Рекомендации по разработке на Java — 4.3. Неизменяемость

// Хорошо: неизменяемый класс
public final class Money {
    private final BigDecimal amount;
    private final Currency currency;
    
    public Money(BigDecimal amount, Currency currency) {
        this.amount = amount.setScale(2, RoundingMode.HALF_UP);
        this.currency = Objects.requireNonNull(currency);
    }
    
    public Money add(Money other) {
        if (!this.currency.equals(other.currency)) {
            throw new IllegalArgumentException("Currencies must match");
        }
        return new Money(this.amount.add(other.amount), this.currency);
    }
    
    public BigDecimal getAmount() {
        return amount;
    }
    
    public Currency getCurrency() {
        return currency;
    }
}