Question

求大神看一下，我这个JAVA程序哪错了？

中国有句俗语叫“三天打鱼两天晒网”。某人从1990年1月1日起开始“三天打鱼两天晒网”，问这个人在以后的某一天中是“打鱼”还是“晒网”。
import java.util.Scanner;
public class kl {
private static final Scanner input = null;
public static void main(String[] args) {
Scanner scanner=new Scanner(System.in);
System.out.println("请输入日期（年月日之间用空格隔开，如1987 10 13：");
scanner.nextInt();
final int BEFORE_YEAR = 1990;
final int BEFORE_MONTH = 1;
final int BEFORE_DAY = 1;
int year;
int month;
int day;
String whatToDo = null;
System.out.println("想知道某人何时打鱼何时晒网吗？");
while (true) {
year = Integer.parseInt(input.next());
month = Integer.parseInt(input.next());
day = Integer.parseInt(input.next());
if ((year >= 1990) && (month >= 1 && month <= 12) && (day >= 1 && day <= kl.maxDayOfYearAndMonth(year, month))) {
break; }
else {
System.err.println("输入的日期错误！请重新输入！"); } }
for (int i = BEFORE_YEAR, dayCal = 0; i <= year; i++) {
for (int j = BEFORE_MONTH; j <= month; j++) {
for (int k = BEFORE_DAY; k <= kl .maxDayOfYearAndMonth(i, j); k++) {
dayCal++; if (dayCal <= 3) {
whatToDo = "打鱼"; }
else if (dayCal <= 5) {
whatToDo = "晒网"; }
if (dayCal == 5) { dayCal = 0; }
if (i == year && j == month && k == day) {
break; } } } }
System.out.println(year + "年" + month + "月" + day + "日这天是在" + whatToDo + "！");
} @SuppressWarnings("unused")
private String next() {
return null;}
public static int maxDayOfYearAndMonth(int year, int month) {
int maxDay = 0; switch (month) {
case 1: case 3: case 5: case 7: case 8: case 10: case 12: maxDay = 31;
case 4: case 6: case 9: case 11: maxDay = 30;
case 2: if (isLeapYear(year)) {
maxDay = 28; }
else {
maxDay = 29; } }
return maxDay;
} /** * 判断是否润年 * * @param year * 年 * @return 润年：true */
private static boolean isLeapYear(int year) {
if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
return true; }
else {
return false;
} } {}{}}

Answer 1

你的case12 后面应该加一个break要不然怎么做maxDay都是等于30

追问

怎么修改呢？

追答

case 1: case 3: case 5: case 7: case 8: case 10: case 12: maxDay = 31;break;
case 4: case 6: case 9: case 11: maxDay = 30; break;

追问

请输入日期（年月日之间用空格隔开，如1987 10 13：
1987 10 13
想知道某人何时打鱼何时晒网吗？
Exception in thread "main" java.lang.NullPointerException
at com.founder.hdbeacon.hhhh.main(hhhh.java:19)
还是不对

Answer 2

package com.founder.hdbeacon;

import java.util.Scanner;
public class kl {
private static final Scanner input = null;
public static void main(String[] args) {
Scanner scanner=new Scanner(System.in);
System.out.println("请输入日期（年月日之间用空格隔开，如1987 10 13：");
int input = scanner.nextInt();
Scanner scanner1=new Scanner(System.in);
int input1 = scanner1.nextInt();
Scanner scanner2=new Scanner(System.in);
int input2 = scanner2.nextInt();
final int BEFORE_YEAR = 1990;
final int BEFORE_MONTH = 1;
final int BEFORE_DAY = 1;
int year;
int month;
int day;
String whatToDo = null;
System.out.println("想知道某人何时打鱼何时晒网吗？");
year = input;
month = input1;
day = input2;
while (true) {

if ((year >= 1990) && (month >= 1 && month <= 12) && (day >= 1 && day <= kl.maxDayOfYearAndMonth(year, month))) {
break; }
else {
System.err.println("输入的日期错误！请重新输入！");
System.out.println("请重新输入日期（年月日之间用空格隔开，如1987 10 13：");
Scanner scanner3=new Scanner(System.in);
year = scanner3.nextInt();
Scanner scanner4=new Scanner(System.in);
month = scanner4.nextInt();
Scanner scanner5=new Scanner(System.in);
day = scanner5.nextInt();
}

}
for (int i = BEFORE_YEAR, dayCal = 0; i <= year; i++) {
for (int j = BEFORE_MONTH; j <= month; j++) {
for (int k = BEFORE_DAY; k <= kl .maxDayOfYearAndMonth(i, j); k++) {
dayCal++; if (dayCal <= 3) {
whatToDo = "打鱼"; }
else if (dayCal <= 5) {
whatToDo = "晒网"; }
if (dayCal == 5) { dayCal = 0; }
if (i == year && j == month && k == day) {
break; } } } }
System.out.println(year + "年" + month + "月" + day + "日这天是在" + whatToDo + "！");
} @SuppressWarnings("unused")
private String next() {
return null;}
public static int maxDayOfYearAndMonth(int year, int month) {
int maxDay = 0; switch (month) {
case 1: case 3: case 5: case 7: case 8: case 10: case 12: maxDay = 31;
case 4: case 6: case 9: case 11: maxDay = 30;
case 2: if (isLeapYear(year)) {
maxDay = 28; }
else {
maxDay = 29; } }
return maxDay;
} /** * 判断是否润年 * * @param year * 年 * @return 润年：true */
private static boolean isLeapYear(int year) {
if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
return true; }
else {
return false;
} } {}{}}

Answer 3

首先呢，忍不住说一个话题外的问题，你的class类名首字母应该大写。好了，附上代码一份，本来想上传图片的，传不上

private static final Scanner input = null;//这个input声明了而没实例化
public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.println("请输入日期（年月日之间用空格隔开，如1987 10 13：");
//                scanner.nextInt();     //这句不要，否则会取不到输入的年份值
                final int BEFORE_YEAR = 1990; 
                final int BEFORE_MONTH = 1; 
                final int BEFORE_DAY = 1; 
                int year; 
                int month; 
                int day; 
                String whatToDo = null; 
                System.out.println("想知道某人何时打鱼何时晒网吗？"); 
                while (true) { 
//                    year = Integer.parseInt(input.next()); //上面说了input没实例
//                    month = Integer.parseInt(input.next()); 
//                    day = Integer.parseInt(input.next()); 
                    year = scanner.nextInt();//程序执行到这里会等待用户输入数据
                    month = scanner.nextInt();//所以前面那句scanner.nextInt();没必要
                    day = scanner.nextInt();
                    if ((year >= 1990) && (month >= 1 && month <= 12) && (day >= 1 && day <= kl.maxDayOfYearAndMonth(year, month))) { 
                        break; 
                    } else { 
                    System.err.println("输入的日期错误！请重新输入！"); 
                    } 
                } 
}

就到这里了，虽然后面还有个异常，没想影响程序执行完

追问

又死循环了

追答

建议你在程序旁边打上断点，debug as java application，然后按F6一步一步去调试

Answer 4

switch 没有返回 导致不管是几月 都是30天

追问

怎么修改呢？

追答

case 1: case 3: case 5: case 7: case 8: case 10: case 12: maxDay = 31;
break;
case 4: case 6: case 9: case 11: maxDay = 30;
break;

追问

不对

追答

应该不会吧