FAQ: Static Variables and Methods - Modifying Static Variables

This community-built FAQ covers the “Modifying Static Variables” exercise from the lesson “Static Variables and Methods”.

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In step 1 I originaly used
It seemed to work fine, but it wasn’t accepted as the right code to use. Was this just a quirk of the system that checks for correct answers, or is there some reason that “+=1” is better than “++”?

In step 2 I originally used
totalMoney += this.Money;
Again it worked fine but it wants me to use
totalMoney += inputMoney;
Again, does this make a difference - is there some reason why it’s better to use inputMoney than this.money? Although the results are the same in practice, it seems slightly neater to me to define totalMoney according to the money value in each ATM, rather than the inputMoney. (Or even better, to have a separate method to set totalMoney according to the sum of money in each ATM, rather than increasing and decreasing it with deposits and withdrawals.)


Indeed this needs to be changed. This sends an incorrect message to the user when the user did nothing wrong.

public class ATM {
    // Static variables
    public static int totalMoney = 0;
    public static int numATMs = 0;

    // Instance variables
    public int money;

    public ATM(int inputMoney) {
        this.money = inputMoney;

        // Steps 1 and 2: Edit numATMs and total money here

    public void withdrawMoney(int amountToWithdraw) {
        if (amountToWithdraw <= this.money) {
            this.money -= amountToWithdraw;
            // Step 3: Edit totalMoney here

    public static void main(String[] args) {

        System.out.println("Total number of ATMs: " + ATM.numATMs);
        ATM firstATM = new ATM(1000);
        ATM secondATM = new ATM(500);
        System.out.println("Total number of ATMs: " + ATM.numATMs);

        System.out.println("Total amount of money in all ATMs: " + ATM.totalMoney);
        System.out.println("Total amount of money in all ATMs: " + ATM.totalMoney);



This should be failing step 1.


same problem. I also used numATMs++


Funny. I also used ++ and it told me it was incorrect but I guess the checker just wanted a different way to write it. Should be changed.

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In this instance, yes because you are using post-increment i++ instead of pre-increment ++i

++i increments and then uses the variable
i++ uses and then increments the variable

Running it with ++numATMs; works fine. And since pre-incrementation hasn’t been covered in the lesson thus far, += 1 is the logical better solution especially in that constructer’s signature.

totalMoney += inputMoney is consistent with the signature of the decrementation in the withdrawMoney method, which first runs an if-then check on this.money. I could imagine a more realistic scenario on step 2 that applies these similar checks on both the existing and the incoming values. And assignment via the incoming value could be the cleanest route to separating concerns

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