1. Testing Object-Oriented Programs
CS 4311
J. McGregor and D. Sykes. A Practical Guide to Testing Object-Oriented
Software, Addison-Wesley, 2001.
I. Burnstein. Practical Software Testing, Springer-Verlag, 2003. 1 1

2. Outline OO testing, why different? Class as unit Test harness
Testing class hierarchies

3. What’s Different about Testing OO Programs?
Encapsulation
Polymorphism
Inheritance
Development

4. Unit Testing OO Programs
Definition
A unit is the smallest possible testable software component, e.g., function, procedure, or module.
What’s unit in OO?
Method vs. class

5. Method As Unit
Methods tend to be small and less meaningful in isolation
Impossible to test some methods (e.g., private methods)
Overhead to test each method separately

6. Class As Unit Purpose Approach Test a class as a whole
Emphasizes interactions among methods
Approach
Test critical methods individually
Test interactions among methods
Test sequences of methods calls, e.g., pop after push

7. Testing Interactions Why test interactions? Intra-class interactions
An OOP is a collection of objects collaborating to solve some problem.
The correct collaboration (or interaction) is critical to the correctness of the program.
Intra-class interactions
E.g., pop after push
Inter-class interactions (integration test)
Test sequences of messages among collaborating objects.
Can use message sequence diagrams, collaboration diagrams, and use case scenarios to generate test cases.

8. Describing Test Cases Conventional definition In OO
A pair of inputs and the expected output, xi, o
In OO
Need to specify the receiver (often called object under test) and its state too, e.g., r in
r.m(x1, x2, …, xn)
Thus, it is a tuple of rpre, xi, rpost, o, e, where
r : is the object under test
rpre : the receiver r’s state in the pre-state
xi : a list of arguments
rpost : the receiver r’s state in the post-state
o: expected return value
e: expected exception

9. Example
A test case for testing the method “void setX(int)” of the class Point with the instance fields x and y, e.g.,
Point r = new Point(10,10);
r.setX(20);
Input Output
receiver (r) args (x) receiver (r) return-value exception
r.x = x = 20 r.x = none none
r.y = 10 r.y = 10

10. Constructing Test Cases
Testing individual methods
Black-box test and white-box test
Testing interactions
Random testing
State-based approach
Can use state diagrams to generate sequences of method calls
Testing exceptions
Design-by-Contract (DBC) vs. defensive design

11. Outline OO testing, why different? Class as unit Test harness
Testing class hierarchies

12. Test Harness
A test harness is the auxiliary code developed to support testing of units and components, consisting of:
Drivers that call the target code, and
Stubs that represents modules it calls.
driver
call
result
unit under test
call
ack
call
ack
stub1
stubn

13. Constructing Test Harness
Conditionally compiled
Test code closely maintained with class code (e.g., in the same file)
Hard to reuse (e.g., to test a subclass)
Requires support for conditional compilation
As static methods or nested classes
Test code closely maintained with class code
Easy to reuse (e.g., by inheritance)
Need to strip down test code before product delivery
As separate tester class

14. Separate Test Class Strengths Weaknesses Guideline
Easy to reuse (e.g., to test a subclass)
Small and fast production code
Weaknesses
Need to create a new tester class
Need to manage changes carefully
Guideline
Use unit testing frameworks such as JUnit for Java, CppUnit for C++, and NUnit of C#.
Tester
CTester C

15. Outline OO testing, why different? Class as unit Test harness
Testing class hierarchies

16. Testing Class Hierarchies
If a new class D is added, what methods need to be tested?
Need test bar() of D?
What about inherited methods, e.g., foo()?
Can use existing test cases of C?
What if D adds a new method? C
foo()
bar() D

17. Organizing Testing Harness
Parallel architecture for class testing (PACT)
Use inheritance to organize test cases and test classes
Tester
CTester C
DTester D

18. Testing Abstract Classes
Can you test abstract classes?
Approaches