Lambda

Lambda is a lightweight framework helping you to split complex business logic and rules into individual atomic functions and assemble them together to any complexity level. It enables functional programming paradigm with Object Oriented features from Java Programming Language. It has been refactored to utilize the Function and Predicate from Google collections. They used to be called Solution and Applicability.

Benefit

So instead of writing code like this,

if (lease.getCountry().equals("GD")) {

    double depreciationFee = (lease.getPrincipal().doubleValue() - lease.getResidualValue().doubleValue()) / (lease.getTerm() * 12);
    double financeFee = (lease.getPrincipal().doubleValue() + lease.getResidualValue().doubleValue()) * lease.getRate().doubleValue() / 100;


    lease.setMonthPayment(new BigDecimal(depreciationFee + financeFee).setScale(2, BigDecimal.ROUND_HALF_UP));
    lease.setStampDuty(
        lease.getMonthPayment().multiply(new BigDecimal(0.02)).setScale(2, BigDecimal.ROUND_HALF_UP)
    );
    lease.setFirstMonthPayment(
        lease.getMonthPayment().add(
            lease.getStampDuty()
        ).setScale(2, BigDecimal.ROUND_HALF_UP)

    );
} else if (lease.getCountry().equals("NR")) {
    double depreciationFee = (lease.getPrincipal().doubleValue() - lease.getResidualValue().doubleValue()) / (lease.getTerm() * 12);
    double financeFee = (lease.getPrincipal().doubleValue() + lease.getResidualValue().doubleValue()) * lease.getRate().doubleValue() / 100;


    lease.setMonthPayment(new BigDecimal(depreciationFee + financeFee).setScale(2, BigDecimal.ROUND_HALF_UP));

    lease.setStampDuty(
        lease.getMonthPayment().multiply(new BigDecimal(0.03)).setScale(2, BigDecimal.ROUND_HALF_UP)
    );

    lease.setFirstMonthPayment(
        lease.getMonthPayment().add(
            lease.getStampDuty().setScale(2, BigDecimal.ROUND_HALF_UP)
        )
    );

    lease.setFirstMonthPayment(lease.getFirstMonthPayment().add(lease.getApplicationFee()));


} else if (lease.getCountry().equals("WL")) {
    double depreciationFee = (lease.getPrincipal().doubleValue() - lease.getResidualValue().doubleValue()) / (lease.getTerm() * 12);
    double financeFee = (lease.getPrincipal().doubleValue() + lease.getResidualValue().doubleValue()) * lease.getRate().doubleValue() / 100;


    lease.setMonthPayment(new BigDecimal(depreciationFee + financeFee).setScale(2, BigDecimal.ROUND_HALF_UP));

    lease.setStampDuty(
        lease.getPrincipal().multiply(new BigDecimal(0.03)).setScale(2, BigDecimal.ROUND_HALF_UP)
    );
    lease.setFirstMonthPayment(
        lease.getMonthPayment().add(
            lease.getStampDuty()
        ).setScale(2, BigDecimal.ROUND_HALF_UP)

    );
}

We write code like this, and you won't see any function composing,

public class LeaseCalculation extends CaseFunction<Lease, Lease, Country> {
    public LeaseCalculation() {
        super(new GetCountry(),
            GD, new GondorLeaseCalculation(),
            NR, new NarniaLeaseCalculation(),
            WL, new WonderlandLeaseCalculation());
    }
}


public final class GondorLeaseCalculation extends CompositeFunction<Lease, Lease> {

    public GondorLeaseCalculation() {
        super(
            new SetMonthlyPayment<>(new MonthlyLeasePayment<>()),
            new SetStampDuty<>(new StampDutyOnMonthlyPayment<>(RATE_2)),
            new SetFirstMonthPayment<>(new SumMonthlyPaymentAndStampDuty<>())
        );
    }
}

Example

Please run LoanStory.java to test. The implementation is LoanCalculation.java. LegacyLoanCalculation.java is an alternative solution without using the framework.

The LoanStory.java demonstrates how to use Lambda to solve the following complex business problems.

In Narnia, if the buyers borrow the application fee, the application fee
will be added to the loan principal, otherwise it will be added to the
first month payment; first time buyers don't pay stamp duty, other buyers
pay stamp duty on the monthly payment at the rate is 3%.

In Wonderland, the buyers pay stamp duty on the loan principal at the
rate of 3% and it will be collected on the first month payment, together
with the application fee.

In Gondor, the buyers pay stamp duty on the monthly payment at the rate
of 2%. The application fee will be added to the first month payment.

The formula to calculate loan payment can be found here

        http://www.kbapps.com/finance.html

Note

When learning this framework, please pay more attention to how to wire atomic functions into a complex function than how individual functions work. The example of wiring can be found in GondorCalculation, NarniaCalculation and WonderlandCalculation.

Exercise

Now we need add functionality to support lease business, assume everything remaining same but there is no first time buyer rule for lease business and application fee need to be paid on first month payment. How to add the new function? The formula to calculate lease payment can be found here

        http://www.leaseguide.com/lease08.htm

Solution

The implementation is LeaseCalculation.java and please run LeaseStory.java to test.