L4: Improving Software Quality by Design

Question 1

What are software bugs?

Answer 1

Errors in design, development, or operation causing unexpected behaviour / results.

Question 2

How do bugs happen?

Answer 2

• Programmer creates a defect
• Defect causes an infection
• Infect propagates
• Infection causes failure

Question 3

How to deal with bugs?

Answer 3

Improve software quality by:
- Properly designing the system
- Quality assuring the code
- Testing the system
- Debugging failures

Question 4

What might lead to bugs in the design process? What about in the software design?

Answer 4

The design process:
- Misunderstanding a customer
- Misinterpreting a written requirement
- Overlooking a case when analysing requirements

The software design:
- Adopting an incorrect algorithm
- Making an unwarranted asssumption about the code
- Making a typing error when entering code
- Implementing incompatible interfaces

Question 5

What is the definition of robustness?

Answer 5

Ability of a system to cope with errors and erroneous input

Question 6

Why do we need robustness and contracts?

Answer 6

• Making developers aware of problematic execution cases
• Avoiding such problematic cases from the start
• Defining where to put blame:
  1. No explicit signaling for violated preconditions implemented
  2. Explicit signaling for violated precodintions implemented

Question 7

How to improve the code and make it more robust?

Answer 7

• Using JavaDoc and natural language
• Parameters:
  1. Definition of the parameters a method receives
  2. Helpful to specify variables more precisely and to add expected units
• Precondition:
  1. Must be true before execution
  2. Can be imposed on state of class, arguments passed, …
  3. Impose obligations on the client
• Postcondition:
  1. Must be true after execution
  2. Can be imposed on state of class, return type, values, …
  3. Impose obligations on the method

Question 8

Writing contracts

Answer 8

Idea of design by contract:
- Formal and verifiable specification
- Supported by some programming languages
- Including preconditions, postconditions, and invariants

For this course:
- @param, @pre, @post, @return, @throws
- Be as precise as possible
- Not fully formal

Question 9

What happens at an exception?

Answer 9

• It will terminate the program execution by returning the defined exception
• A method is robuts if it always throws an exception if its preconditions are violated
• The returned exception must be handled by the client

Question 10

When to use exceptions?

Answer 10

• Make a method robust
• Avoid encoding information in special values
• Signal special (erroneous) situations in non-local (non-private) methods
• Ensure that a failure cannot be ignored

Question 11

White-box and black-box testing

Answer 11

White-box: test internal logic
- Consider only the code, ignore specification
- Used to test internal state of the system

Black-box: test intended functionality
- Consider only specification and requirements, ignore code
- Used to test correct behaviour

Question 12

Test-driven development

Answer 12

Core Idea:
- Develop tests first to specify new behaviour
- Develop code to fulfill tests
- Improve code quality