Testing Object-Oriented Software –
Part One
Object-Oriented Principles
from a testing perspective
Test early, test often, test enough.
Software Engineering of Standalone Programs
University of Colorado
January 20, 2002
ECEN5033 - OO Testing University of Colorado
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Primary reference
• A Practical Guide to Testing Object-Oriented
Software
• John McGregor and David A. Sykes
• Addison Wesley – Object Technology Series, 2001
• ISBN 0-201-32564-0
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What is software testing?
• The evaluation of the work products created during a
software development effort
– Done throughout development effort
– Applied to all development products (models) before
as well as after code is written
• More specifically
– The process of uncovering evidence of defects
– Since a defect can be introduced at any phase,
testing efforts find defects in all phases
• Testing is not the debugging, isolation, or repair of
bugs
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What is software?
• The instruction codes and data necessary to accomplish
some task on a computer or microprocessor
• All representations of those instructions and data
• Analogy
– Architects and builders can examine blueprints to
spot problems
– We can examine analysis and design models before
the code is written with a form of “execution.”
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Assumptions
• Development process is incremental with iterations
within each increment
• Models are expressed in UML
• Software design in accordance with good design
principles
– inheritance
– data hiding
– abstraction
– low coupling
– high cohesion
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Testing vs. Quality Assurance
• Quality Assurance
– Responsible for test plans and system testing
– Monitor testing during development
– Keep statistics
• Testing is a necessary but insufficient part of a QA process
• QA addresses activities designed to
– prevent defects
– remove defects
• Testing helps in identifying problems and failures
• Testing helps QA by identifying them early in dev.
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What’s special about testing OO software?
• Features such as class inheritance and interfaces
support polymorphism in which code manipulates
objects without their exact class being known
– Testers must ensure the code works no matter what
the exact class of such objects might be.
• Features that support data hiding complicate testing
because operations must be added to a class interface
(by the developer) just to support testing
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OO Testing Is Still Testing
• We still do
– unit testing but we change the definition of unit
– integration testing to make sure subsystems work
correctly together
– system testing to verify that requirements are met
– regression testing to make sure previous
functionality still works after new functionality is
added
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OO Testing Is Not Just Old Style Testing
• Fundamental aspect of OO software
• OO Software is designed as a set of objects that
essentially model a problem and then collaborate to
effect a solution
• While the solution may change over time, the structure
and components of the problem do not change as
frequently
– a program structured from the problem is more
adaptable to changes later
– components derived from the problem can be reused
in development of other programs to solve related
problems
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Benefit
• Many analysis models map straightforwardly to design
models which, in turn, map to code
• Start testing during analysis
• Refine the same tests for design
• Refine those tests for code
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Advantages of testing
analysis and design models
• Test cases can be identified earlier in the process, even
while determining requirements
• Early test cases help analysts and designers to
– better understand and express requirements
– ensure that specified requirements are testable
• Bugs can be found early – saving time and money
• Test cases can be reviewed for correctness early in the
project
– If test cases are applied to models early,
misunderstandings of requirements on the part of
testers can be corrected early
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Avoid the “bugging phase”
• In other words, model testing helps to ensure that
testers and developers have a consistent understanding
of the system requirements early in the project.
• However, code testing is still important
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Categories of OO Testing
•
•
•
•
•
•
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Model testing
Class testing instead of unit testing
Class interaction testing instead of integration testing
System and subsystem testing
Acceptance testing
Self-testing
Should you try to apply all of these? Probably not if
you want to be taken seriously and be employed.
• You should learn to recognize approaches and
techniques that will apply to your project in a useful
and affordable way.
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Testing perspective
• Skeptical, objective, thorough, systematic
• Look at any development product and question its
validity
• Attitude that should be held by a developer as well as a
full-time tester
• To ensure
– a. the software will do what it is supposed to do
– b. the software will not do what it is not supposed to
do
– Ensuring “a.” does not automatically ensure “b.”
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Object
• An operational entity that encapsulates both specific
data values and the code that manipulates those values.
• Provides the mechanisms needed to
– receive messages
– dispatch methods
– return results
– associates instance attributes with methods
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Objects from a testing perspective
• Encapsulates – the complete definition of the object is
easy to identify, easy to pass around, easy to manipulate
• Hides information – can make changes to the object hard
to observe which makes checking test results difficult
• Has a state that persists for its life. This state can become
inconsistent and can be the source of incorrect behavior
• Has a lifetime – can be examined during its lifetime to
check if it is in the right state based on its lifetime.
– Common source of failures – construction of an object
too late or destruction of it too early
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Message
• Message – a request that an operation be performed by
some object.
– can include actual parameters used to perform that
operation
– receiver can return a value
• OO program is a community of objects that collaborate
to solve a problem.
• This is achieved by sending messages to one another
– Can result in a return value
– Can result in an exception from receiver to sender
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Messages from a testing perspective
• A message
– has a sender who determines when to send and may
make an incorrect decision about this
– has a receiver
• may not be ready for the specific msg it receives
• may not take the correct action if msg is unexpected
– may include actual parameters
• used by or updated by the receiver
• objects passed as parameters must
– be in correct states before and after the message is
processed
– implement the interfaces expected by the receiver
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Those issues are the primary focus of
interaction testing
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Interface
• Aggregation of behavioral declarations
• Example: a set of behaviors related to being a moving
item on a screen such as the ball in (the old game of)
Pong
• Building block for specifications
• A specification is the total set of public behaviors for a
class.
--------------• Java: has a syntactic construct interface; doesn’t allow
declaration of any state variables
• C++: declare an abstract base class with only public, pure
virtual methods
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Interfaces from a testing perspective
• Interface encapsulates operation specifications which
– build the specifications of larger groupings such as
classes
– If it contains behaviors that do not belong with the
other behaviors, implementations of the interface
will have unsatisfactory designs
• Interface has relationships with other interfaces and
classes.
– may be specified as the parameter type for a
behavior to allow any implementer of that interface
to be passed as a parameter
• Interface describes a set of behavior declarations
whether or not we use the interface syntax
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Class
• Set of objects that share a common conceptual basis.
• Class definition says what members (objects) of the set
look like, what they have in common.
• Objects form the basic elements for executing OO
programs
• Classes are the basic elements for defining OO
programs
– Any concept to be represented in a program must
first be defined in a class.
– Then objects defined by that class are created
(instantiation) and are called instances.
object = instance
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Class as object
Note:
• OO languages usually allow a class to be an object itself
and can have operations and attributes defined for it
• In C++ and Java, operations and data values associated
with a class are identified by the keyword static and these
operations are called static operations
• Public static operations in a class specification mean
– the class itself is an object that can be messaged
– we must treat the class as an object and create tests for
the class as well as for its instances
• Scary thought: non-constant static data associated with a
class can affect the behavior of the instances (yikes!)
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ClassA
SubClassA1
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SubClassA2
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Operations
• A class specification includes a specification for each
of the operations that can be performed by each of its
instances
• An operation is an action that can be applied to an
object to obtain a certain effect.
– Accessor (inspector) operations – provide
information about the object but do not change the
object
– Modifier (mutator) operations – change the state of
the object by setting one or more attributes to have
new values (perhaps not every time)
• Accessors are tested differently than modifiers.
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Two special operations
• Constructor – a class object operation used to create a
new object
– includes initializing a new instance when it comes
into existence
• Destructor – an instance object operation used to
perform any processing needed just prior to the end of
the object’s lifetime
• Differ from accessors & modifiers
– invoked implicitly as a result of the birth and death
of objects
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What do we expect of a class specification?
• A description of what a class represents.
– It’s either a concept in the problem being solved or
– in the solution to that problem
• Some meaning and constraints to be associated with
each of the operations defined in the class specification
– So ... each operation should have a specification that
describes what it does, including its preconditions
and invariants
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Reminder
• Preconditions
– Conditions that must hold before the operation can be
performed.
• Post conditions
– Conditions that must hold after the operation is
performed.
• Invariants
– Conditions that must always hold within the lifetime of
the object
– An operation’s method may violate invariants during
execution but it must “hold” again by completion.
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Preconditions
• Usually stated in terms of one or more of the
following:
– attributes of the object containing the operation
– attributes of any actual parameters in the message
requesting that an operation be performed
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Post conditions
• Usually stated in terms of one or more of the
following:
– attributes of the object containing the operation
– attributes of any actual parameters in the message
requesting that the operation be performed
– the value of any reply
– the exceptions that might be raised
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Invariants
• A class invariant describes a set of operating
boundaries for an instance of a class
• It is possible to define interface invariants and
operational invariants
• A class invariant can be treated as an implied post
condition for EACH operation in the class
• Usually stated in terms of
– attributes of an object
– states of an object
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Behavior of the instances of a class
• The aggregate of the specifications of all of the
operations in a class provides
– part of the description of the behavior of its
instances
• Behavior
– difficult to infer from operation specifications alone
– typically designed and represented at a higher form
of abstraction
• defining a set of states for an instance
• describing how various operations effect transitions from
state to state
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To write a specification for an operation
• To define the interface between the receiver and the
sender
– Contract approach – emphasizes preconditions but
has simpler post conditions
– Defensive programming approach -- emphasizes
post conditions but has simpler preconditions
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Contract approach to class design from a
testing perspective
• Preconditions specify obligation of the sender
• If met, receiver is obligated to meet the requirements
set form in the post conditions and class invariant
• Care must be taken in the design of the class interface
to ensure that
– the preconditions are sufficient to allow a receiver to
meet the post conditions
– a sender can determine whether all preconditions are
met before sending a message
– post conditions address all possible outcomes of an
operation, assuming preconditions were met
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Defensive approach to class design from a
testing perspective
• Interface defined primarily in terms of the receiver and any
assumptions it makes on its own state and the values of any
inputs (arguments or global data) at the time of the request.
• Operation typically returns some indication re status of the result
of the request (success or failure),
– traditionally as a return code associating a value with each
possible outcome
– can provide to sender an object that encapsulates the status of
the request
• Identify “garbage in” and eliminate “garbage out” by checking
for improper values coming in and reporting the status of
processing the request to the sender
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What does this mean for tester?
• The approach used in an interface determines the types
of testing that need to be done.
• Contract approach
– simplifies class testing
– complicates interaction testing – must ensure any
sender meets the preconditions
• Defensive approach
– complicates class testing – test cases must address
all possible outcomes
– complicates interaction testing – must ensure all
possible outcomes are produced and that they are
properly handled by the sender
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During design and design inspections of a
class – how maintain testing perspective?
• Review the preconditions and post conditions for
testability
• Are the constraints clearly stated?
• Does the specification include the means by which one
can check preconditions? (the sender does not want to
be an expert on the receiver; receiver should explain
how to check for the preconditions)
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Class implementation
• Describes how an object represents its attributes and carries
out its operations. It is made of several components:
• A set of data values stored in data members (aka instance
variables or variables) – some or all of the values associated
with the attributes of an object.
• A set of methods (aka member functions) – code used to
implement an algorithm to accomplish an operation
declared in the public or private class specification.
• A set of constructors to initialize a new instance.
• A destructor to handle any processing associated with
destruction of an instance
• A set of private operations in a private interface – provide
support for the implementation of public operations.
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Importance of class testing
• Classes define the building blocks for OO programs
• A class is an abstraction of the commonalities among
its instances – therefore, the testing process must
ensure that a representative sample of members are
selected for testing.
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Classes from a testing perspective
• A class specification contains operations to construct
instances. They may not properly initialize the
attributes of new instances.
• Class relies on collaboration to define its behaviors and
attributes. The other classes may be implemented
incorrectly and contribute to failure of the class that
relies on them.
• A class’ implementation “satisfies” its specification –
does not mean the specification is correct.
• Might not support all required operations; may perform
them incorrectly.
• Might not provide a way for a precondition to be
checked by a sender before sending a message
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Inheritance
• Relationship between classes that allows the definition
of a new class based on the definition of an existing
class.
– allows reuse of both specification & implementation
– important advantage: the preexisting class does not
have to be modified or made aware of the new class
• New class is called subclass or derived class
• Parent class is called superclass or base class
• Each class (except the root) has one or more ancestors;
the chain of ancestors up to the root is called
inheritance hierarchy
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Good OO Design Use of Inheritance
• Used only to implement an is-a or is-a-kind-of
relationship
• Best use: with respect to specifications and not
implementation – inclusion polymorphism, for example
(more on that in a moment)
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Inheritance from a testing perspective
• Provides a mechanism by which bugs can be
propagated from a class to each of its descendants 
– Important reason to test classes as they are
developed to eliminate fault propagation
• Provides a mechanism by which we can reuse test
cases.
– subclass inherits part of its specification and
implementation from its superclass, potentially can
reuse test cases from superclass to subclass
• Models an is a kind of relationship
– Use of inheritance solely for code reuse will
probably lead to maintenance difficulties
– Common mistake in OO development
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Inheritance models is-a-kind-of relationship
• If D is a subclass of C, then D is a kind of C
• If so, an instance of D can be used whenever an instance
of C is expected
• To work, the behavior of D must somehow conform to
that which is associated with C
• Behavior of a class
– observable states of an instance
– the semantics associated with the operations defined
for an instance of that class
• Behavior of a subclass – incremental changes to the
observable states and operations defined by its superclass
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Substitution Principle
• Only the following changes are allowed in defining the
behavior associated with a new subclass:
– Preconditions for each operation must be the same
or weaker – less constraining – than those of the
superclass
– Post conditions for each operation must be the same
or stronger – do at least as much as defined by the
superclass
– Class invariant – must be the same or stronger; add
more constraints
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Substitution Principle of Inheritance
from a testing perspective
• Developers must enforce (in inspections, if not before) the
constraints of this principle on behavior changes
– Observable states and all transitions between them
associated with the superclass must be preserved by the
subclass
– The subclass may add transitions between these states
– The subclass may add observable states as long as each
is either concurrent or a substate of an existing state
• In other words, don’t use inheritance because you are too
lazy to specify a class that is similar but is-not-a-kind-of
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Polymorphism
• Ability to treat an object as belonging to more than one
type.
– Not necessarily the safest approach to programming
– Supports designs that are flexible
• Inclusion polymorphism is the occurrence of different
forms in the same class
– Can substitute an object whose specification
matches another object’s specification for the latter
object in a request for an operation
– i.e., a sender can use an object as a parameter based
on its implementation of an interface rather than its
full class
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A class is a set of objects that share a common
conceptual basis.
• This definition is influenced primarily by associating
inheritance and inclusion polymorphism
• The class at the root (top of tree graph) establishes a
common conceptual basis for all objects in the set.
• A descendant refines the behavior established by the
root class and intermediate ancestors
• Objects in the descendant class are still in the set of
objects in the root class – a subset of each of the sets
defined by its ancestors
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Two perspectives of sets representing classes
– both are useful during testing
• Class’ perspective – each set contains all instances,
maybe an infinite number; most easily represented with
Venn diagrams
• Executing program’s perspective – each set is drawn
with one element per instance in existence.
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A Matter of Perspective
• When a class is to be tested
– outside context of any application program, we test
it by selecting arbitrary instances using the class
perspective
– in context of an executing application program or in
context of object persistence, we use the other
perspective
• ensure size of set is correct
• ensure elements correspond to appropriate objects
in the problem or solution
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Inclusion polymorphism – powerful capability
• Can perform all design and programming to interfaces
– without regard to exact class of the object sent to a
message to perform an operation
• Takes design and programming to a higher level of
abstraction
• Can define classes for which no instances exist but for
which its subclasses have instances
– An abstract class’ purpose is to define an interface
that is supported by all of its descendants
– Exploits polymorphism during design to extend a
system incrementally by adding classes instead of
modifying existing ones
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Inclusion polymorphism from a testing perspective
• A polymorphic reference hides the actual class of a referent
(referent is the thing being referred to).
• All referents are manipulated through their common
interface.
• Allows systems to be extended by adding classes rather than
modifying existing ones – unanticipated interactions can
occur in the extensions
• Allows any operation to have 1 or more parameters of a
polymorphic reference – increases the number of possible
kinds of actual parameters that should be tested
• Allows operation to specify replies that are polymorphic
references – actual class of referent could be unanticipated
by the sender
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Impact of this dynamic nature of OO
• Puts more importance on testing a representative
sample of runtime configurations
• Static analyses provide potential interactions that might
occur
• Only runtime configuration illustrates what actually
happens
• In the McGregor & Sykes book, they explain a
statistical technique to assist in determining which
configurations will expose the most faults for the least
cost of resources
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Each subclass must be a subtype
• That is, each specification for the subclass must fully meet all
specifications of its direct ancestor
• This is an enforceable design requirement when these rules are
applied (refers to diagram on next slide):
• The tryit method in A, the sender, satisfies the preconditions of
the doIt operation of B before tryit calls doIt. If an instance of C
or D is to be substituted, the preconditions for C’s doIt or D’s
doIt must not add any new conditions to those for B’s doIt.
(Why?)
• If an instance of C or D is to be substituted when A’s tryit sends
a message to B’s doIt, the post conditions on B’s doIt must still
be true although C or D can add additional post conditions.
• Similarly for the invariant for B – must still be true in instances
of C and D although additional invariants may be added.
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Design solution with inclusion polymorphism
A
tryit(B b)
tryit(B b) {
b.doIt( );}
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B
doIt( ) {...}
C
doIt( ) {...}
ECEN5033 - OO Testing University of Colorado
D
doIt( ) {...}
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Parametric Polymorphism
• The capability to define a type in terms of one or more
parameters – rather like a macro
• C++ provides this with the concept of templates
– a compile-time ability to instantiate a “new” class
– “new” because an actual parameter is provided for the
formal parameter (at compile-time) in the definition
– Instances of the new class can then also be created
– Used a lot in the C++ Standard Template Library
• Almost looks like a kind of inheritance but it isn’t
– If the template works for one instantiation, no
guarantee it will work for another
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Parametric Polymorphism
from a testing perspective
• Need to inspect details of the template code to
understand what it will do with various parameters
• It is possible to write templated drivers for testing
many parts of templates
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Abstraction
• The process of removing detail from a representation.
• Allows us to look at a problem in various levels of
detail.
– leave out details that are irrelevant for a given
consideration
• OO technologies use abstraction extensively
– inheritance hierarchy, for example
– system models whose detail increases during
development
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Layers of abstraction
from a testing perspective
• Layers of abstraction in the development process are
paralleled by layers of testing analysis
• If we begin testing analysis with the highest levels of
abstraction of development models,
– we provide a more thorough examination of the
development product
– and, therefore, a more effective and accurate set of
tests
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Testing Object-Oriented Software Test early, test often