Test Driven Development

Improving Application Quality Using Test-Driven Development (TDD) 3


Adding Functionality

What we’ve seen so far, whilst sound TDD practices, hasn’t resulted in much demonstrable code. We know that MagazineIndex is going to manage a collection of zero or more instances of the class Magazine, so it stands to reason that we might have an AddMagazine method. So let us set about writing the test for AddMagazine.
Listing 4 presents our new test case – remember, we must write our tests before we write any implementation code! I’ve done the simplest thing possible, added CheckAddMagazine that creates an instance of MagazineIndex, adds a magazine name, and then checks that the magazine name was added correctly. Listing 4 is not going to compile – there’s no AddMagazine method defined, nor is there an IsMember function.
[Test]
public void CheckAddMagazine()
{
MagazineIndex cbtMagazineIndex;
cbtMagazineIndex = new MagazineIndex();
cbtMagazineIndex.AddMagazine("Methods & Tools");
Assert.IsTrue(cbtMagazineIndex.IsMember("Methods & Tools"), "Magazine was not added to collection!");
}
Listing 4: Adding functionality
Doing the simplest thing to Listing 2 will allow our new test in Listing 4 to compile. This means adding the AddMagazine and IsMember methods to the class MagazineIndex. Listing 5 presents MagazineIndex with these modifications.
namespace WinApp
{
public class MagazineIndex
{
public void AddMagazine(string MagazineName)
{
}
public bool IsMember(string MagazineName)
{
return false;
}
}
}
Listing 5: Doing the simplest thing
With Listing 5 in place, we can re-compile the solution and re-run the test. Prepare yourself for some bad news.

Figure 6: Red bar, test fails
Bad news, the test failed. What do we need to do in order to fix this? What might move us in the right direction would be a change to the AddMagazine and the IsMember methods, and the introduction of a string called MagazineList, this is the simplest thing. Listing 6 presents the updated MagazineIndex class.
public class MagazineIndex
{
private string MagazineList;
public void AddMagazine(string MagazineName)
{
MagazineList = MagazineName;
}
public bool IsMember(string MagazineName)
{
return MagazineList == MagazineName;
}
}
Listing 6: Small steps, tests pass

Figure 7: One magazine, the test passes
Adding Two Magazines
Like all good developers, we will write some test code that exercises adding more than one magazine. Listing 7 presents a new test that does just this.
[Test]
public void CheckAddTwoMagazines()
{
MagazineIndex cbtMagazineIndex;
cbtMagazineIndex = new MagazineIndex();
cbtMagazineIndex.AddMagazine("Methods & Tools");
cbtMagazineIndex.AddMagazine("The Delphi Magazine");
Assert.IsTrue(cbtMagazineIndex.IsMember("Methods & Tools"), "M&T was not added to collection!");
Assert.IsTrue(cbtMagazineIndex.IsMember("The Delphi Magazine"), "TDM was not added to collection!");
}
Listing 7: Adding two magazines to the collection

Figure 8: Two magazines, the test fails
Our simple approach of using a string to manage the MagazineList has caused a test to fail. That is part of the TDD process – let the tests dictate (drive) which code is developed and how it is developed. In this case, CheckAddTwoMagazines dictates that we need to develop a collection or list of some sort to replace the simple string. The TDD mantra of "red, green, refactor" seems to stand true. It is time to refactor the AddMagazine and IsMember methods.
namespace WinApp
{
using System.Collections;
public class MagazineIndex
{
private ArrayList MagazineList = new ArrayList();
public void AddMagazine(string MagazineName)
{
MagazineList.Add(MagazineName);
}
public bool IsMember(string MagazineName)
{
return (MagazineList.Contains(MagazineName));
}
}
}
Listing 8: Introducing an ArrayList
After the introduction of Listing 8 , re-compiling and re-running the tests reveals Figure 9 and a green bar.

Figure 9: Two magazines and a green bar
ArrayLists in .NET languages are a useful mechanism that allows us to manage groups of object references, in our case strings. An ArrayList has the ability to grow as new object references are added – hence it is the ideal abstraction for managing a collection of magazine titles. ArrayLists support useful methods like Add, Insert, Remove, and conveniently for us a Contains method (we have used this in our IsMember implementation.)

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