object oriented software development 2009-2010 class diagrams: towards implementation oosad booklet...
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Object Oriented Software
Development2009-2010
Class diagrams: Towards implementation
OOSAD Booklet Chapter 4Seminar: Week 4Brian Farrimond
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Session Topics Object diagrams – snapshots for before
and after Role names Implementing classes in Java Implementing associations
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Object diagram Object diagrams represent actual
objects and the links between them They give useful snapshots of the
state of the system to help understand what changes to the system the functionality will cause
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Class diagram
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Object diagram
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Object diagram after Fred borrows “Sing with Bing”
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What has happened between the two object diagrams A new LP object with name A416
has been created and given the attribute value “Sing with Bing”.
This new object has been linked to the Fred object.
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What has happened between the two object diagrams A new LP object with name A416
has been created and given the attribute value “Sing with Bing”.
This new object has been linked to the Fred object.
.. This is what the code for borrowing an LP must
achieve
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Object diagrams Objects are connected by links A link is an instance of an
association just like an object is an instance of a class
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Using role names Role names are an alternative to
association names More useful when we come to
implementation
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Using role names
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Using role names
If only one association between two classes then some UML authors permit the role names to be omitted
The convention is that if role name omitted then a default role name is the name of the class at this end of the association
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Using role names
If only one association between role names then some UML authors permit the role names to be omitted
The convention is that if role name omitted then a default role name is the name of the class at the this end of the association
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Using role names Exercise 4.2 on Page 56
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Using role names Exercise 4.2 on Page 56
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Attributes and operations in class diagrams - reminder Three layered
cake Class name Attribute list Operations list
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Attributes and operations in class diagrams - reminder Things we miss
out Attributes to
indicate association with other classes in the diagram …
Because … the association
line provides that information already so we avoid clutter by avoiding duplication
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Attributes and operations in class diagrams - reminder Things we miss
out Operations to set
and retrieve attribute values …
Because … we can take
those for granted and so save clutter
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Implementing the Member class in Java We leave implementation in C++
until later in the module – it needs knowledge of pointers - scary
Java hides the presence of pointers from us. It was one of the design goals of the creators of Java
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Implementing the Member class in Java
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Implementing the Member class in Java
Attributes -> Java fields
Operations -> Java methods
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Implementing the Member class in Java
Keywords
private: fields not visible outside the class – need messages to access them
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Implementing the Member class in Java
Keywords
public: the class and the methods can be used outside
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Implementing associations in Java
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Implementing associations in Java
With role names:
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Implementing associations in Java
With role names:
public class Ambulance {
private Crew[] crew;
:
public class Crew {
private Ambulance ambulance;
:
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Implementing associations in Java
With role names:
public class Ambulance {
private Crew[] crew;
:
public class Crew {
private Ambulance ambulance;
:
An array since there is more than one crew in each ambulance
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Sending messages between objects
Class diagrams record the way messages can pass between objects
A B
CD
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Sending messages between objects
In this diagram messages can be sent: between A and
B between A and D between B and C
A B
CD
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Sending messages between objects
In this diagram messages can be sent: between A and B between A and
D between B and C
A B
CD
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Sending messages between objects
In this diagram messages can be sent: between A and B between A and D between B and
C
A B
CD
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Sending messages between objects
To send a message from A to C we must send a message
to B
that causes B to send a message to C
A B
CD
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Sending messages between objects
To send a message from A to C we must send a
message to B
that causes B to send a message to C
A B
CD
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Sending messages between objects
To send a message from A to C we must send a message
to B
that causes B to send a message to
C
A B
CD
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public class Ambulance {
private Crew[] crew;
:
crew[1].doThis();
public class Crew {
private Ambulance ambulance;
:
public void doThis() {
}
Sending messages between objects
doThis()
Sending the message Method to handle message
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public class Ambulance {
private Crew[] crew;
:
public void doThat(){
}
public class Crew {
private Ambulance ambulance;
:
ambulance. doThat() ;
Sending messages between objects
doThat()
Sending the messageMethod to handle message
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Navigation If we know that messages only
travel one way along an association we can modify the class diagram to indicate this …
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Navigation If we know that messages only
travel one way along an association we can modify the class diagram to indicate this …
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Navigation If we know that messages only
travel one way along an association we can modify the class diagram to indicate this …
Arrow head indicates direction of messages
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public class Ambulance {
private Crew[] crew;
:
public class Crew {
:
No attribute needed for Ambulance since we are not sending messages from
here to the “owning” Ambulance
Navigation
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If a class diagram shows no arrowheads than assume the navigation is both ways
If any arrow heads are present then the only navigability allowed is that indicated by the arrow heads
Navigation
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Summary Object diagrams provide snapshots for before
and after Role names are an alternative to association
names Attributes and operations can be recorded in
class diagrams We can implement classes in Java We can implement associations in Java taking
into account navigability
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Exercises Ex 4.4 page 67 Ex 4.5 page 67 Ex 4.6 page 68