Java 1.4 has a new library that deals with XML built into it called JAXP. In order to parse an XML document, you need a Document object. You can get a Document object from a DocumentBuilder object and you get DocumentBuilder objects from a DocumentBuilderFactory object. Yeah, I'm not sure if I like that either but it's the way they do it.

This is where my 1 single line of uncovered code is: when you use the newInstance() method of the DocumentBuilderFactory to create a new DocumentBuilder if a "builder cannot be created with the configuration requested" the newDocumentBuilder() method throws ParserConfigurationException. This exception is a checked exception and should be dealt with and not terminate the program.

Here was my method. How could I throw that exception and cover the catch line?

protected void setupBuilder()
{
   try
   {
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      this.builder = factory.newDocumentBuilder();
   }
   catch (ParserConfigurationException e)
   {
      e.printStackTrace();
   }
}

I'm going to turn it into an unchecked runtime exception by catching it and throwing IllegalArgumentException. In the context I'm using the factory it would be a programming error to misconfigure it and then try to use it to create a builder. Programming errors, like using a method improperly, usually throw unchecked runtime exceptions like IllegalArgumentException.

You might be wondering why I don't just make this function throw ParserConfigurationException. Because then I'd have to explicitly use a try/catch whenever I used it. If I change the exception to a runtime exception, that is not the case. So here's what I came up with:

protected void setupBuilder(DocumentBuilderFactory factory)
{
   try
   {
      this.builder = factory.newDocumentBuilder();
   }
   catch (ParserConfigurationException e)
   {
      throw new IllegalArgumentException("parameter factor is invalid: " + e);
   }
}

The key is to give the method a parameter that will change it's behaviour or output. Without an input parameter you cannot control the output and have good unit testing. Here are the test methods I wrote to completely cover the method:

public void testSetupBuilder_Null()
{
   XMLRepository repository = XMLRepository.getInstance();

   fail("Expected IllegalArgumentException");
}

The last test just implements a custom factory that throws ParserConfigurationException when newDocumentBuilder() is called. It's a pretty simple way to cover the catch block. So now I have 100% code coverage for AudioMan. Sweeeet.