Consider the simple class hierarchy of the design model in Figure 17.1 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 model 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. In this tutorial, we only discuss the simpler case of Class Hierarchy Merge re-factoring for disjoint segmentations, where
we take the following re-factoring steps:

Add an

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

.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.Whenever the segmentation is

**rigid**(does not allow dynamic classification), 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.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 attribute`subjectArea`

from`TextBook`

and`about`

from`Biography`

to`Book`

, making them optional, that is [0..1].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.Drop the segment subclasses from the model.

In the case of our example, the result of this design re-factoring is shown in Figure 17.7 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.