Testing Object-Oriented Programs

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.

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 • 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? • 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 interaction diagrams (e.g., message sequence diagrams and communication diagrams) and use case scenarios to generate test cases.

8. Describing Test Cases Conventional definition 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’s, rpost, o, e, where r : is the object under test rpre : the receiver r’s state in the pre-state xi’s : the arguments rpost : the receiver r’s state in the post-state o: expected return value e: expected exception

9. Input Output receiver (r) args (x) receiver (r) return-value exception r.x = 10 x = 20 r.x = 20 none none r.y = 10 r.y = 10 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);

10. Constructing Test Cases • Testing individual methods • Black-box test and white-box test • Testing interactions • Random testing • State-based approach • Can use state machine 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. unit under test stub1 driver stubn 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. call result ack call call ack

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 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 May not be able to access all features of the class under test Guideline Use unit testing frameworks such as xUnit: JUnit, PHPUnit, CppUnit, NUnit (C#). Tester CTester C

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

16. D bar() C foo() bar() 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?

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

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