Prefactoring

2.6. Abstracting

In creating a description of a use case or a model of a possible class, avoid using primitive data types. Pretend that ints or doubles do not exist. Almost every type of number can be described with an abstract data type (ADT). Items are priced in Dollars (or CurrencyUnits, if you are globally oriented). The number of physical copies of an item in an inventory is a Count. The discount that a good customer receives is denoted with a Percentage. The size of a CDDisc is expressed as a Length (or LengthInMeters or LengthInInches if you are going to be sending a satellite into space). The time for a single song on a CDRelease could be stored in a TimePeriod.

Using an ADT places the focus on what can be done with the type, not on how the type is represented. An ADT shows what you intend to do with the variable. You can declare the variable as a primitive data type and name the variable to reflect that intent. However, a variable declared as an abstract data type can have built-in validation, whereas a variable declared as a primitive cannot.

Each ADT needs a related description. For example, a Count represents a number of items. A Count can be zero or positive. If a Count is negative, it represents an invalid count. Declaring a variable as a Count conveys this information. You can create variations of Count. You may have a CountWithLimit data type with a maximum count that, if exceeded, would signal an error.

You can place limits on many different data types. For ...