Real World Example

Example

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        PaymentWallet asha = new PaymentWallet("Asha", 1000);
        PaymentWallet ravi = new PaymentWallet("Ravi", 200);

        asha.transferTo(ravi, 300);
        ravi.transferTo(asha, 900);

        System.out.println(asha.getOwner() + ": " + asha.getBalance());
        System.out.println(ravi.getOwner() + ": " + ravi.getBalance());
        System.out.println("Asha history: " + asha.getHistory());
    }
}

class PaymentWallet {
    private final String owner;
    private long balance;
    private final List<String> history = new ArrayList<>();

    PaymentWallet(String owner, long openingBalance) {
        this.owner = owner;
        this.balance = openingBalance;
    }

    public void transferTo(PaymentWallet to, long amount) {
        if (amount <= 0 || amount > balance) {
            System.out.println("Transfer of " + amount + " from " + owner + " declined");
            return;
        }
        balance -= amount;
        to.balance += amount;           // same class, so private access is allowed
        history.add("-" + amount + " to " + to.owner);
        to.history.add("+" + amount + " from " + owner);
    }

    public String getOwner() { return owner; }
    public long getBalance() { return balance; }
    public List<String> getHistory() { return List.copyOf(history); }
}

Output

Transfer of 900 from Ravi declined
Asha: 700
Ravi: 500
Asha history: [-300 to Ravi]