Copy and array to greater size array then return to original size array

Question

Hi for my program I trying to make an array bigger by creating another array to copy the array to and then returning those values back to the original array.

import java.util.Scanner;
import java.util.Arrays;

public class Bank {

    public static void main(String[] args) {

        Scanner command = new Scanner(System.in);

        BankAccount [] arrayAcc = new BankAccount[1];
        BankAccount [] arrayAcc1 = new BankAccount[arrayAcc.length + 1];

        for(int z = 0; z < arrayAcc.length; z++){
            arrayAcc[z] = arrayAcc1[z];
        }
        arrayAcc = arrayAcc1;

        System.out.println("Enter new account number: ");
        int accNum = command.nextInt();
        arrayAcc[count++].setAccountID(accNum);
    }
}

When I later try to get an object from this array it is giving me an error:

Exception in thread "main" java.lang.NullPointerException
at Bank.main(Bank.java:47)

Post the relevant code to replicate the problem. Seems like your problem is about reinitializing a non-field variable. — Luiggi Mendoza
– Luiggi Mendoza, Commented Jul 14, 2014 at 0:43
@Victor exactly the comment's point. Just post the relevant code, so viewers don't have to read through all of them. — josephus
– josephus, Commented Jul 14, 2014 at 1:13

Keenle · Accepted Answer · 2014-07-14 00:49:36Z

2

You copy items from arrayAcc1 to arrayAcc while should be conversely.

Should be:

arrayAcc1[z] = arrayAcc[z];

BTW, why not to use List<BankAccount> accs = new ArrayList<BankAccounts>()?

answered Jul 14, 2014 at 0:49

Keenle

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4 Comments

th3r1singking Over a year ago

thanks for catching that! @Keenle what would 'List<BankAccount> accs = new ArrayList<BankAccounts>()' do?

Keenle Over a year ago

List<T> automatically resizes as you add items. So you can just call accs.add(new BankAccount()) and no need to deal with array resizing routine.

th3r1singking Over a year ago

@keenle I tried to do that but how would I add a new object to the array?

Keenle Over a year ago

You can remove BankAccount[] array and create accs ArrayList<BankAccount> and work with it instead of dealing with array routines.

McLovin · Accepted Answer · 2014-07-14 01:11:17Z

0

You don't need the for loop, because Arrays.copyOf already does the work for you. You also don't need the cast, because the method returns an array of the same type as its argument.

The NullPointerException is thrown when you try to access an array index that has not had a value assigned to it. You haven't filled your array with any BankAccount objects, so when you execute arrayAcc[count++].setAccountID(accNum);, there is not a BankAccount to set the account ID of. To add an account to the list, call the constructor after the array has been resized, and add the object to the last index. The code should be similar to this:

BankAccount []arrayAcc1 = Arrays.copyOf(arrayAcc, arrayAcc.length + 1);
arrayAcc = arrayAcc1;
arrayAcc[arrayAcc.length - 1] = new BankAccount(/*arguments here*/);

answered Jul 14, 2014 at 1:11

McLovin

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1 Comment

th3r1singking Over a year ago

Thank you! You caught my problem!

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