Generalization (inheritance)

How to read it

Generalization relates a specific class to a more general one. In UML 2.5.1, every instance of the specific classifier is also an instance of the general one, and the specific classifier inherits the general one's features. The arrowhead is a hollow triangle, and it points at the general class. When several subclasses share a parent, each can have its own line and arrowhead, which the specification calls the separate target style, or the lines can join into one arrowhead, the shared target style. Virginia Tech's OpenDSA notes add that merged lines don't mean the subclasses depend on each other, only that they share an ancestor. Robert C. Martin's 1997 tutorial describes the arrowhead as looking "rather like a slice of pizza".

Also searched as: generalization (inheritance) symbol, inheritance arrow, uml generalization (inheritance), uml inheritance.

Look-alikes

The same hollow triangle on a dashed line is Realization: a class implementing an interface, not inheriting from a class. An open arrowhead made of two strokes is something else: on a dashed line it is a Dependency, and on a solid line it marks a navigable end of an Association. A small filled triangle beside an association's name only gives the direction to read the name in.

Differences in course notes

The University of Washington's 2016 software engineering slides vary the arrow by what the parent is: a solid line and black arrow for a class, a solid line and white arrow for an abstract class, and a dashed line and white arrow for an interface. The UML specification uses a hollow triangle for every generalization, whatever the parent.