SummaryInh Metrics

The SummaryInh metrics measure the "amount" of inheritance that takes place in a system. (For measurements relating to inheritance for individual modules see the ScalarInh metrics suite.) Specifically they measure the proportion of modules in the system that participate in an inheritance relationship, either Defined Using Inheritance (DUI) or being Inherited From (IF). This can be thought of as the degree to which the software engineer chose to use inheritance.

As with the ModuleCount metrics, the measurements can be broken down in different ways. In the case of DUI, modules can be distinguished according to where the parent module(s) come from, such as the standard library (SL), third party (TP), or user defined (UD). The kinds of the two modules at each end of an inheritance relationship can also be distinguished, such as class-extends-class (CC), class-implements-interface (CI), or interface-extends-interface (II). The nesting level of the module that is defined using inheritance can also be distinguished. See below for references for dicussions on the rationale for why these metrics might be useful.

This gives the following metrics.

Using (Defined Using Inheritance) Used (Inherited From)

Class-Class SLCCDUI, TPCCDUI, UDCCDUI CCIF

Class-Interface SLCIDUI, TPCIDUI, UDCIDUI CIIF

Interface-Interface SLIIDUI, TPIIDUI, UDIIDUI IIIF

Interace-Annotation SLIADUI, TPIADUI, UDIADUI IAIF

Enum-Interface SLEIDUI, TPEIDUI, UDEIDUI EIIF

Exception-Interface SLExIDUI, TPExIDUI, UDExIDUI ExIIF

Exception-Exception SLExExDUI, TPExExDUI, UDExExDUI ExExIF

Metric components Each row is one of the 7 relationships identified above (whether it's extends or implements is implied by the combination of kinds of type). The columns show the two directions of the relationship. The cells of the "using" relation have the metrics in the order of: using Standard Library, using Third Party, or using User Defined.

**Metric components** Each row is one of the 7 relationships identified above (whether it's `extends` or `implements` is implied by the combination of kinds of type). The columns show the two directions of the relationship. The cells of the "using" relation have the metrics in the order of: using Standard Library, using Third Party, or using User Defined.
	Using (Defined Using Inheritance)	Used (Inherited From)
Class-Class	SLCCDUI, TPCCDUI, UDCCDUI	CCIF
Class-Interface	SLCIDUI, TPCIDUI, UDCIDUI	CIIF
Interface-Interface	SLIIDUI, TPIIDUI, UDIIDUI	IIIF
Interace-Annotation	SLIADUI, TPIADUI, UDIADUI	IAIF
Enum-Interface	SLEIDUI, TPEIDUI, UDEIDUI	EIIF
Exception-Interface	SLExIDUI, TPExIDUI, UDExIDUI	ExIIF
Exception-Exception	SLExExDUI, TPExExDUI, UDExExDUI	ExExIF

Various potentially useful combinations can also be identified.

CCDUI: Proportion of classes that extend other classes.
UDDUI: Proportion of user defined modules that are defined using inheritance
CIDUI: Proportion of classes that implement interfaces.
TCCIF: Proportion of classes that are extended
TCIIF: Proportion of interfaces that are implemented
TIIIF: Proportion of interfaces that are extended
DUI: Proportion of modules that are defined using inheritance.
IF: Proportion of modules that are inherited from.

Further Information

An early essay on Measuring Inheritance in Java.
Full definitions and an empirical study using these metrics on various Java open source systems can be found in:
Ewan Tempero, James Noble and Hayden Melton 'How do Java Programs Use Inheritance? An Empirical Study of Inheritance in Java Software' 22nd European Conference on Object-Oriented Programming (ECOOP), Springer Berlin / Heidelberg Paphos, Cyprus. July 2008. pp. 667-691. [DOI] [Earlier TR]