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Foundations of Programming: Object-Oriented Design
Illustration by Mark Todd

Using aggregation and composition


From:

Foundations of Programming: Object-Oriented Design

with Simon Allardice

Video: Using aggregation and composition

We've already seen what are referred to as Associations in a diagram. Drawing any kind of line between objects simply suggests there is some kind of interaction. One object knows about or interacts with a different object. We can add a note to explain this, it might just be that one object is calling a method of another object. But we can get more specialized with what we're trying to illustrate. We've seen the way already that we show Inheritance with the empty arrow pointing at the superclass or parent class.
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  1. 11m 35s
    1. Welcome
      1m 25s
    2. Who this course is for
      1m 15s
    3. What to expect from this course
      3m 6s
    4. Exploring object-oriented analysis, design, and development
      1m 41s
    5. Reviewing software development methodologies
      4m 8s
  2. 26m 14s
    1. Why we use object-orientation
      2m 42s
    2. What is an object?
      5m 22s
    3. What is a class?
      4m 43s
    4. What is abstraction?
      2m 45s
    5. What is encapsulation?
      3m 45s
    6. What is inheritance?
      3m 35s
    7. What is polymorphism?
      3m 22s
  3. 12m 16s
    1. Understanding the object-oriented analysis and design processes
      4m 13s
    2. Defining requirements
      6m 9s
    3. Introduction to the Unified Modeling Language (UML)
      1m 54s
  4. 23m 35s
    1. Understanding use cases
      6m 11s
    2. Identifying the actors
      4m 16s
    3. Identifying the scenarios
      5m 7s
    4. Diagramming use cases
      4m 18s
    5. Employing user stories
      3m 43s
  5. 16m 36s
    1. Creating a conceptual model
      1m 59s
    2. Identifying the classes
      2m 27s
    3. Identifying class relationships
      2m 38s
    4. Identifying class responsibilities
      6m 43s
    5. Using CRC cards
      2m 49s
  6. 22m 25s
    1. Creating class diagrams
      6m 11s
    2. Converting class diagrams to code
      4m 57s
    3. Exploring object lifetime
      5m 55s
    4. Using static or shared members
      5m 22s
  7. 19m 49s
    1. Identifying inheritance situations
      6m 49s
    2. Using inheritance
      2m 43s
    3. Using abstract classes
      2m 2s
    4. Using interfaces
      4m 20s
    5. Using aggregation and composition
      3m 55s
  8. 9m 23s
    1. Creating sequence diagrams
      5m 18s
    2. Working with advanced UML diagrams
      2m 3s
    3. Using UML tools
      2m 2s
  9. 10m 39s
    1. Introduction to design patterns
      2m 40s
    2. Example: the singleton pattern
      4m 53s
    3. Example: the memento pattern
      3m 6s
  10. 21m 47s
    1. Introduction to object-oriented design principles
      2m 50s
    2. Exploring general development principles
      3m 55s
    3. Introduction to SOLID principles
      6m 43s
    4. Introduction to GRASP principles
      8m 19s
  11. 7m 1s
    1. Reviewing feature support across different object-oriented languages
      3m 50s
    2. Additional resources
      2m 27s
    3. Goodbye
      44s

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Foundations of Programming: Object-Oriented Design
3h 1m Intermediate May 22, 2012

Viewers: in countries Watching now:

Most modern programming languages, such as Java, C#, Ruby, and Python, are object-oriented languages, which help group individual bits of code into a complex and coherent application. However, object-orientation itself is not a language; it's simply a set of ideas and concepts.

Let Simon Allardice introduce you to the terms—words like abstraction, inheritance, polymorphism, subclass—and guide you through defining your requirements and identifying use cases for your program. The course also covers creating conceptual models of your program with design patterns, class and sequence diagrams, and unified modeling language (UML) tools, and then shows how to convert the diagrams into code.

Topics include:
  • Why use object-oriented design (OOD)?
  • Pinpointing use cases, actors, and scenarios
  • Identifying class responsibilities and relationships
  • Creating class diagrams
  • Using abstract classes
  • Working with inheritance
  • Creating advanced UML diagrams
  • Understanding object-oriented design principles
Subjects:
Developer Design Patterns Programming Foundations
Software:
Java
Author:
Simon Allardice

Using aggregation and composition

We've already seen what are referred to as Associations in a diagram. Drawing any kind of line between objects simply suggests there is some kind of interaction. One object knows about or interacts with a different object. We can add a note to explain this, it might just be that one object is calling a method of another object. But we can get more specialized with what we're trying to illustrate. We've seen the way already that we show Inheritance with the empty arrow pointing at the superclass or parent class.

And we have another couple of terms here to explore and their supporting diagrams. We are going to talk about aggregation and composition, both are very long words for simple ideas. And they describe an obvious relationship between our objects that one object can often be built of other objects. So Aggregation is often referred to as a HAS A relationship as opposed to the IS A relationship of Inheritance. We would never say something like a customer is a address, but we might say a customer has a address or a car has a engine.

Well, correctly I'd say a car has an engine, but this will work. Now HAS A can implicitly suggest HAS MANY, so a bank has many bank accounts or a university has many students. But what we're exploring is the HAS A relationship not the IS A relationship. In UML we can display it like this, there is an official diagram for the HAS A aggregation, it's the unfilled diamond. So, for example, we might have a classroom object that will contain an array of student objects that might be important to diagram that relationship.

In this case we'd read it as a classroom has a student, potentially classroom has many students. So as with other diagrams we can optionally have a multiplicity indicated to say that one classroom can have--and we use the Asterisk to represent zero to many students. This is aggregation, and it's very common, and to be honest it's not always worth showing it on a class diagram unless there is something interesting or unusual about it, but you will see this empty diamond, and that's what it means.

Now Composition is also Aggregation, Composition is a more specific form of Aggregation. And it implies ownership, now what does that mean? Let's say we have a Document class and a Page class, and what we imagine is that a Document object will be composed of Page objects, made up of page objects, this is Composition. Now what's the difference between Aggregation and Composition or Composition implies ownership. And what that means is when the owning object is destroyed so are the contained objects.

It's still a HAS A relationship here, Document has a Page or has many pages. But if I were to delete the Document object all the associated Page objects should be deleted too. I would not expect then those page objects to be shared with any other part of the application. On the other hand in a plain aggregation situation as with the Classroom and Student relationship, if I deleted the Classroom object, perhaps the class got canceled, I would not expect all the Student objects to be destroyed, they may be used in different classrooms or just be able to live on their own.

And that's the difference, Composition implies ownership, Aggregation does not. Now Aggregation is not usually worth showing on a diagram, but Composition often can be. If the lifetime of an object is dependent on another object existing that can be worth showing, even if we are just prompting the idea that when you're defining the owning class, say here the Document class, you may need to write a constructor and a destructor that would take care of creating and/or deleting the internal objects.

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