If the world were perfect, you would have exactly one unique name for each concept in a system. In this imperfect world, having two concepts with the same name leads to confusion. In Sam's case, the term CD was applied to both a CDRelease and a CDDisc. Separating the two concepts with two names clarified the requirements.

Using two different names for a single idea can also be confusing, albeit less so than two ideas with a single name. Referring to a physical CD as both a CDDisc and a CDPhysical might be justified by political measures. ("This department calls it this and that department calls it that.") Sam referred to the act of renting a CD as both renting a CD and checking out a CD. If these two terms really encompass the same operation, the duality of reference can be annoying, but might not be confusing.

Sometimes it is hard to determine whether you have two independent concepts or one. Try making up a one-line definition for a name. If it is difficult to create a simple definition, go ahead and use two names. Later on, if you find that the distinction was meaningless, you

AIRLINE FLIGHTS

I worked with a group responsible for developing an airline reservation system. There is a lot of interesting terminology in the airline business. Think of your concept of a "flight" as a passenger. When you fly from Boston to Peoria, do you say that you are taking a flight to Peoria? Do you say that, even if you make a connection in Chicago? In that case, would you refer to it as a "connecting flight"? If you were on Flight 80 from Boston to Chicago and on Flight 100 from Chicago to Peoria, would you say that you are on two different flights? Suppose that Flight 80 landed in Albany on the way to Chicago. Are you on three different flights?

The reservations people agreed on the following definitions:

Segment

A trip with a single departure and arrival. For example:

Boston to Albany on Flight 80

Albany to Chicago on Flight 80 Flight

Flight

A numerically designated set of segments, with the departure location of the subsequent segment the same as the arrival location of the previous segment. For example:

Flight 80 Miami to Boston to Albany to Chicago to Seattle

Flight 100 Dallas to Chicago to Peoria

Leg

A set of one or more consecutive flight segments on a flight. For example:

Flight 80 Boston to Albany to Chicago

Journey

A set of one or more legs for a passenger that takes a passenger from a departure location to an arrival location. For example:

Flight 80 Boston to Albany to Chicago and Flight 100 Chicago to Peoria

These consensus definitions made it easy to create higher-level concepts, such as "marriage." A marriage is a journey, for which the cancellation of one leg results in the cancellation of all legs of the journey.

can always declare the two names to be synonyms. Suppose that Sam and I came up with the terms CDAlbum and CDRelease. We might distinguish them by stating that a CDAlbum is a collection of songs with a title given to the set, and a CDRelease is a collection of songs that was released on a single CDDisc.

The conversion from one style of architecture, design, or coding to another is not necessarily symmetrical. Suppose that a single name has been used to denote two ideas. Later you decide that you need to replace that name with appropriate names for each idea. You need to examine each usage of the term carefully to determine which of the two concepts it represents. On the other hand, suppose that you have used two different names for a single concept. If you want to combine those into a single name, you can do a simple global replacement.

For example, suppose we have a class called Message, which represents messages displayed to the user. We think at the beginning that these messages are going to behave differently, so we divide them into WarningMessages, ErrorMessages, SevereErrorMessages, and ReallySevereErrorMessages. We make every message an object of one of these four classes. Later on, we realize that SevereErrorMessages and ReallySevereErrorMessages really do not behave differently. We can eliminate the distinction using a simple search and replace. Conversely, if we had not distinguished the two and later found that there should be a difference, we would have to look closely at each object of SevereErrorMessage to determine whether it should be categorized as ReallySevereErrorMessage.

SPLITTERS CAN BE LUMPED MORE EASILY THAN LUMPERS CAN BE SPLIT

It is easier to combine two concepts than it is to separate them.

 

Please check back next week for the conclusion to this article.