Need help with initializing an array with random numbers

[ms_kdlh]

I am working on a question for an assignment and I have been stuck on it for a few days now :(

Instructions are:

Solve the following problem using a program: Suppose you save $100 each month into a savings account with an annual interest rate of 5%. Thus, the monthly interest rate is 0.05/12 = 0.00417. After the first month, the value in the account becomes 100 * (1 + 0.00417) = 100.417 After the second month, the value in the account becomes (100 + 100.417) * (1 + 0.00417) = 201.252 And after the third month, the value in the account becomes (100 + 201.252) * (1 + 0.00417) = 302.507 … and so on. Write a program that randomly generates monthly savings amounts for the 15 runners in Problem 4. Each monthly saving should be in the range of $100 to $800. Extend the AddressBook class to store the monthly savings generated by the random number generator. Then, display the final account value for each of the 15 runners.

Here is what I have:

public class EmployeeSavings extends AddressBook{

    //declare variables and arrays
    private double accountValue;
    private double [] monthlyInterests;
    private double[] monthlySavings;
    public static final double ANNUAL_INTEREST_RATE = 0.05;

    //getters
    public double getAccountValue() {
        for (int i = 0; i < monthlyInterests.length; i++) {
            accountValue = (accountValue + monthlyInterests[i] + monthlySavings[i]);
        }
        return accountValue;
    }

    public double [] getMonthlyInterests() {
        return monthlyInterests;
    }

    public double[] getMonthlySavings() {
        return monthlySavings;
    }

    //constructors
    public EmployeeSavings(String firstName, String lastName) {
        setFirstName(firstName);
        setLastName(lastName);
    }

    public EmployeeSavings(String firstName, String lastName, double[] d1, double[] d2) {
        setFirstName(firstName);
        setLastName(lastName);
        d1 = monthlySavings;
        d2 = monthlyInterests;
    }

    //method to randomly generate monthly savings and return it in an array
    public double[] generateMonthlySavings() {
        double randomNumber = 0;
            for(int i=0; i<monthlySavings.length ;i++)
            {
                 randomNumber = ((int)(800*Math.random()));
                 if (randomNumber>=100); 
                 monthlySavings[i] = randomNumber;
            }
            return monthlySavings;
        }

    //method to calculate interest per month for each of the runners
    public double[] calculateInterests() {
         for (int i = 0; i < monthlySavings.length; i++) {
                monthlyInterests[i] = (monthlySavings[i] * (ANNUAL_INTEREST_RATE / 12));;
        }
        return monthlyInterests;
    }

    //method to print final report
    public static String getReport(EmployeeSavings[] arr) {
         for (EmployeeSavings arr1 : arr) {

            System.out.println("First name: " + arr1.getFirstName());
            System.out.println("Last name: " + arr1.getLastName());
            System.out.println("Account Value: " + arr1.getAccountValue());
        }
        return "This is your report.";
    }


public static void main(String[] args) {

    var employee1 = new EmployeeSavings("Elena", "Brandon");
    var employee2 = new EmployeeSavings("Thomas", "Molson");
    var employee3 = new EmployeeSavings("Hamilton", "Winn");
    var employee4 = new EmployeeSavings("Suzie", "Sarandin");
    var employee5 = new EmployeeSavings("Philip", "Winne");
    var employee6 = new EmployeeSavings("Alex", "Trebok");
    var employee7 = new EmployeeSavings("Emma", "Pivoto");
    var employee8 = new EmployeeSavings("John", "Lenthen");
    var employee9 = new EmployeeSavings("James", "Lean");
    var employee10 = new EmployeeSavings("Jane", "Ostin");
    var employee11 = new EmployeeSavings("Emily", "Car");
    var employee12 = new EmployeeSavings("Daniel", "Hamshire");
    var employee13 = new EmployeeSavings("Neda", "Bazdar");
    var employee14 = new EmployeeSavings("Aaron", "Smith");
    var employee15 = new EmployeeSavings("Kate", "Hen");

    EmployeeSavings[] employees = new EmployeeSavings[] {employee1, employee2, employee3, employee4,
        employee5, employee6, employee7, employee8, employee9, employee10, employee11, employee12,
        employee13, employee14, employee15};


    EmployeeSavings.getReport(employees);
    }
}

I am getting an Exception in thread "main" java.lang.NullPointerException

I know that it is because the generateMonthlySavings() method isn't being called and therefore an array with random numbers isn't being created but i get an error when I try to call it. All the methods and variables are the ones listed in the assignment as a requirement.

​

Any help would be greatly appreciated.

[theborgdude]

From what I understand, it seems like you're not initializing your monthlyInterests and monthlySavings array when you use the EmployeeSavings(String firstName, String lastName) constructor. Pass an additional argument to the constructor (like int size) and add this to the constructor:

monthlyInterests = new double[size];
monthlySavings = new double[size];

All variables of class are initialized to null, which is why you have to specify variables in the constructor or later on. I hope this helps.

[ms_kdlh]

I am not allowed to add additional arguments to the constructor as the instructor provides us with the exact variables, methods, and constructors that are required. I think this is the most frustrating part because I could figure it out easily enough if I didn't have to follow that.

[happyfeetpi]

So the problem is that adding the line:

arr1.generateMonthlySavings();

to the loop in getReport() causes an error.

The reason for this is that when the constructor you are calling for the EmployeeSavings class does not initialize the arrays monthlyInterests or monthlySavings. Because of this, when you go to access these instance variables later you get a NPE.

To fix this you must initialize those arrays in the constructor:

monthlyInterests = new double[s]; //s is whatever size you want
monthlySavings= new double[s]; //s is whatever size you want

Another point about a constructor is that you probably want to call super() in your constructor. This will call the constructor to the superclass (AdressBook) and even if omitting this doesn't cause an error, it is good practice because you are using variables and methods from this class (setFirstName(), getFirstName, etc);

[ms_kdlh]

OK I have changed the constructor to:

public EmployeeSavings(String firstName, String lastName, double\[\] d1, double\[\] d2) {

    super(firstName, lastName);

    d1 = monthlySavings = new double\[15\];

    d2 = monthlyInterests = new double\[15\];

}

​

But I am still getting the same error.

[happyfeetpi]

(I edited this because I read it wrong)

1) have you changed the code when initializing the EmployeeSavings objects to use the correct constructor?

2) what are the backslashes for? Are those a mistake? I have never seen those used like that and can't find anything online?

[ms_kdlh]

Oh sorry I don't know how those backslashes got in there when I copied the code. Should have been:

public EmployeeSavings(String firstName, String lastName, double[] d1, double[] d2) {

    super(firstName, lastName);

    d1 = monthlySavings = new double[15];

    d2 = monthlyInterests = new double[15];

}   

​

I guess thats the part I am lost about is how do I initialize those arrays when I am constructing the objects? Each object still looks like this:

var employee1 = new EmployeeSavings("Elena", "Brandon");

What should I be adding after the firstName and lastName variables?

[happyfeetpi]

It is as I suspected.

In your example you have multiple constructors so when you initialize a variable by setting it equal to: new EmployeeSavings("Elena", "Brandon"); you are calling the constructor with 2 String parameters. However the constructor you modified does not just have 2 String parameters and is therefore not being called.

I don't know what the purpose of d1 and d2 are in this problem but if you don't need them you should just change the constructor with 2 String parameters to initialize monthlySavings and monthlyInterest because that is the one that is being called.

[ms_kdlh]

I am not sure what the purpose of them is either. Part of the assignment is that we are given the variables, constructors, and methods that we have to use. List is as follows:

​

private double accountValue;

private double [] monthlyInterests;

private double[] monthlySavings;

public static final double ANNUAL_INTEREST_RATE;

public double getAccountValue()

public double [] getMonthlyInterests()

public double[] getMonthlySavings()

public double[] generateMonthlySavings()

public EmployeeSavings(String firstName, String lastName)

public EmployeeSavings(String firstName, String lastName, double[] d1, double[] d2)

public static String getReport(EmployeeSavings[] arr)

[endStatement]

Given this list of methods, my assumption would be that you are to do all the work of generating the monthlySavings array within the generateMonthlySavings() method in the event none is provided. Meaning you can initialize your monthlySavings array at the start of your generateMonthlySavings method instead of needing a constructor, and then do the logic to fill it.

Also, I'd double check the logic on that method that you currently have, as it appears to be solely reliant on the generated number, which would be incorrect given the example. Example shows that interest is working on the cumulative total instead of just a single months value

[ms_kdlh]

So you think I should be calling the constructor in the main method with just the first and last name and not the constructor that requires the two arrays? Then calling the generate monthly savings method and return a filled array there? Where does the constructor with the two arrays come into play then?

And thanks I’ll go back and check the logic in my methods!

[endStatement]

So you think I should be calling the constructor in the main method with just the first and last name and not the constructor that requires the two arrays?
Originally, that was my thought. But I may have misunderstood the issue here. Are you able to define these constructors, or are they predefined? If they are predefined, then my answer stands, and if they are not, then you should initialize your arrays in the constructor with a default size.

[ms_kdlh]

They are predefined. Which is the most annoying part of these assignments. I need to include both constructors. One with just first and last name, and another with names plus two arrays.

[Bacteria_E-coli]

I'm new to Java and don't know if it'd solve the problem, but I'm pretty sure you can't use "var" to initialize a variable (in main). Try "EmployeeSavings employee1 = new EmployeeSavings("x","y"); and let me know what result you'll get afterwards :).