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Contract based programming

Contract based programming. using pre- and post-conditions. Design by contract. Idea A program is correct if given correct input the program produces correct output. Correct input → PROGRAM → correct output A program is considered a kind of “black box”

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Contract based programming

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  1. Contract based programming using pre- and post-conditions Pre- and post-conditions, Using exceptions

  2. Design by contract • Idea • A program is correct if given correct input the program produces correct output. • Correct input → PROGRAM → correct output • A program is considered a kind of “black box” • Same idea applies to parts of a program • Methods, functions, etc. • Correct input → METHOD → correct output • Precondition • Specification of correct input • Postcondition • Specification of correct output Pre- and post-conditions, Using exceptions

  3. Pre- and postconditions • Statement that evaluates to true or false • Many Java methods have a precondition • Class.forName(String className) • Assumes className != null and that the class exists. • Integer.parseInt(String s) • Assumes that the String s contains a number • If you don’t respect the precondition the methods will most likely throw some kind of (runtime) exception • IllegalArgumentException • NumberFormatException • A subclass of IllegalArgumentException • In Java pre- and postconditions are not part of method signatures • You can specify pre- an postconditions as comments Pre- and post-conditions, Using exceptions

  4. class S { // pre: A; post: B method(int p) { … } } class T extends S { // pre: X; post: Y method(int p) { … } } X can be weaker than A Y can be stronger than B Example A: p > 0 disallows 0 X: p ≥ 0 allows 0 B result is true or false Example: Collection.add() Y result is always true Example: List.add() Pre- and postconditions used with method overriding Pre- and post-conditions, Using exceptions

  5. Invariant • An invariant is a statement that is invariably true. • Class invariant • Statement about the objects state between method invocation • Loop invariant • Statement about the state of variables in a loop Pre- and post-conditions, Using exceptions

  6. Proof of post condition • The idea of introducing pre- and postconditions is to formally (mathematically) prove the postcondition from the preconditions • Precondition AND program implies post condition • An invariant may help doing the proof. • However, often the proof can be quite hard to do. • Usually only done in critical systems • Controlling hospital equipment, satellites, etc. • Usually more errors in the proof than in the program • But gives you a chance to rethink you program. Pre- and post-conditions, Using exceptions

  7. Stating pre- and post-conditions • In Java pre- and post-conditions are usually described in the JavaDoc comments • /** PRE: a > 0*/ • void method(int a) {…} Pre- and post-conditions, Using exceptions

  8. Checking pre-conditions • In Java pre-conditions can be checked using simple if-statements • If (parameter is not legal) throw SomeException • The exception thrown is often an IllegalArgumentException, or one of its subclasses • NumberFormatException Pre- and post-conditions, Using exceptions

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