--- title: "Four class relationships from figure" subject: "Object Oriented Analysis and Design" chapter: 2 key: "semester_6/object_oriented_analysis_and_design:class-relationships-from-figure" source: https://www.bctengineer.com/q/semester_6/object_oriented_analysis_and_design/class-relationships-from-figure site: BCT Engineering --- # Four class relationships from figure Object Oriented Analysis and Design · Chapter 2: Object Oriented Analysis - Asked in: 2069 Bhadra, 8 marks - Video: [UML Class and Object Diagrams | Association vs. Aggregation vs. Composition | Geekific](https://www.youtube.com/watch?v=sN2_CoB_kbw) — Geekific - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/object_oriented_analysis_and_design/class-relationships-from-figure - Whole chapter: https://www.bctengineer.com/md/semester_6/object_oriented_analysis_and_design/chapter-2 ## Question Explain four different types of relationships that we model in object-oriented analysis, which exists between two different classes represented as shown in below figure. [Figure: two class boxes connected by relationship arrows; the figure is not legible in the scan.] ## Short answer The four key relationships are dependency (dashed arrow for a uses-a relationship), association (solid line with multiplicity, including aggregation hollow diamond and composition filled diamond), generalization (hollow triangle for is-a inheritance), and realization (dashed hollow triangle for implementing an interface). ## Answer The figure is not legible, so the four standard relationships that UML models between two classes are explained with their notation: **dependency, association, generalization and realization** (the four listed in the UML User Guide). Aggregation and composition are covered as special forms of association. #### 1. Dependency A weak, temporary "uses" relation: a change in the supplier may affect the client, e.g. when the supplier is a method parameter or local variable. Drawn as a dashed arrow towards the supplier. ```text +---------+ - - - - -> +--------+ | Printer | <> | Report | +---------+ +--------+ ``` #### 2. Association (with aggregation and composition) A structural link between objects of two classes ("knows/is linked to"). Drawn as a solid line, with a name, role names and multiplicity. ```text +---------+ 1 0..* +---------+ | Teacher |-------------| Student | +---------+ teaches +---------+ ``` A teacher teaches many students. Aggregation is a "has-a" (whole-part) association where the part can exist without the whole (hollow diamond at the whole). Composition is a strong form where the part dies with the whole (filled diamond). ```text +-------+ <>-------- +--------+ hollow: aggregation | Team | | Player | +-------+ <#>------- +--------+ filled: composition | House | | Room | ``` #### 3. Generalization (inheritance) An "is-a" relation between a general superclass and specialised subclasses. Drawn as a solid line with a hollow triangle at the superclass. ```text +--------+ | Person | +--------+ ^ +-----+-----+ +---------+ +---------+ | Student | | Teacher | +---------+ +---------+ ``` #### 4. Realization A relation between an interface (a specification) and the class that implements it; the class promises to provide all the interface's operations. Drawn as a dashed line with a hollow triangle pointing to the interface. ```text +---------------+ | <> | | Printable | +---------------+ ^ : (dashed, hollow triangle) +---------------+ | Invoice | +---------------+ ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the difference between aggregation and composition? A. Aggregation: part can exist without the whole (hollow diamond); composition: part dies with the whole (filled diamond) B. Aggregation: filled diamond; composition: hollow C. Both mean 'is-a' D. Aggregation is dashed Answer: A. Team-Player is aggregation; House-Room is composition. (Key point: Aggregation vs composition) 2. How is generalization drawn in UML? A. Dashed arrow B. Solid line with a hollow triangle at the superclass C. Solid line with a filled diamond D. Plain line with multiplicity Answer: B. Generalization is the 'is-a' relationship, e.g. Student is a Person. (Key point: Generalization notation) 3. Which describes dependency? A. A strong whole-part link B. A weak, temporary 'uses' relation (e.g. a parameter), drawn as a dashed arrow to the supplier C. An 'is-a' relation D. A relation with multiplicity 1..* Answer: B. A change in the supplier may affect the client. (Key point: Dependency) 4. What is an association? A. A structural link between objects of two classes, drawn as a solid line with role names and multiplicity B. A dashed arrow C. An inheritance link D. A realisation of an interface Answer: A. E.g. a Teacher teaches many Students. 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