Person.java

public class Person {
    private final String name;
    private int balance;

    public Person(String name, int balance) {
        if (name == null || name.trim().isEmpty()) {
            throw new IllegalArgumentException("Name cannot be empty.");
        }
        if (balance < 0) {
            throw new IllegalArgumentException("Balance cannot be negative in Person model.");
        }
        this.name = name;
        this.balance = balance;
    }

    public String getName() {
        return name;
    }

    public int getBalance() {
        return balance;
    }

    public void addToBalance(int delta) {
        this.balance += delta;
        if (this.balance < 0) {
            throw new IllegalStateException("Balance cannot become negative.");
        }
    }

    @Override
    public String toString() {
        return name + ":" + balance;
    }
}