Help with standard deviation assignment

Question

I am having trouble with the std deviation portion of one of my assignments. Plus, only 5 test scored will print, not all 6. Could you please take a look and see what I am doing wrong? Is there a way to add the find the std dev. and mean for each semester?
----------------------Start Code------------------------------
public class Main {
    int grades[][]={{15,20,15,20,15,20},
    {14,21,14,21,14,21},{13,22,13,22,13},
    {14,21,14,21,14,21},{15,20,15,20,15}};
    int students, tests;
    String output;
    /** Creates a new instance of Main */
    public Main() {
     * @param args the command line arguments
    public static void main(String[] args) {
        Main m = new Main();
        m.students = m.grades.length;
        m.tests = m.grades[0].length;
        m.output = "The scores are:\n";
        m.buildString();
        m.output += "\nMean:       " + m.mean();
// +                "\nStd Dev:   " + m.StdDev() + "\n";
        System.out.print(m.output);
    public String mean(){
        double mean = 0;
        String outputMean = "";
        for (int column = 0; column < students; column++){
        int sumOfGrades = 0;
            for (int row = 0; row < students; row++){
                sumOfGrades += grades[row][column];
            mean = sumOfGrades/students;
            outputMean += mean + "   ";
        return outputMean;
//    public double StdDev(){
//        Main m = new Main();
//        int sumOfGrades = 0;
//        double StdDev = 0;
//        for (int column = 0; column < students; column++){
//            for (int row = 0; row < students; row++){
//                sumOfGrades += Math.pow((grades[row][column]-mean()), 2);
//            StdDev = Math.sqrt(mean/students) + "   ";
//        return StdDev;
    public void buildString(){
        output += "           |       Fall        |       Spring       |\n";
        output += "           ";
        for (int counter = 0; counter < tests; counter++)
            output += "Test " + (counter + 1) + " ";
        for (int row = 0; row < students; row++){
            output += "\nStudent(" + (row + 1)  + ")   ";
            for (int column = 0; column < tests -1; column++)
                output += grades[row][column] + "     ";
----------------------end code-----------------------------
------------------ Displays----------------
The scores are:
           |       Fall        |       Spring       |
           Test 1 Test 2 Test 3 Test 4 Test 5 Test 6
Student(1)   15     20     15     20     15    
Student(2)   14     21     14     21     14    
Student(3)   13     22     13     22     13    
Student(4)   14     21     14     21     14    
Student(5)   15     20     15     20     15    
Mean:       14.0   20.0   14.0   20.0   14.0 
-----------------end displays----------------

Answer 1

What I was thinking was that mean() would be a lot more useful if
(1) you passed it an argument saying what test you were interested in. This would
take the place of "column" in your code.
(2) it returned a double value which was the mean of the given test. The snippet you
posted is OK, except that it should return a double not an int. Ie it should look like:    /** Returns the mean for a given test. */
public double mean(int column) {
    double mean = 0.0;
    // do calculation
    return mean;
}Of course if you do this you will have to change how mean() is used from within main().
Instead of a simple "m.output += "\nMean: " + m.mean();" you will need a loop:    // start the string
m.output += "\nMean:  ";
for(int ndx = 0; ndx < tests; ++ndx) {
        // append each mean
    m.output += "  " + mean(ndx);
}The standard deviation method will be very similar to the mean() one, and
used in a similar way:    /** Returns the sd for a given test. */
public double mean(int column) {
    double sd = 0.0;
    // do calculation based on what was posted,
    // or what you have been told to do.
    return sd;
}