TDD for a new Java application - Where should I start? - tdd

Question

Goal : testing TDD in a typical enterprise Java environment.
Context :
Frameworks used (even if it's overkill, I practice project based learning) :
DAO : Hibernate
Spring IoC
Front : Spring MVC + Twitter Boostrap (if possible)
TDD : JUnit
DB : PostgreSQL
My project is a simple billing system which would help freelancers create, edit and print/send bills to customers.
Once my project is created and configured, I don't know where to start.
Let's say my first my first feature is to create a bill with a unique number and a title.
Question : What should I test first ?
the Domain layer with a createBill(String title) method which would generate a unique Serial Number ? I'd mock the DB layer.
the UI first, mocking the service layer ? I don't know how to do it.
Thanks in advance for your answers,
Cheers

Answer 1

Start with a test :-)
What does your system do?
public class BillingSystemTest {
#Test
public void generatesBills() {
Bill bill = new BillingSystem().generate()
assertNotNull(bill)
}
}
First test complete!
Make it pass, move on to the next test...
#Test
public void generatesAnInvoiceNumberForEachBill() {
Bill bill = new BillingSystem().generate()
assertEquals(1, bill.getNumber())
}
// ...and the next
#Test
public void generatesUniqueInvoiceNumbersForEachBill() {
BillingSystem bs = new BillingSystem()
assertEquals(1, bs.generate().getNumber())
assertEquals(2, bs.generate().getNumber())
}
#Test
public void generatesAnInvoiceSubjectWhenNoneIsSpecified() {
Bill bill = new BillingSystem().generate()
assertEquals("Invoice #1 from ACME Corp.", bill.getSubject())
}
#Test
public void allowsForCustomSubjectsOnBills() {
Bill bill = new BillingSystem().generate("Custom subject")
assertEquals("Custom subject", bill.getSubject())
}
I've obviously skipped the refactoring steps here, but now that you have the tests, and the code that goes with it, you need to evaluate it for more opportunity. I'm imagining the code looking something like this.
public class BillingSystem {
private nextInvoiceNumber = 1;
public Bill generate() {
return generate("Invoice #" + nextInvoiceNumber + " from ACME Corp.");
}
public Bill generate(String subject) {
Bill bill = new Bill(nextInvoiceNumber, subject)
nextInvoiceNumber++
return bill;
}
}
Looking at this code, it seems ok, but may violate the Single Responsibility Principle (SRP). Here the BillingSystem generating a bill as well as managing the invoice number. This is an opportunity for refactoring. After the refactoring, your design may look something like this:
public class BillingSystem {
private InvoiceNumbering invoiceNumbering = new InvoiceNumbering()
public Bill generate() {
return generate("Invoice #" + invoiceNumbering.peekNext() + " from ACME Corp.");
}
public Bill generate(String subject) {
Bill bill = new Bill(invoiceNumbering.generateNext(), subject)
nextInvoiceNumber++
return bill;
}
}
Your design is better and your tests all pass. Next thing to do is refactor out the tests to remove the implementation details from them. They may end up looking something like:
#Test
public void generatesBills() {
Bill bill = new BillingSystem().generate()
assertNotNull(bill)
}
#Test
public void generatesAnInvoiceNumberForEachBill() {
// Using hand rolled mocks
MockInvoiceNumbering in = new MockInvoiceNumbering()
in.generateNextShouldReturn(4)
Bill bill = new BillingSystem(in).generate()
assertEquals(4, bill.getNumber())
}
#Test
public void generatesUniqueInvoiceNumbersForEachBill() {
MockInvoiceNumbering in = new MockInvoiceNumbering()
BillingSystem bs = new BillingSystem(in)
bs.generate();
bs.generate();
assertEquals(2, in.numberOfTimesGenerateNextWasCalled)
}
#Test
public void generatesAnInvoiceSubjectWhenNoneIsSpecified() {
Bill bill = new BillingSystem().generate()
assertEquals("Invoice #1 from ACME Corp.", bill.getSubject())
}
#Test
public void allowsForCustomSubjectsOnBills() {
Bill bill = new BillingSystem().generate("Custom subject")
assertEquals("Custom subject", bill.getSubject())
}
As a part of this refactoring, you would likely create some tests around your InvoiceNumbering class.
Hope that's enough of a start. Left a lot out. :-)
Hope that helps!
Brandon