Java Program for Leap Years in a Range

Question

I cannot seem to figure out why it will not print more than one year in a range. I've got everything except that up and running so far.

The starting year cannot be before 1752 and the ending year cannot be past 9999. I need the program to find all of the leap years in between the chosen starting year and the chosen ending year and then print all of those leap years off for the user.

Everything seems to work fine until I added the else if at the end to print a message if there are no years in the given range. I would just remove it if I could but that message is a part of the requirements for my assignment.

import java.util.Scanner;
class Final_Project_Mendenhall {
  public static void main(String[] args)
    {
        int startYear = 1752;
        int endYear = 9999;
        int i;
        int j = 0;
 
        Scanner in = new Scanner(System.in); //Scanner object to get user input for Start Year
        System.out.print("Enter the Start Year (Greater or equal to 1752):");
        startYear = in.nextInt(); //Get Start Year from user
            while (startYear < 1752){
            System.out.println("Enter a valid start year (Greater or equal to 1752):");
            startYear = in.nextInt(); //Get valid Start Year from user
        } //End while
        
        System.out.print("Enter the End Year (Less than 9999):");
        endYear = in.nextInt(); //Get End Year from user
            while (endYear > 9999){
            System.out.println("Enter a valid end year (Less than 9999):");
            endYear = in.nextInt(); //Get valid End Year from user
        } //End while
       
        System.out.println("Leap years:"); //Print Leap Years
 
        for (i = startYear; i <= endYear; i++){ //Loop through years between Start and End Years
          if ((0 == i % 4) && (0 == i % 400)){ //Find and print Leap Years, if there are
            System.out.println(i);
            j++;
          }//End if
        //End for
         else if(j == 0){
            System.out.println("There are no leap years in that range");
        }//End if
      }
      }
    }

Answer 1

Your condition for the leap year is not correct.

Given below are the conditions for a year to qualify as a leap year:

1. Either divisible by 400 -OR-
2. Divisible for 4 but not by 100

Replace

if ((0 == i % 4) && (0 == i % 400))

with

(0 == i % 400) || (0 == i % 4 && 0 != i % 100)

For production code, I also recommend you use the OOTB java.time API:

for (i = startYear; i <= endYear; i++) {
    if (java.time.Year.isLeap(i)) {
        System.out.println(i);
    }
}

---

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.[2]

— United States Naval Observatory

Source: https://en.wikipedia.org/wiki/Gregorian_calendar

Answer 2

It seems as if the specific line of code you need to fix is this:

if ((0 == i % 4) && (0 == i % 400))

If we analyze this, you can see that only if i is divisible by 400, this will return true. If you change it to this:

if ((0 == i % 4 && 0 != i % 100) || (0 == i % 400))

you should see it work. Cleaning up the code we get:

import java.util.Scanner;
class Final_Project_Mendenhall {
  public static void main(String[] args)
    {
        int startYear = 1752;
        int endYear = 9999;
        int i;
        int j = 0;
 
        Scanner in = new Scanner(System.in);
        System.out.print("Enter the Start Year (Greater or equal to 1752):");
        startYear = in.nextInt(); //Get Start Year from user
        while (startYear < 1752){
            System.out.println("Enter a valid start year (Greater or equal to 1752):");
            startYear = in.nextInt(); //Get valid Start Year from user
        }
        
        System.out.print("Enter the End Year (Less than 9999):");
        endYear = in.nextInt(); //Get End Year from user
        while (endYear > 9999){
            System.out.println("Enter a valid end year (Less than 9999):");
            endYear = in.nextInt(); //Get valid End Year from user
        } //End while
       
        System.out.println("Leap years:"); //Print Leap Years
 
        for (i = startYear; i <= endYear; i++){
          if ((0 == i % 4 && 0 != i % 100) || (0 == i % 400)){
            System.out.println(i);
            j++;
          }
        }
        if(j == 0){
            System.out.println("There are no leap years in that range");
        }
    }
}

Answer 3

if ((0 == i % 4) && (0 == i % 400))

searches for is divisible by 4 and 400 at the same time, so practically 400.
What you presumably wanted to check for are is divisible by 4, but not divisible by 100, unless they are divisible by 400. One way to write that would be

if (((0 == i % 4) && (0 != i % 100)) || (0 == i % 400))

And that no-leap-year-found message should be after the loop, not in an else if. You can detect that nothing is found only after checking the entire range:

for (i = startYear; i <= endYear; i++){ //Loop through years between Start and End Years
  if (((0 == i % 4) && (0 != i % 100)) || (0 == i % 400) { //Find and print Leap Years, if there are
    System.out.println(i);
    j++;
  } //End if
} //End for <--- the } was missing
if(j == 0) { // <--- no else
  System.out.println("There are no leap years in that range");
}