比较和循环未按预期工作

Question

I am writing a program in Java, which basically find the number of days between 2 specific dates. The user inputs two dates in DD/MM/YYYY format, and I take the sub strings of the day/month/year and calculate the number of days between the 2. I also did a sort of validation, where if the format is wrong (for example d/mm/yyyy) then the person has to re-write the date in the right format. I did this by making sure that the / in the date are in their right position. If the 4 / match, then a boolean variable gives me true, and the program continues, if not it gives me false and it asks for the date again.

The problem is that it is always giving me false, and even then, it is not re-looping back.

import java.util.*;
 public class DateDifference {
 int day1, day2, month1, month2, year1, year2;
 String startdate, enddate, day1S, day2S, month1S, month2S, year1S, year2S;
 String datevalidation = "/";
 boolean dval;
public static void main(String args[]){
    DateDifference difference = new DateDifference();
}
DateDifference() {
    Calendar cal1 = Calendar.getInstance();
    Calendar cal2 = Calendar.getInstance();

    Scanner sc = new Scanner (System.in);
    do {
    System.out.println("Enter Start Date [dd/mm/yyyy]:");
    startdate = sc.next();
    System.out.println("Enter End Date [dd/mm/yyyy]:");
    enddate = sc.next();

    int l1 = startdate.length();
    int l2 = enddate.length();
    day1S = startdate.substring(0,2);
    month1S = startdate.substring(3,5);
    year1S = startdate.substring(6,l1);

    day2S = enddate.substring(0,2);
    month2S = enddate.substring(3,5);
    year2S = enddate.substring(6,l2);

    if (startdate.substring(2,3).equals(datevalidation) & startdate.substring(5,6).equals(datevalidation) & enddate.substring(2,3).equals(datevalidation) & enddate.substring(5,6).equals(datevalidation)) 
        dval = true;

    else 
        dval = false;
        System.out.println("Wrong date format. Try again");

   } while (dval = false);

   day1 = Integer.parseInt(day1S);
   month1 = Integer.parseInt(month1S);
   year1 = Integer.parseInt(year1S);
   day2 = Integer.parseInt(day2S);
   month2 = Integer.parseInt(month2S);
   year2 = Integer.parseInt(year2S);

    cal1.set(year1, month1 - 1, day1); 
    cal2.set(year2, month2 - 1,day2);
    System.out.println("Days= "+daysBetween(cal1.getTime(),cal2.getTime()));
}
public int daysBetween(Date cal1, Date cal2){
    return (int)( (cal2.getTime() - cal1.getTime()) / (1000 * 60 * 60 * 24));
}

}

Here are some images on how it is working:

When input is correct: http://vvcap.net/db/2100RYdaSM1T6EXLqgq5.png

When input is not correct: http://vvcap.net/db/9qSEwDKSTIdLAWmxS_Cy.png

When not correct, it just gets stuck when its trying to make 1/ in an integer, which I can understand.

Any help?

Thanks a lot.

Answer 1

Your problem, from what I see, is when you are taking the substring() of the dates. You are checking to see if the position equals "/" BUT you are taking 2 positions (hence the error showing that the string you are converting to and int is "1/" from the error code.

Try changing all the If statement line looks like the following:

From

 if (startdate.substring(2,3).equals(datevalidation) & startdate.substr...
       dval = true;
 ...

To

 if ( !startdate.substring(2,3).contains(datevalidation) &
      !startdate.substring(5,6).contains(datevalidation) &
      !enddate.substring(2,3).contains(datevalidation) &
      !enddate.substring(5,6).contains(datevalidation)) 
       dval = true;
 ...

AND the while() statement should be == not just = . You're setting it rather than checking on each iteration of the loop.

Rather than checking if 2 characters equals 1 character (eg "1/".equals("/")) you are now checking to see if those 2 characters DO NOT contain the separator.

Also you want want a checker or a try-catch block just in-case the two digits entered are not numeric to stop the NumberFormatExcpetion from returning.

Hope this helps

Answer 2

You have a typo in this row

 } while (dval = false);

operation dval = false is always false so your do .. while runs only once. I think you meant to use

 } while (dval == false);

to have real loop.

---

To verify the loop behavior you can run this test and look what happens:

@Test
public void testLoop() {

    int iteration = 0;
    boolean flag;
    do {
        flag = false; // unused assignment
        ++iteration; // iteration counter
        System.out.println( "Running iteration " + iteration );
    } while ( flag = false );

    System.out.println( "Loop finished. Total number of iterations is " + iteration );
}