CS616 – Software Engineering II

Lecture 6

Larman: Chapter 13



·       Contracts for operations define system behavior

·       Describe the outcome of executing system operation in terms of state changes to domain objects.


·       Contracts describe detailed system behavior in terms of state changes to objects in the Domain Model, after a system operation has executed.

·       Contracts are defined for system operations

operations that the system offers in its public interface to handle incoming system events.

·       System operations can be identified by discovering these system events


Figure: System operations handle input system events.


·       Entire set of system operations, across all use cases, defines the public system interface

o      UML - the system as a whole can be represented by a class.


Example Contract: enteritem


Contract C02: enteritem


Cross References:



enterltem(itemlD: ItemID, quantity: integer)

Use Cases: Process Sale

There is a sale underway.



— A SalesLineltem instance sli was created (instance creation).

— sli was associated with the current Sale (association formed).

— sli.quantity became quantity (attribute modification).

— sli was associated with a ProductSpecification, based on itemlD match (association formed).


Contract Sections - Template

A description of each section in a contract is shown in the following schema.


Name of operation, and parameters


Cross References:

(optional) Use cases this operation can occur within



Noteworthy assumptions about the state of the system or objects in the Domain Model before execution of the opera­tion. These will not be tested within the logic of this operation, are assumed to be true, and are non-trivial assumptions the reader should know were made.



The state of objects in the Domain Model after completion of the operation. Discussed in detail in a following section.



Postconditions – General Discussion

·       Postconditions are declarations about the Domain Model objects that are true when the operation has finished

·       Domain Model state changes include:

·       Instance creation and deletion.

·       Attribute modification.

·       Associations formed and broken

·       Advantage of Postconditions

o      Describes state changes required of a system operation without having to describe how they are to be achieved.

o      e.g. postconditions:

— A SalesLineltem instance sli was created (instance creation).

— sli was associated with the current Sale (association formed).

— sli.quantity became quantity (attribute modification).

— sli was associated with a ProductSpecification, based on itemlD match (association formed).

·       Express postconditions in the past tense, to emphasize they are declarations about a state change in the past.

1.     For example:

·    (better) A SalesLineltem was created.

·    (worse) Create a SalesLineltem.


Postconditions –enterItem Postconditions Discussion

Instance Creation and Deletion

After the itemlD and quantity of an item have been entered, the new object SalesLineltem is created:


·         A SalesLineltem instance sli was created (instance creation).


Attribute Modification

After the itemlD and quantity of an item have been entered by the cashier, the quantity of the SalesLineltem should have become equal to the quantity parameter:


·         sli.quantity became quantity (attribute modification).


Associations Formed and Broken

After the itemlD and quantity of an item have been entered by the cashier, the new SalesLineltem should have been related to its Sale, and related to its ProductSpecification:

·                 sli was associated with the current Sale (association formed).

·                 sli was associated with a ProductSpecification, based on itemlD match (association formed).


·       When Are Contracts Useful ?

Use cases are main repository of project’s requirements.

Situations where details and complexity of required state changes are awkward to capture in use cases.

e.g. airline reservation system and the system operation addNewReservation.

Guidelines for creating Contracts

1.       Identify system operations from the SSDs.

2.       Construct a contract for system operations that are complex or subtle in their results, or which are not clear in the use case

3.               Describe postconditions, using the following categories:

o        instance creation and deletion
o        attribute modification
o        associations formed and broken


·                 State postconditions in a declarative, passive past tense form to emphasize the declaration of a state change rather than a design of how it is going to be achieved.

o      e.g.

o   (better) A SalesLineltem was created.

o   (worse) Create a SalesLineltem.


·                 Establish a memory between existing objects or those newly created by defining the forming of an association.

o      e.g.,

§       Not enough that new SalesLineltem instance is created when the enterltem operation occurs.

§       After the operation is complete, it should be true that the newly created instance was associated with Sale:


The SalesLineltem was associated with the Sale (association formed).

NextGen POS Example: Contracts

System Operations of Process Sale


Contract COl: inakeNewSale


Cross References:




Use Cases: Process Sale





— A Sale instance s was created (instance creation).

— s was associated with the Register (association formed).

— Attributes of s were initialized.



Contract CO2: enterltem


Cross References:



enterltem(itemlD : ItemID, quantity: integer)

Use Cases: Process Sale

There is a sale underway.




A SalesLineltem instance sli was created (instance creation).

       s/i was associated with the current Sale (association tormed).

       sli.quantity became quantity (attribute moditication).

sli was associated with a ProductSpecification, based on itemlD match (association tormed).


Contract C03: endSale


Cross References:




Use Cases: Process Sale

There is a sale underway


- Sale.isComplete became true (attribute modification)

Contract C04: makePayment


Cross References:



makePayment( amount: Money)

Use Cases: Process Sale

There is a sale underway.




       A Payment instance p was created (instance creation).

       p.amount Tendered became amount (attribute moditication).

       p was associated with the current Sale (association tormed).

       The current Sale was associated with the Store (association tormed); (to add it to the historical log ot completed sales)



Contracts, Operations, and the UML

·       UML formally defines operations:

An operation is a specification of a transformation or query that an object may be called to execute


·       A method (in the UML) is an implementation of an operation.

·       UML operation has a:

1.     signature (name and parameters)

2.     operation specification - describes the effects produced by executing the operation (postconditions).


·       Contracts applied to operations at any level of granularity:

o      e.g.

§       the public operations (or interface) of a subsystem

§       an abstract class


Operation Contracts Within the UP


Inception —— No - too detailed.

Elaboration ——

·       Most contracts written during elaboration, when most use cases are written.

·       Only write contracts for the most complex and subtle system operations.


Larman: Chapter 14




UP guidelines –

·       10% of the requirements investigated in inception

·       slightly deeper investigation started in first iteration of elaboration.

·       Now shift emphasis toward designing a solution in terms of collaborating software objects.


Iteratively Do the Right Thing, Do the Thing Right

·       In iterative development, a transition from primarily a requirements focus to primarily a design and implementation focus will occur in each iteration.

·       Over course of early elaboration iterations, requirements discovery will stabilize,

·       By end of elaboration, 80% of the requirements are reliably defined in detail.


On to Object Design

·       During object design, a logical solution based on the object-oriented paradigm is developed.

·       Heart of solution is the creation of interaction diagrams, which illustrate how objects collaborate to fulfill the requirements.

·       After drawing interaction diagrams, class diagrams are drawn that summarize the definition of the software classes (and interfaces) that are to be implemented in software.

·       These artifacts are part of the Design Model.

·       Interactions diagrams are the most important for developing a good design

·       Creation of interaction diagrams requires the application of principles for assigning responsibilities and the use of design principles and patterns.

Larman: Chapter 15




·       UML includes interaction diagrams to illustrate how objects interact via messages.

·       This chapter introduces notation.


Sequence and Collaboration Diagrams

Interaction diagram refers to:

·   collaboration diagrams

·   sequence diagrams


·       Collaboration diagrams show object interactions in a graph or network format


Figure: Collaboration diagram.


·       Sequence diagrams illustrate interactions in a “fence format,” where each new object is added to the right


Figure 15.2 Sequence diagram.


·       Strengths and weaknesses:

o Diagrams for narrow pages - collaboration diagrams

§       Allow vertical expansion for new objects

o Collaboration diagram examples make it harder to easily see the sequence of messages.






clearly shows sequence or time ordering of messages

simple notation

forced to extend to the right when add ing new objects; consumes horizontal space


space economical—flexibility to add new objects in two dimensions


better to illustrate complex branching, iteration, and concurrent behavior

difficult to see sequence of messages

more complex notation


Example Collaboration Diagram: makePayment

Figure: Collaboration diagram.


·       Read the collaboration diagram shown above as follows:

1.  The message makePayment is sent to an instance of a Register. The sender is not identified.

  1. The Register instance sends the makePayment message to a Sale instance.
  2. The Sale instance creates an instance of a payment


Example Sequence Diagram: makePayment

Figure 15.4 Sequence diagram.


·       The sequence diagram shown above has the same intent as the prior collaboration diagram.


Common Interaction Diagram Notation

UML has adopted a simple and consistent approach to illustrate instances vs. classifiers

·   For any kind of UML element (class, actor, ...), an instance uses the same graphic symbol as the type, but the designator string is underlined.

Figure 15.5 Class and instances.


·       To show an instance of a class in an interaction diagram, the regular class box graphic symbol is used, but the name is underlined.

o A name can be used to uniquely identify the instance.

o If none is used, a “:“ precedes the class name.


·       UML has a standard syntax for message expressions:

return := message(parameter : parameterType) : returnType



spec := getProductSpect(id)

spec := getProductSpect(id:ItemID)

spec := getProductSpect(id:Ite.mID) : ProductSpecification


Basic Collaboration Diagram Notation

·   Links

o      it indicates that some form of navigation and visibility between the objects is possible (see Figure 15.6).

o      More formally, a link is an instance of an association.

e.g., there is a link from a Register to a Sale, along which messages may flow, such as the makePayment message.

Figure: Link lines.


o      Multiple messages can flow along the same single link in both directions.


·       Messages

o   Each message between objects is represented with a message expression and small arrow indicating the direction of the message.

o   Many messages may flow along this link

o   A sequence number is added to show the sequential order of messages in the current thread of control.

Figure: Messages.


·   Messages to “self” or “this”

o A message can be sent from an object to itself

o Shown by a link to itself, with messages flowing along the link.


Figure : Messages to “this.”


·   Creation of Instances

o      Any message can be used to create an instance

o In UML - use a message named create for this purpose.

o If another (less obvious) message name is used, the message may be annotated with a special feature called a UML stereotype, like so: «create».

o The create message may include parameters, indicating the passing of initial values.

o UML property [new] may optionally be added to the instance box to highlight the creation.

Figure : Instance creation.


·   Message Number Sequencthg

o The order of messages is illustrated with sequence numbers

o The numbering scheme is:

1.     The first message is not numbered.

msg1( ) is unnumbered.

2.     The order and nesting of subsequent messages is shown with a legal numbering scheme

·       Nested messages have a number appended to them.

·       Nesting is denoted by prepending the incoming message number to the outgoing message number.



Figure :Sequence numbering.


·   Conditional Messages

o A conditional message is shown by following a sequence number with a conditional clause in square brackets, similar to an iteration clause.

o The message is only sent if the clause evaluates to true.


Figure : Conditional message.


·   Mutually Exclusive Conditional Paths

Figure :Mutually exclusive messages


o Must modify the sequence expressions with a conditional path letter.

o The first letter used is a by convention.


o Figure states that either 1a or 1b could execute after msg1.

o Both are sequence number 1 since either could be the first internal message.


o Subsequent nested messages are still consistently prepended with their outer message sequence.

1b.1 is nested message within 1b.


·   Iteration or Looping

·       If the details of the iteration clause are not important to the modeler, a simple ‘*’ can be used.


·      Iteration Over a Collection (Multiobject)

o      Common algorithm - iterate over all members of a collection (e.g. a list or map), sending a message to each.

o      Some kind of iterator object is used

o      UML - term multiobject - used to denote a set of instances — a collection.


Figure : Iteration over a multiobject.


o      The “*“ multiplicity marker is used at the end of the link to indicate that the message is being sent to each element of the


·   Messages to a Class Object

o      Messages may be sent to a class itself to invoke class or static methods.

o      A message is shown to a class box whose name is not underlined, indicating the message is being sent to a class rather than an instance


Figure: Messages to a class object (static method invocation)


Basic Sequence Diagram Notation


o      Unlike collaboration diagrams, sequence diagrams do not show links.



o      Each message between objects is represented with a message expression on an arrowed line between the objects

o      Time ordering is organized from top to bottom.


Figure: Messages and focus of control with activation boxes.


o      Sequence diagrams may show the focus of control (that is, in a regular blocking call, the operation is on the call stack) using an activation box.

o      The box is optional, but commonly used by UML practitioners.


Illustrating Returns


A sequence diagram may show the return from a message as a dashed open-arrowed line at the end of an activation box


o Many practitioners exclude them.

o Some annotate the return line to describe what is being returned (if anything) from the message.


Messages to “self” or “this”

o      Use nested activation box.


Creation of Instances


Figure : Instance creation and object lifelines.


·       object lifelines —vertical dashed lines underneath the objects

·       Indicate the extent of the life of the object in the diagram.

·       In some circumstances it is desirable to show explicit destruction of an object  

·       UML lifeline notation provides a way to express this destruction


Figure: Object Destruction


Conditional Messages

A condition message is shown in Figure



Figure : A conditional message.


Mutually Exclusive Conditional Messages

Notation is a angled message line emerging from a common point



Figure :Mutually exclusive conditional messages.


Iteration for a Single Message

Iteration notation for one message is shown.



Iteration of a Series of Messages

Notation to indicate iteration around a series of messages is shown



Iteration Over a Collection (Multiobject)

In sequence diagrams, iteration over a collection is shown



·       A ‘*‘ multiplicity marker at the end of the role (next to the multiobject) to indicate sending a message to each element rather than repeatedly to the collection itself.


Messages to Class Objects

·       Class or static method calls are shown by not underlining the name of the classifier, which signifies a class object rather than an instance


Figure: Invoking class or static methods.



Larman: Chapter 16



·       The GRASP patterns are a learning aid to help understand essential object design, and apply design reasoning in a methodical, rational, explainable way

·       This approach is based on patterns of assigning responsibilities.


Responsibilities and Methods

·       UML defines a responsibility as “a contract or obligation of a classifier”

·       Responsibilities are related to the obligations of an object in terms of its behavior.

Two types:

·  knowing

·  doing


·        Doing responsibilities of an object include:

·        Knowing responsibilities of an object include:


·       Responsibilities are assigned to classes of objects during object design.

e.g. “

·   “a Sale is responsible for creating SalesLineltems”   (a doing)

·   “a Sale is responsible for knowing its total”               (a knowing).

·        Relevant responsibilities related to “knowing” are often inferable from the domain model, because of the attributes and associations it illustrates.


·        Responsibilities are implemented using methods that either act alone or collaborate with other methods and objects.


Responsibilities and Interaction Diagrams

·        Interaction diagrams show choices in assigning responsibilities to objects

Figure : Responsibilities and methods are related.


·        Figure shows that Sale objects have been given a responsibility to create Payments, which is invoked with a makePayment message and handled with a corresponding makePayment method.

·        Fulfillment of this responsibility requires collaboration to create the SalesLineltem object and invoke its constructor.



·        Patterns are a collection of general principles and idiomatic solutions that guide programmers in the creation of software.

·        Sample pattern:


Pattern Name:

Information Expert


Assign a responsibility to the class that has the information needed to fulfill it.

Problem It Solves:

What is a basic principle by which to assign responsibilities to objects?


·        Object technology - a pattern is a named description of a problem and solution that can be applied to new contexts

o  it provides advice in how to apply it in varying circumstances, and considers the forces and trade-offs.’

o  Many patterns provide guidance for how responsibilities should be assigned to objects, given a specific category of problem.


·        All patterns have suggestive names.


·  Supports chunking and incorporating that concept into understanding and memory.

·  Facilitates communication.

·        GRASP patterns have concise names such as Information Expert, Creator, Protected Variations.


GRASP:   Patterns of General Principles in Assigning Responsibilities

Question:     What are the GRASP patterns?

Answer:       They describe fundamental principles of object design and responsibility assignment, expressed as patterns.


·        GRASP is an acronym that stands for General Responsibility Assignment Software Patterns.


·        First five GRASP patterns:

·   Information Expert

·   Creator

·   High Cohesion

·   Low Coupling

·   Controller


The UML Class Diagram Notation

·        A UML class box illustrates software classes by showing three compartments


Figure : Software classes illustrate method names.


Information Expert (or Expert)

Solution       Assign a responsibility to the information expert — the class that has the information necessary to fulfill the responsibility.


Problem       What is a general principle of assigning responsibilities to objects?


Example       In the NextGEN POS application, some class needs to know the grand total of a sale.


Start assigning responsibilities by clearly stating the responsibility.


Ask: Who should be responsible for knowing the grand total of a sale?


·        By Information Expert, look for that class of objects that has the information needed to determine the total.


Key question: Do we look in the Domain Model or the Design Model to analyze the classes that have the information needed?

·        The Domain Model illustrates conceptual classes of the real-world domain

·        The Design Model illustrates software classes.



1.  If there are relevant classes in the Design Model, look there first.

2.  Else, look in the Domain Model, and attempt to use (or expand) its representations to inspire the creation of corresponding design classes.


·   e.g.

o      assume we have begun design work and there is no or a minimal Design Model.

o      Therefore, look to the Domain Model for information experts;

§       perhaps the real-world Sale is one.

o      Then add a software class to the Design Model called Sale, and give it the responsibility of knowing its total, expressed with the method named getTotal.

·   This approach supports low representational gap in which the software design of objects appeals to our concepts of how the real domain is organized.


·   To examine this case in detail, consider the partial Domain Model in Figure


Figure : Associations of Sale.


·   What information is needed to determine the grand total?

·   It is necessary to know about all the SalesLineltem instances of a sale and the sum of their subtotals.

·   A Sale instance contains these

·   therefore, by the guideline of Information Expert, Sale is a suitable class of object for this responsibility;

·   It is an information expert for the work.


·   Interaction diagram creation raises questions of responsibility.


·        we are starting to work through the drawing of diagrams in order to assign responsibilities to objects.

·        A partial interaction diagram and class diagram


·        What information is needed to determine the line item subtotal?

o      SalesLineltem. quantity and ProductSpecification.price

§       SalesLineltem knows its quantity and its associated ProductSpecification;

§       Ttherefore, by Expert, SalesLineltem should determine the subtotal; it is the information expert.


In terms of an interaction diagram, this means that the Sale needs to send get-Subtotal messages to each of the SalesLineltems and sum the results;


Figure : Calculating the Sale total.



·       To fulfill the responsibility of knowing and answering its subtotal, a SalesLineltem needs to know the product price.

·       The ProductSpecification is an information expert on answering its price; therefore, a message must be sent to it asking for its price.



Figure: Calculating the Sale total


·       So, to fulfill the responsibility of knowing and answering the sale’s total, three responsibilities were assigned to three design classes of objects as follows.

Design Class



knows sale total


knows line item subtotal


knows product price


The context in which these responsibilities were considered and decided upon was while drawing an interaction diagram. The method section of a class diagram can then summarize the methods.


The principle by which each responsibility was assigned was Information Expert—placing it with the object that had the information needed to fulfill it.


·        Information Expert is frequently used in the assignment of responsibilities

o      Basic guiding principle used continuously in object design.


·        Fulfillment of a responsibility often requires information that is spread across different classes of objects.

o      Many “partial” information experts who will collaborate in the task.

o      For example, the sales total problem ultimately required the collaboration of three classes of objects.


·        Whenever information is spread across different objects, they will need to interact via messages to share the work.



·        Situations where a solution suggested by Expert is undesirable, usually because of problems in coupling and cohesion

o      e.g. who should be responsible for saving a Sale in a database?

o      Much of the information to be saved is in the Sale object, and thus by Expert an argument could be made to put the responsibility in the Sale class.

o      Logical extension of this decision is that each class has its own services to save itself in a database.

o      Leads to problems in cohesion, coupling, and duplication.

§       the Sale class must now contain logic related to database handling

§       The class is no longer focused on just the pure application logic of “being a sale;” it now has other kinds of responsibilities, which lowers its cohesion.

§       The class must be coupled to the technical database services of another subsystem, rather than just being coupled to other objects in the domain layer of software objects, which raises its coupling.

§       Likely that similar database logic would be duplicated in many persistent classes.