Pre- and postconditions, Using assertions and exceptions 1 Pre- and postconditions Using assertions and exceptions

Pre- and postconditions, Using assertions and 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 postconditions, Using assertions and 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 –Integer.parseInt(String s) Assumes s contains a number –If you don’t respect the precondition the methods will most likely throw some kind of (runtime) exception NullPointerException 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 postconditions, Using assertions and exceptions 4 Pre- and postconditions used with method overriding class S { // pre: A; post: B method(int p) { … } } class T extends S { // pre: X; post: Y method(int p) { … } } X can be less strong the A Y can be stronger than B Example –A: p > 0disallows 0 –X: p ≥ 0allows 0 –B result is true or false –Y result is always true

Pre- and postconditions, Using assertions and 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 postconditions, Using assertions and exceptions 6 Proof of post condition The idea of introducing pre- and postconditions is to formally (mathematically) prove the postcondition from the preconditions –Postcondition & 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 postconditions, Using assertions and exceptions 7 Assertions in Java Assertions is a relatively new feature if Java –Since Java 1.4 –Prior to Java 1.4 programmers had to program their own assertion facility. That is no longer necessary or advisable. New keyword: assert –assert booleanExpression; –Assert booleanExpression : errorMessage; Example –boolean isMale(char sex) { – assert sex == ‘M’ || sex == ‘F’ : “Illegal sex: “ + sex; – return sex == ‘M’ –}

Pre- and postconditions, Using assertions and exceptions 8 Enabling assertions The Java compiler must be explicitly informed that you have assertions in your program –javac –source 1.4 SomeClass.java You tell the compiler that your source code is Java version 1.4 –Reason Assert is a new keyword. Some older program may have used assert to name a variable or a method. The Java virtual machine (JVM) must be explicitly informed to check assertions –java –ea SomeClass -ea means “enable assertions” –Reason Checking assertions take time. You only want to check assertions during testing and debugging, not when the program is running at the customers site.

Pre- and postconditions, Using assertions and exceptions 9 Enabling assertions in NetBeans

Pre- and postconditions, Using assertions and exceptions 10 When not to use assertions Checking parameters to public methods –Don’t use assertions to check parameters on public methods –Assertion checking disabled when the program is running at the customers site. –You cannot otherwise control the validity of parameters to public methods. –Check (using an if-statement) that the parameters are valid. If not throw NullPointerException IllegalArgumentException, or one of its subclasses

Pre- and postconditions, Using assertions and exceptions 11 When to use assertions Checking parameters to private methods –You control who calls private methods. –When things work (you are no longer debugging) then you can safely disable assertion checking. Checking conditions in methods –Where parameters have been checked without using exceptions.

Pre- and postconditions, Using assertions and exceptions 12 References Sun Microsystems Programming with Assertions – /assert.htmlhttp://java.sun.com/j2se/1.4.2/docs/guide/lang /assert.html Qusay H. Mahmoud Using Assertions in Java Technology, Sun Microsystems 2005 – s/JavaLP/assertions/ s/JavaLP/assertions/