Lecture 8: Avoiding Debugging

1 First Defense: Make Bugs Impossible

• Static checking
• Dynamic checking
• Immutability
  
  • Use immutable type
  • Use immutable reference: use final for declaring the parameters of a method and as many local variables as possible.

2 Second Defense: Localize Bugs

• Fail fast is a design princeple:: Checking preconditions is an example of defensive programming. For example:

 /**
* @param x
requires x >= 0
* @return approximation to square root of x
*/
public double sqrt(double x) {
if (! (x >= 0)) throw new AssertionError();
...
}

3 Assertions

• For most applications, assertions are not expensive compared to the rest of the code, and the benefit they provide in bug-checking is worth that small cost in peformance.
• Java assert statement is a different mechanism from the JUnit methods assertTrue(), assertEquals() , etc. They are designed for use in different contexts.

3.1 What to Assert

• Method argument requirements
• Method return value requirements

public double sqrt(double x) {
assert x >= 0;
double r;
... // compute result r
assert Math.abs(r*r - x) < .0001;
return r;
}

• Covering all cases

switch (vowel) {
case 'a':
case 'e':
case 'i':
case 'o':
case 'u': return "A";
default: assert false;
}

3.2 What Not to Assert

• If an assertion is obvious from its local context, leave it out.
• Never use assertions to test conditions that are external to your program, such as the existence of files, the availability of the network, or the correctness of input typed by a human user. Use Exception instead.
• Java’s assert statement are designed so that assertions are executed only during testing and debugging, and turned off when the program is released to users.
• Asserted expressions should not have side-effects, since assertions may be disabled.

4 Modularity & Encapsulation

• Modularity: Modularity means dividing up a system into components, or modules, each of which can be designed, implemented, tested, reasoned about, and reused separately from the rest of the system.
• Encapsulation: Encapsulation means building walls around a module (a hard shell or capsule) so that the module is responsible for its own internal behavior, and bugs in other parts of the system can’t damage its integrity.
  
  • Access control: Keeping things private as much as possible.
  • Variable scope: Keeping variable scopes as small as possible.
    
    • Avoid global variables.
    • Always declare a loop variable in the for-loop initializer.
    • Declare a variable only when you first need it, and in the innermost curly-brace block that you can. But in dynamic language, like python, the scope of a variable is normally the entire function anyway, so you can’t restrict the scope of a variable with curly braces.

Reference

[1] 6.005 — Software Construction on MIT OpenCourseWare | OCW 6.005 Homepage at https://ocw.mit.edu/ans7870/6/6.005/s16/

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