Java：我的有效日期检查程序不适用于闰年

Question

目前，我正在编写一个程序，要求用户输入日、月、年。然后程序检查日期是否有效。但是，对于二月，当输入是闰年时，我无法让它输出正确的响应。

import javax.swing.*;

public class ValidDate{

public static void main(String[]args){

    int day,month,year;

    day = Integer.parseInt(JOptionPane.showInputDialog("Enter a day"));
    month = Integer.parseInt(JOptionPane.showInputDialog("Enter a month"));
    year = Integer.parseInt(JOptionPane.showInputDialog("Enter a year"));

    switch(month){  //this statement will check whether or not the days are valid
        case 1:
            if ( day >= 1 && day <= 31 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
            break;

        case 2:
            if ( day >= 1 && day <= 28 || month % 4 == 0 && day >= 1 && day <= 29 ){   //look at 
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 3:
            if ( day >= 1 && day <= 31 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 4:
            if ( day >= 1 && day <= 30 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 5:
            if ( day >= 1 && day <= 31 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 6:
            if ( day >= 1 && day <= 30 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 7:
            if ( day >= 1 && day <= 31 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 8:
            if ( day >= 1 && day <= 31 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 9:
            if ( day >= 1 && day <= 30 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 10:
            if ( day >= 1 && day <= 31 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 11:
            if ( day >= 1 && day <= 30 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }
        case 12:
            if ( day >= 1 && day <= 31 ){
                System.out.println("Date is valid");
            }
            else{
                System.out.println("Date is not valid");
            }//case ended
        }
    }
}

我输入闰年得到的输出是

Date is not valid
Date is valid
Date is valid
Date is valid
Date is valid
Date is valid
Date is valid

Answer 1

变化：

month % 4 == 0

到：

year % 4 == 0

如果您出于学习目的尝试此操作，那很好。否则，您可以使用以下方法验证闰年：

GregorianCalendar gc = new GregorianCalendar();
if (gc.isLeapYear(year)) 

GregorianCalendar

Answer 2

没有

休息;

语句，一旦找到正确的案例，它就会失败并执行下一个案例。除非您正在创建批量案例，否则请添加 break 语句 我所说的散装箱：

enum Day {
  MONDAY,
  TUESDAY,
  WEDNESDAY,
  THURSDAY,
  FRIDAY,
  SATURDAY,
  SUNDAY;

  public boolean isWorkDay() {
    switch(this) {
      case SATURDAY:
      case SUNDAY:
        return false;
      default:
        return true;
    }
  }
}