开头。您应该将Schedule更改为schedule。（或者您可以遵循其中一个更具体的命名约定。有些项目使用f启动所有字段名称 - 在您的情况下fSchedule;其他项目使用{{启动所有字段名称1}} for＆＃34; member＆＃34; - 在您的情况下，m。如果您采用其中一种惯例，请务必保持一致。）

Answer 2

我认为方法isValidDOW应该是：

public boolean isValidDOW(String d){
    for  (int i=0; i < daysofTheWeek.length; i++)
        if ( d.equals(daysofTheWeek[i])) // why i?
        {
            return true;
    }
    return false;
}

使用 daysofTheWeek.length 而不是 dayofweek.length（），例如，如果dayofweek =＆＃39; Sat＆＃39;，for循环将只执行<强> 3 次，但周六＆＃39;在ArrayList daysofTheWeek中排在第6位，值 5 。

Arraylist的空指针异常？

2 个答案: