In the UML class diagram, which of the following represents the relationship between the classes as shown in the figure below? [Figure: UML class diagram: class Class1 on the left and class Class2 on the right, joined by a line from Class2 that ends in a hollow triangle pointing at Class1.]

ITPEC FE Morning April 2019 Question 46

In the UML class diagram, which of the following represents the relationship between the classes as shown in the figure below?

a)Aggregationb)Associationc)Compositiond)Generalization

Explanation

Generalization — a solid line ending in a hollow triangle that points at the superclass.

In the figure the line leaves Class2 and ends in a hollow triangle at Class1. That is the UML notation for generalization: Class2 is a subclass of Class1 and inherits its attributes and operations ("Class2 is a Class1"). The triangle always points at the more general class.

Relationship	Notation
Generalization	solid line, hollow triangle at the superclass
Aggregation	solid line, hollow diamond at the whole
Composition	solid line, filled diamond at the whole
Association	plain solid line, optionally an open arrowhead

Answer (d)

Why not others:

•(a) Aggregation — a whole-part relation drawn with a hollow diamond, not a triangle
•(b) Association — a plain line (or open arrowhead for navigability), with no triangle
•(c) Composition — a strong whole-part relation drawn with a filled diamond

Key rule: Triangle means "is a" (generalization); diamond means "has a" (hollow for aggregation, filled for composition), always drawn at the whole.

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