Singleton

package org.example;

public class AVAX extends Currency {
    private static AVAX instance;
    private AVAX() {}

    public static AVAX getInstance() {
        if (instance == null) {
            instance = new AVAX();
        }
        return instance;
    }

    @Override
    public String toString() {
        return super.toString();
    }
}
package org.example;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Wallet {
    private HashMap<Currency, Double> balances = new HashMap<>();
    private List<History> histories = new ArrayList<>();

    public void deposit(Currency c, Double quantity) {
        balances.put(c, balances.getOrDefault(c, 0.0) + quantity);
        histories.add(new History(c, quantity, TransferType.DEPOSIT));
    }

    public void send(Currency c, Double quantity, Wallet targetWallet) {
        double balance = balances.getOrDefault(c, 0.0) - quantity;

        if (balance < 0.0)  {
            throw new RuntimeException("Insufficient balance");
        }

        balances.put(c, balance);
        histories.add(new History(c, quantity, TransferType.SEND));

        targetWallet.deposit(c, quantity);
    }

    /**
     * To make it work, need to add a few toString methods
     */
    public void printBalance() {
        for (Map.Entry<Currency, Double> entry : balances.entrySet()) {

        }
    }

    /**
     * To make it work, need to add a few toString methods
     */
    public void printHistory() {
         for (History history : histories) {

         }
    }
}