Exception Contracts Material Transformation
Rationale for exceptions, transforming preconditions to defined behavior, handling undefined behavior, reflection on exceptions and contracts, with code examples illustrating exceptional postconditions and masking techniques.
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Presentation Transcript
Exception Contracts Material from Liskov Chapter 4 Paul Ammann
Rationale for Exceptions Preconditions document undefined behavior Undefined behavior Partial specification In any implementation, something happens Should clients rely on undocumented behavior? What happens in next release? Bottom line: Preconditions are usually undesirable But sometimes unavoidable Exception handling: Transform preconditions to defined behavior One of several possible mechanisms 2
Transforming a Precondition to an Exceptional Postcondition Key: Postcondition defines behavior for inputs excluded by precondition public double sqrt (double x) // precondition: x >= 0 // postcondition: public double sqrt (double x) // precondition: // postcondition: If x < 0 throw IllegalArgumentException // else public double sqrt (double x) /** * @param x the square of the intended result * @return approximate square root of x * @throws IllegalArgumentException if x < 0 */ 3
Exceptions and Contracts An exception visible to client must appear in the contract We say that the exception is reflected An exception not visible to client must not appear in the contract We say that the exception is masked One exception does not follow this rule: AssertionError Used to indicate that the computation is broken 4
Reflection Example public static int min (int[] a) throws NPE,EE { // Effects if a is null throw NPE else if a is empty throw EE else return min value in a int m; try { m = a[0];} catch (IOOBE e) { throw new EE ( Arrays.min ); } for (int i=0; i < a.length; i++) if (a[i] < m) m = a[i]; return m;} 5
Masking Example public static boolean sorted (int[] a) throws NPE { // Effects if a is null throw NPE else if a is sorted in ascending order return true else return false int p; try { p = a[0];} catch (IOOBE e) { return true; } for (int i=0; i < a.length; i++) if (p < a[i]) {p = a[i];} else {return false;} return true;} 6
