Java String Equals() method and multidimensional array

Question

Here is what I'm trying to do:

Write an application that stores at least four different course names and meeting days and times in a two-dimensional array. Allow the user to enter a course name (such as "CS 110") and display the day of the week and time that the course is held (such as Th 3:30). If the course does not exist, display an error message.

Here's the code I have:

import javax.swing.JOptionPane;
import java.util.Scanner;

public class Schedule {
    public static void main(String[] args) {
        //declare variables and arrays
        final int NUM_RANGES = 3;
        int sub = NUM_RANGES - 1;

        String[][] classNames = {
            {"CS 2401", "TS 5697", "UO 7896"},
            {"Tue 8:30", "Thu 7:30", "Fri 9:15" }
        };

        //get input
        String classInput = JOptionPane.showInputDialog("Please input a class name: ");

        //match to output and print
        while(sub >= 0)
            --sub;

        if (classInput.equals(classNames[0])) {
            JOptionPane.showMessageDialog(null, "Class time is: " + classNames[0][0]);
            System.exit(0);
        } else if (classInput.equals("TS 5697")) {
            JOptionPane.showMessageDialog(null, "Class time is: " + classNames[1][1]);
            System.exit(0);
        } else if (classInput.equals("UO 7896")) {
            JOptionPane.showMessageDialog(null, "Class time is: " + classNames[2][2]);
            System.exit(0);
        } else {
            JOptionPane.showMessageDialog(null, "Please enter a valid class     name.");
            System.exit(0);
        }
    }
}

As you can see, I'm trying to do the equals method on the first IF but can't figure it out. Thank you for your help.

I tagged this question with 'homework' as it's obviously that... — home
– home, Commented Nov 12, 2011 at 17:41
As a side note, it seems like the sub variable is redundant and can be removed. It will be assigned the value -1 (after the while(sub >= 0) loop has completed) and then it is never used again. — matsev
– matsev, Commented Nov 12, 2011 at 17:44
What is sub good for, does not make sense to me. Your whole approach is fairly static - just retrieve user input, iterate through the array of courses and compare them with user input (e.g. equals or equalsIgnoreCase). — home
– home, Commented Nov 12, 2011 at 17:49

ruakh · Accepted Answer · 2011-11-12 17:27:48Z

1

You need classNames[0][0] rather than bare classNames[0].

But really, you shouldn't be using a two-dimensional array for this; you should either use a Map<String, String> to map from course-names to meeting-times, or better yet, create a small Course class that contains both the course-name and the meeting-time (and any other necessary information), and use a Map<String, Course> to map from course-names to instances of Course.

answered Nov 12, 2011 at 17:27

ruakh

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Comments

ben · Accepted Answer · 2011-11-12 17:27:31Z

0

In a 2d array (arr) you've got to specify the row and column of the index you want to access (arr[row][column]). Every time. So it looks like you need:

if (classInput.equals(classNames[0][0]))

answered Nov 12, 2011 at 17:27

ben

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Comments

itun · Accepted Answer · 2011-11-12 17:33:54Z

0

Every Java object inherits from java.lang.Object which has toString() method. Which returns String of the object. every class can override it so a new toString() will return a desirable string. But as default it returns some runtime code of this Object.

All in all, your classNames[0] converts to String. Do I catch your doubts?

But you have an error here:

else if (classInput.equals("UO 7896")) {
    JOptionPane.showMessageDialog(null, "Class time is: " + classNames[2][2]);
    System.exit(0); }

You don't have classNames[2].

answered Nov 12, 2011 at 17:33

itun

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Comments

viktor · Accepted Answer · 2011-11-12 17:34:44Z

0

You can't compare String with String[] this way. It requires you to iterate through all elements.

for(String className : classNames[0]) {
   if(className.equals(classInput)) {
      //found ... do something
   }
}

You should use Map<String, String> for Key-Value then code would be simple. Also if you have String constants use them on left side of equals "Constant string".equals(var) this guarantees that NullPointerException never happens.

answered Nov 12, 2011 at 17:34

viktor

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Dimitry · Accepted Answer · 2011-11-12 17:38:29Z

0

Hey equals is case sensitive try using equalsIgnoreCase. Also use trim() to trim the string so it doesn't have white spaces.

answered Nov 12, 2011 at 17:38

Dimitry

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