6. The Class Hierarchy Merge Design Pattern

Consider the simple class hierarchy of the design model in Figure 12.1 above, showing a disjoint segmentation of the class Book. In such a case, whenever there is only one level (or there are only a few levels) of subtyping and each subtype has only one (or a few) additional properties, it's an option to re-factor the class hierarchy by merging all the additional properties of all subclasses into an expanded version of the root class such that these subclasses can be dropped from the model, leading to a simplified model.

This Class Hierarchy Merge design pattern comes in two forms. In its simplest form, the segmentations of the original class hierarchy are disjoint, which allows to use a single-valued category attribute for representing the specific category of each instance of the root class corresponding to the unique subclass instantiated by it. When the segmentations of the original class hierarchy are not disjoint, that is, when at least one of them is overlapping, we need to use a multi-valued category attribute for representing the set of types instantiated by an object. We only discuss the simpler case of Class Hierarchy Merge re-factoring for disjoint segmentations, where we take the following re-factoring steps:

  1. Add an enumeration datatype that contains a corresponding enumeration literal for each segment subclass. In our example, we add the enumeration datatype BookCategoryEL.

  2. Add a category attribute to the root class with this enumeration as its range. The category attribute is mandatory [1], if the segmentation is complete, and optional [0..1], otherwise. In our example, we add a category attribute with range BookCategoryEL to the class Book. The category attribute is optional because the segmentation of Book into TextBook and Biography is incomplete.

  3. Whenever the segmentation is rigid, we designate the category attribute as frozen, which means that it can only be assigned once by setting its value when creating a new object, but it cannot be changed later.

  4. Move the properties of the segment subclasses to the root class, and make them optional. We call these properties, which are typically listed below the category attribute, segment properties. In our example, we move the attributes subjectArea from TextBook and about from Biography to Book, making them optional, that is [0..1].

  5. Add a constraint (in an invariant box attached to the expanded root class rectangle) enforcing that the optional subclass properties have a value if and only if the instance of the root class instantiates the corresponding category. In our example, this means that an instance of Book is of category "TextBook" if and only if its attribute subjectArea has a value, and it is of category "Biography" if and only if its attribute about has a value.

  6. Drop the segment subclasses from the model.

In the case of our example, the result of this design re-factoring is shown in Figure 12.7 below. Notice that the constraint (or "invariant") represents a logical sentence where the logical operator keyword "IFF" stands for the logical equivalence operator "if and only if" and the property condition prop=undefined tests if the property prop does not have a value.

Figure 12.7. The result of applying the Class Hierarchy Merge design pattern