Array counter doesn't work with java

Question

I'm making a Java Program that uses JOptionPane about days. I'm not talking about date (10/14/2016 etc.), but days: Sunday, Monday etc.

My problem is the function that adds a specific number of days to the current day (e.g. today is Tuesday then add 5 days into it).

I'm using arrays to access/refer to a specific day for display and output purposes.

I've referenced Sunday to Saturday, with array indexes of 0 to 6 respectively.

The problem is that, assuming the current day is Saturday, if the user added 3 days into it, the program crashes.

I believe this crashes because since Saturday is located at index 6 and the algorithm attempted to access the 9th index, which doesn't exist. Since Java is a safe-to-go language, it doesn't display 'null', it decided to crash instead.

What's supposed to happen in that situation is that from Saturday, the program would display Tuesday, not:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 9

I'm using Netbeans as my IDE and I'm self learning java since my college teaches me c++, visual basic, c# for my current year and curriculum. If you want the details, then,

Here's the source code for the main class:

package weekdaysprogram;

import java.util.*;

public class weekdayParametersProgramMain 
{

    static Scanner console = new Scanner (System.in);

    public static void main(String[] args) 
    {
        weekdayParametersProgram firstObject = new weekdayParametersProgram();
        firstObject.inputMainMenu();
    }
}

Here's the source code for the second class:

package weekdaysprogram;

import javax.swing.JOptionPane;

public class weekdayParametersProgram
{
    int currentDay; //variable used for referencing a certain day

    String[] day = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}; //Array of days

    public void inputMainMenu() //Main menu function
    {
        int choice;

        String inputStr;
        String mainMenuStr = "Day Class\n"
                + "Enter choice: \n"
                + "1. Set Day \n"
                + "2. Print Day \n"
                + "3. Print Next Day \n"
                + "4. Print Previous Day \n"
                + "5. Calculate Day \n"
                + "6. Exit";

        inputStr = JOptionPane.showInputDialog(mainMenuStr);
        choice = Integer.parseInt(inputStr);

        switch (choice)
        {
            case 1:
                setDay();
                break;
            case 2:
                printDay();
                break;
            case 3:
                printNextDay();
                break;
            case 4:
                printPreviousDay();
                break;
            case 5:
                calculateDay();
                break;
            case 6:
                exit();
                break;
            default:
                JOptionPane.showMessageDialog(null, "Error Try Again", "Error", JOptionPane.ERROR_MESSAGE);
                inputMainMenu();
                break;
        }
    }


    public void calculateDay() //the 5th function I'm getting an error at aka the culprit of my error
    {
        String message = "Current Day: " + currentDay + " - " + day[currentDay] + "\n"
                + "Please enter the amount of days to be added: ";

        int tempNum1 = currentDay;
        String tempNum2 = JOptionPane.showInputDialog(message);
        int tempNum3 = Integer.parseInt(tempNum2);

        for (int count=0; count <= tempNum3; count++)
        {
            if (count == 7) tempNum1 = 0;
            else if (count==0) continue; //this is a patch, since there would be a confusion if the user inputted 1 for the addtional day
            else tempNum1++;
        }

        String nextMessage = day[currentDay] + " + " + tempNum3 + " days = " + day[tempNum1];

        JOptionPane.showMessageDialog(null, nextMessage, "New Day", JOptionPane.INFORMATION_MESSAGE);

        inputMainMenu();
    }

    public void setDay() //1st function for set day
    {

        String inputStr;

        String message = "Enter day index: \n"
                + "0 = " + day[0] + "\n"
                + "1 = " + day[1] + "\n"
                + "2 = " + day[2] + "\n"
                + "3 = " + day[3] + "\n"
                + "4 = " + day[4] + "\n"
                + "5 = " + day[5] + "\n"
                + "6 = " + day[6] + "\n";
        inputStr = JOptionPane.showInputDialog(message);

        switch (inputStr)
        {
            case "0":
                currentDay = 0;
                break;
            case "1":
                currentDay = 1;
                break;
            case "2":
                currentDay = 2;
                break;
            case "3":
                currentDay = 3;
                break;
            case "4":
                currentDay = 4;
                break;
            case "5":
                currentDay = 5;
                break;
            case "6":
                currentDay = 6;
                break;
            default:
                JOptionPane.showMessageDialog(null, "Please try again.", "Error", JOptionPane.ERROR_MESSAGE);
                setDay();
                break;
        }

        inputMainMenu();
    }

    public void printDay() //2nd function for printing out the current day
    {
        JOptionPane.showMessageDialog(null, day[currentDay], "Current Day", JOptionPane.INFORMATION_MESSAGE);

        inputMainMenu();
    }

    public void printNextDay() //3rd function for printing out the next day
    {
        int tempNum = currentDay;

        if (tempNum == 6) tempNum = 0;
        else tempNum += 1;

        JOptionPane.showMessageDialog(null, day[tempNum], "Next Day", JOptionPane.INFORMATION_MESSAGE);

        inputMainMenu();
    }

    public void printPreviousDay() //4th function for printing out the previous day
    {
        int tempNum = currentDay;

        if (tempNum == 0) tempNum = 6;
        else tempNum -= 1;

        JOptionPane.showMessageDialog(null, day[tempNum], "Previous Day", JOptionPane.INFORMATION_MESSAGE);

        inputMainMenu();
    }

    public void exit()
    {
        System.exit(0);
    }
}

Answer 1

The problem is in calculateDay() method

Lets discus your code line by line.

Suppose currentDay=5 before entering calculateDay() method

Now read the comments carefully inside the code

    String message = "Current Day: " + currentDay + " - " + day[currentDay] + "\n"
            + "Please enter the amount of days to be added: ";//here currentDay =6

    int tempNum1 = currentDay;//tempNum1=6
    String tempNum2 = JOptionPane.showInputDialog(message);//suppose input "3"
    int tempNum3 = Integer.parseInt(tempNum2);//tempNum3 = 3

    for (int count=0; count <= tempNum3; count++)
    {
        if (count == 7) tempNum1 = 0;
        else if (count==0) continue; 
        else tempNum1++;//for count value 1,2,3 tempNum1 will increase so it will be 7, 8 finally 9. So tempNum1=9
    }

    String nextMessage = day[currentDay] + " + " + tempNum3 + " days = " + day[tempNum1];//here tempNum1=9 So ArrayIndexOutOfBound occurs

To avoid this error add another checking like you did in your other methods

    JOptionPane.showMessageDialog(null, nextMessage, "New Day", JOptionPane.INFORMATION_MESSAGE);


        for (int count=0; count <= tempNum3; count++)
        {
            if (count == 7) tempNum1 = 0;
            else if (count==0) continue; 
            else {
                tempNum1++

                if(tempNum1=7) tempNum=0;
            }
        }

Answer 2

In the calculateDay() method the execution can fail for some inputs.

• If the user enters "6", tempNum3 will be the integer 6.
• In the for-loop, count will start at 0 and loop until tempNum3, which is 6.
• For values of count from 1 to 6, tempNum1 will be incremented each time.
• tempNum1 was initialized with currentDay (which could have been also 6) and tempNum1 will have a value greater than 6, leading to an IndexOutOfBoundsException when the String nextMessage is built.

Edit: Loris' answer also provides a solution (use the modulo operator), which I suggest you do everytime you change an index of the days array. (Be careful with negative values though)

Answer 3

Instead of

    for (int count=0; count <= tempNum3; count++)
    {
        if (count == 7) tempNum1 = 0;
        else if (count==0) continue; //this is a patch, since there would be a confusion if the user inputted 1 for the addtional day
        else tempNum1++;
    }

you could use

tempNum1 = (currentDay + tempNum3) % 7;

The % is the modulo operator, so the result will always be between 0 and 6.