1. Aggregation and Inheritance
Lecture 8.2
Aggregation and Inheritance

2. Designing with Aggregation and Inheritance
Software Design
Step 1
Identify the objects.
Step 2
Find/design a class for each object.
One of the Challenges
deciding when to use aggregation and when to use inheritance
SupplierClass
SuperClass
ClientClass
SubClass
© 2006 Pearson Addison-Wesley. All rights reserved

3. What is AGGREGATION? public class SomeAggregate { } SomeAggregate
private ClassA partA;
private ClassB partB;
private ClassC partC1;
private ClassC partC2;
// methods omitted }
SomeAggregate
ClassA
ClassB
ClassC
© 2006 Pearson Addison-Wesley. All rights reserved

4. Class Diagram Object Diagram public class SomeAggregate { }
private ClassA partA;
private ClassB partB;
private ClassC partC1;
private ClassC partC2;
// methods omitted }
Class Diagram
Object Diagram
SomeAggregate
ClassA
ClassB
ClassC
:SomeAggregate
partA : ClassA
partB : ClassB
partC1 : ClassC
partC2 : ClassC
© 2006 Pearson Addison-Wesley. All rights reserved

5. When to aggregate? One class contains_a other. contains_a
An equilateral triangle contains_a vertex.
A dog has_a tail.
A sandal includes_a sole.
A company’s work roster contains_an employee.
Cherry pie contains_a cherry pit.
contains_a
© 2006 Pearson Addison-Wesley. All rights reserved

6. When to inherit? One class is_a another class is_a
An equilateral triangle is_a triangle.
A dog is_an animal.
A sandal is_a special kind of footware.
An hourly employee is_an employee.
Cherry pie is_a dessert.
is_a
Generalizations can be discovered from common attributes.
© 2006 Pearson Addison-Wesley. All rights reserved

7. Create the best relationship from...
record button
camcorder
record button
tape recorder
lens
on/off switch
viewfinder
tape recorder
on/off switch
lens
camcorder
viewfinder
© 2006 Pearson Addison-Wesley. All rights reserved

8. A Dilemma (which is it?) A Camcorder is_a tape recorder. or
A Camcorder contains_a tape recorder.
Either aggregation or inheritance
will work in this case (see next slide).
© 2006 Pearson Addison-Wesley. All rights reserved

9. public class Camcorder {
private TapeRecorder rec;
•••
//somewhere in the code
rec.startRecording(); }
public class Camcorder
extends TapeRecorder {
•••
//somewhere in the code
startRecording(); }
© 2006 Pearson Addison-Wesley. All rights reserved

10. java.awt.Container
Container is an awt class well suited to inheritance.
A Container object is a transparent rectangle. It is useful
for grouping other graphical objects into a single unit.
An Container subclass can reuse...
 add and remove
 setBounds and setLocation
 getX, getY, getWidth, getHeight, getBackground
 repaint
A Container subclass may want to override...
 setSize and setBounds
 setBackground
© 2006 Pearson Addison-Wesley. All rights reserved