Java - 几个月和几天？

Question

First of, I have been searching to find anything close to my question but I couldn't find anything. I am not a very good programmer, I have just started to play with it a little, and my interest for it is growing a lot.

I was thinking if I could make a basic program with basic and easy understandable language, to a month and days "calculator".

Like if I have a sysout print which says: Write month number , and I'll type in 11 , and then write a day number in the month and someone writes 27 it will say date correct!

But if it asks me for month and I'll type 6 , as June and I write in 31 as days it will print which would say Month 6 doesn't have day 31 .

I want to keep it simple so I understand like basic language not too hard! I'm just a fresh starter!

Thanks for all help.

Answer 1

If you just want to get the job done, I'd suggest you go have a look at the Calendar API or perhaps JodaTime .

If you're more interested in learning how to do it yourself, here's one suggestion for an approach:

Hard-code an array like

int[] daysInMonths = { 31, 27, 31, ... };

and then check using something along the following lines:

// Get month and day from user
Scanner s = new Scanner(System.in);
int month = s.nextInt();
int day = s.nextInt();

int monthIndex = month - 1;     // since array indices are 0-based

if (1 <= day && day <= daysInMonths[monthIndex])
    print ok
else
    print does not exist

Answer 2

This program should get you going...

import java.util.*;

class Date {
    public static void main(String[] args) {
        int[] days = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        System.out.print("Enter a month number and a day number (separate using \';\':_");
        Scanner sc = new Scanner(System.in).useDelimiter(";");
        int month = sc.nextInt();
        int day = sc.nextInt();
        sc.close();
        System.out.println((day == days[month - 1]) ? "Date correct!" : "Date incorrect: Month " + month + " does not have day " + day);
    }
}

Answer 3

tl;dr

Month.of( 6 ).maxLength()

java.time

The modern approach uses the java.time classes that supplant the troublesome old Date / Calendar classes.

Use the Month enum.

Month month = Month.of( 6 ) ;  // Months numbered sanely, 1-12 for January-December.
if( dayOfMonthInput > month.maxLength() ) {
    … bad input
}

Of course for February the last day of month depends on leap year. So if you want to be precise you need to use YearMonth .

YearMonth ym = YearMonth.of( 2017 , 2 ) ;
if( dayOfMonthInput > ym.lengthOfMonth() ) {
    … bad input
}

---

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date , Calendar , & SimpleDateFormat .

The Joda-Time project, now in maintenance mode , advises migration to the java.time classes.

To learn more, see the Oracle Tutorial . And search Stack Overflow for many examples and explanations. Specification is JSR 310 .

Where to obtain the java.time classes?

• Java SE 8 , Java SE 9 , and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport .
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • See How to use ThreeTenABP… .

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval , YearWeek , YearQuarter , and more .

Answer 4

in Date class, the first month January is "0", ZERO. So you think June and you write 6, but must be 5; as

January 0
February 1
March 2
April 3
May 4
June 5

Answer 5

Leveraging the functionality of the Calendar class is a far better way to do this.

    int day = 31;
    int month = 6; // JULY

    Calendar calendar = Calendar.getInstance();
    if(month < calendar.getActualMinimum(Calendar.MONTH) 
            || month > calendar.getActualMaximum(Calendar.MONTH))
        System.out.println("Error - invalid month");
    else 
    {
        calendar.set(Calendar.MONTH, month);
        if (day <  calendar.getActualMinimum(Calendar.DAY_OF_MONTH) 
                || day > calendar.getActualMaximum(Calendar.DAY_OF_MONTH))
        {
            System.out.println("Error - invalid day for month");
        }
        else
        {
            System.out.println("Date is correct");
        }
    }
}

All you have to be aware of is that Calendar currently treats January as month 0.