I am searching for a new TDD example

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re: I am searching for a new TDD example

Scott Bellware — Sat, 28 Aug 2004 00:12:00 GMT

I just did an app as a part of a TDD and OO mentoring and consulting gig that parsed fixed length text files. It turned out to be a great TDD experience where I was able to demonstrate micro-incremental development and evolving code. The goal was to create simple business objects that would encapsulate a row of fixed length text data (and ultimately to write a reader that would pull data from a file and feed it into the parsing objects).

The example worked really well because we could start with a simple test fixture that has a string member (called “rowData” or what-have-you), and write test methods that asserted that certain values were located at certain indexes on the row. The simplest test fixture looked like the following:

[TestFixture]
public class RowFixture
{
string rowData = "1JimNewkirk";

[Test]
public void Id()
{
Assert.AreEqual("1", this.rowData.Substring(0, 1));
}

[Test]
public void FirstName()
{
Assert.AreEqual("Jim", this.rowData.Substring(1, 3));
}

[Test]
public void LastName()
{
Assert.AreEqual("Newkirk", this.rowData.Substring(4, 7));
}
}

This was before we even had a target class to test. Next we identified some repetition that we wanted to encapsulate. Rather than have to invoke String.Substring(), we decided to encapsulate these calls, which led to encapsulating the row data:

public class Row
{
private string data;

public string Id
{
get{return this.data.Substring(0, 1);}
}

public string FirstName
{
get{return this.data.Substring(1, 3);}
}

public string LastName
{
get{return this.data.Substring(4, 7);}
}

public Row(string data)
{
this.data = data;
}
}

With the fixture:

public class RowFixture
{
string rowData = "1JimNewkirk";
Row row;

[SetUp]
public void CreateRow()
{
this.row = new Row(this.rowData);
}

[Test]
public void Id()
{
Assert.AreEqual("1", this.row.Id);
}

[Test]
public void FirstName()
{
Assert.AreEqual("Jim", this.row.FirstName);
}

[Test]
public void LastName()
{
Assert.AreEqual("Newkirk", this.row.LastName);
}
}

We then tackled the duplication of calls to Substring() by creating a ParsedField class:

public class ParsedField
{
private readonly int startIndex;
private readonly int length;
private readonly Row row;

internal int StartIndex
{
get{return this.startIndex;}
}

internal int Length
{
get{return this.length;}
}

public string Value
{
get{return this.row.Data.Substring(this.startIndex, this.length);}
}

public ParsedField(int startIndex, int length, Row row)
{
this.startIndex = startIndex;
this.length = length;
this.row = row;
}
}

And the fixture:

[TestFixture]
public class ParsedFieldFixture
{
[Test]
public void Value()
{
Row row = new Row("1");
ParsedField parsedField = new ParsedField(0, 1, row);
Assert.AreEqual("1", parsedField.Value);
}
}

Which caused us to create the Data property on Row:

public string Data
{
get{return this.data;}
}

We then changed the Row class to use the ParsedField class:

public class Row
{
private string data;

private ParsedField idField;
private ParsedField firstNameField;
private ParsedField lastNameField;

public string Data
{
get{return this.data;}
}

public string Id
{
get{return this.idField.Value;}
}

public string FirstName
{
get{return this.firstNameField.Value;}
}

public string LastName
{
get{return this.lastNameField.Value;}
}

public Row(string data)
{
this.data = data;

this.idField = new ParsedField(0, 1, this);
this.firstNameField = new ParsedField(1, 3, this);
this.lastNameField = new ParsedField(4, 7, this);
}
}

We ultimately refactored the tests to reuse the Row instance through inheritance, made the Row abstract and created a TestRow with the specifics, etc. It was a good way for me to teach TDD and OO in a context that the developers could identify with, without getting into data connections, etc.

The framework later evolves to use strongly-typed readers, type discriminators, and all kinds of simple yet complicated things for a non-OO person to whet their OO appetite with.

I’ll be using this example in some upcoming presentations on TDD in my area of the world this fall.

Cheers!

(And I too love the book… one of the best books on programming – period – since Refactoring. Much thanks from me and my team!)