Homework: "Exception in thread "main" java.lang.NullPointerException"

Question

Okay so I have a homework assignment. I thought I had everything good on it, and I am getting an

"Exception in thread "main" java.lang.NullPointerException at HW3.main<HW3.java:18>

That line is:

tempMembers[i/4].setFirstName(args[i]);

Now I am still pretty much a noob at programming, and everything up until this point I have done was in Eclipse. This program was to be created in a text editor and then compiled and ran at the command prompt. I do not know if maybe I am just inputting the arguments incorrectly or what not.

So for this error my command prompt entry was

java HW3 Bill Smith 2009 Football Jane Doe 2000 Tennis David Jones 1995 Baseball

So is the error in my code or in my input? And if the error is in my code can you point me in the correct direction? Like I said command line arguments are completely new to me, and my class doesn't do examples of any of this, just talks about concepts.

public class HW3 {


    public static void main(String[] args) throws Exception {

        if (args.length % 4 != 0) {
            throw new Exception(
                    "First Name, Last Name, Year Inducted, Sport not entered correctly");
        }

        HallOfFame hallOfFameList = new HallOfFame();
        hallOfFameList.setNumberOfMembers(args.length / 4);

        HallOfFameMember[] tempMembers = new HallOfFameMember[args.length / 4];


        for (int i = 0; i < args.length; i += 4) {
            tempMembers[i/4].setFirstName(args[i]);
            tempMembers[i/4].setLastName(args[i+1]);
            tempMembers[i/4].setYearInducted(Integer.parseInt(args[i+2]));
            tempMembers[i/4].setSport(args[i+3]);
        }

        hallOfFameList.setMembers(tempMembers);
        HallOfFameMember[] sortedMembers = null;
        hallOfFameList.sortMembers(sortedMembers);
        HallOfFame.printReport(sortedMembers);


    }


}


public class HallOfFameMember implements Comparable<HallOfFameMember> {
    private String firstName;
    private String lastName;
    private String sport;
    private int yearInducted;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getSport() {
        return sport;
    }

    public void setSport(String sport) {
        this.sport = sport;
    }

    public int getYearInducted() {
        return yearInducted;
    }

    public void setYearInducted(int yearInducted) {
        this.yearInducted = yearInducted;
    }

    @Override
    public int compareTo(HallOfFameMember o) {

        return this.getYearInducted() - o.getYearInducted();
    }

}


public class HallOfFame {
    private HallOfFameMember[] members;
    private int numberOfMembers;

    public HallOfFameMember[] getMembers() {
        return members;
    }

    public void setMembers(HallOfFameMember[] members) {
        this.members = members;
    }

    public int getNumberOfMembers() {
        return numberOfMembers;
    }

    public void setNumberOfMembers(int numberOfMembers) {
        this.numberOfMembers = numberOfMembers;
    }

    public void sortMembers(HallOfFameMember[] sortedMembers) {

        boolean bool = true;
        HallOfFameMember temp;

        while (bool) {

            bool = false;

            for (int i = 0; i < sortedMembers.length - 1; i++) {
                if (sortedMembers[i].compareTo(sortedMembers[i + 1]) > 0) {
                    temp = sortedMembers[i];
                    sortedMembers[i] = sortedMembers[i + 1];
                    sortedMembers[i + 1] = temp;
                    bool = true;

                }
            }
        }

    }

    public static void printReport(HallOfFameMember[] print) {
        System.out.println("Java Sports Hall of Fame Inductees\n\n");
        System.out.printf("%-30s\t%-30s\t%-30s\n", "Name", "Year Inducted",
                "Sport");
        for (int i = 0; i < print.length; i++)
            System.out.printf("%-30s\t%-30s\t%-30s\n", print[i].getLastName()
                    + "," + print[i].getFirstName(),
                    print[i].getYearInducted(), print[i].getSport());
    }

}

krock · Accepted Answer · 2011-09-25 02:31:05Z

5

You initialised your array on this line:

HallOfFameMember[] tempMembers = new HallOfFameMember[args.length / 4];

but the array will contain null elements. You need to construct each element in the array before you try to call methods on it:

tempMembers[i/4] = new HallOfFameMember();
tempMembers[i/4].setFirstName(args[i]);

answered Sep 25, 2011 at 2:31

krock

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wchargin · Accepted Answer · 2011-09-25 02:34:29Z

3

When you declare tempMembers, you initialize the array but not the elements. Add this to your for loop:

tempMembers[i / 4] = new HallOfFameMember();

Then you'll get another NPE because sortedMembers is null (as you are explicitly setting it to be).

I also notice some other strange areas in your code. For example, in HallOfFame#sortMembers(HallOfFameMember[]), why do you use a while loop that will run exactly once in any condition (setting bool to true beforehand and false during the loop)?

Change the last four lines to :

hallOfFameList.setMembers(tempMembers);
final HallOfFameMember[] sortedMembers = tempMembers;
hallOfFameList.sortMembers(tempMembers);
HallOfFame.printReport(sortedMembers);

to resolve the last exception and get the desired output.

answered Sep 25, 2011 at 2:34

wchargin

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2 Comments

Zankorel Over a year ago

Thank you. I got the sort error. Not sure exactly what you mean about the bool. But I did get an error. And your code fixed it. I had to write my own sort method. And I figured the way I set it up would work. Ill give it another look to make sense of it. Thank you again

wchargin Over a year ago

Sure thing! This is a help site, after all. Sorry about the boolean thing; I misunderstood the code. I hope your assignment works out!

Ryan Stewart · Accepted Answer · 2011-09-25 02:33:08Z

0

The exception means that tempMembers[i/4] evaluates to null, and when you try to call a method on null, you get a NullPointerException. In this case, you're getting it because you haven't actually put anything into tempMembers yet. In Java, creating an array doesn't populate it. Creating the array with new HallOfFameMember[args.length / 4] creates a new array of some length with all of its positions set to null. So after creating the array, you need to create and store a new HallOfFameMember in each array position.

answered Sep 25, 2011 at 2:33

Ryan Stewart

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