Removing object element from HashSet

Question

I am working on a Wallet and have started with the basics. I am adding Penny objects into a HashSet which stores the money. I initialise the capacity to 5 and start with a full wallet.

When trying to spend a Penny, null is returned (as expected) but then when calling the moneyCheck() method, there are still 5 coins.

I am not sure what to do as I am trying to remove a Penny using the.remove() method and the object is not being removed from the HashSet.

Any direction would be appreciated. Thank you

import java.util.HashSet;

public class Wallet{
    private HashSet<Penny> walletPenny;

    public Wallet(){
        int walletCapacity = 5;
        walletPenny = new HashSet<>();

        int counter = 0;
        while (counter < pocketCapacity){
            pocketPenny.add(new Penny());
            counter++;
        }
    }

    public Penny removePenny(){
        Penny spentPenny = null;

        if (walletPenny.isEmpty()){
            System.out.print("No more pennies!\n");
            return spentPenny;
        }
        else {
            pocketPenny.remove(spentPenny);
            System.out.println(("Spent penny!"));
        }
        return spentPenny;
    }
}

    public int moneyCheck(){
        System.out.print("Money remaining: " + walletPenny.size() + "\n");
        return walletPenny.size();
    }

Answer 1

Found an answer and thought I will update this in case anyone else has the same issue.

To remove the penny I initialised a Penny object with an iterator to find the next available object in the HashSet. I then returned that value to be used elsewhere in the program.

    public Penny removePenny(){
        Penny spentPenny;

        if (walletPenny.isEmpty()){
            System.out.print("No more pennies!\n");
            spentPenny = null;
            return spentPenny;
        }
        else {
            spentPenny = walletPenny.iterator().next();
            walletPenny.remove(spentPenny);
            System.out.println(("Spent penny!"));
            return spentPenny;
        }
    }

Answer 2

When you try to remove the penny with pocketPenny.remove(spentPenny) , Java tries to find which penny to remove. In order to do that, it uses equals and iterates the HashSet until it finds a penny which is equal (identity wise) to your spentPenny . More on how remove() works . Since you provide null as spentPenny , it will match nothing (your 5 pennies are initialized, so they are not null). You can verify this if you check the return value of .remove() method - it will be false .

In order to fix this, you need to:

1. Provide a non-null value when you call the remove() method
2. Implenent equals() in your Penny class
3. Keep track of which Pennies you created so you can remove them later

Full example:

public class Penny {

    private final int serialNum;

    public Penny(int serialNum) {
        this.serialNum = serialNum;
    }

    @Override
    public int hashCode() {
        int hash = 5;
        hash = 79 * hash + this.serialNum;
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Penny other = (Penny) obj;
        return this.serialNum == other.serialNum;
    }

}

and then create your Pennies:

int counter = 0;
        while (counter < pocketCapacity){
            pocketPenny.add(new Penny(counter));
            counter++;
        }

Now you have 5 pennies, numbered 0 to 4. You can remove one, for example #3:

Penny spentPenny = new Penny(3);
pocketPenny.remove(spentPenny);