Pre- and postconditions

1
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

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 dont 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

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 weaker than A
• Y can be stronger than B
• Example
• A p gt 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()

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

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.

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
• if (direction LEFT) doLeft()
• else if (direction RIGHT) doRight()
• else
• assert false Bad direction

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 takes time. You only want to
  check assertions during testing and debugging,
  not when the program is running at the customers
  site.

9
Enabling assertions in NetBeans
10
When not to use assertions

• Checking parameters to public methods
• Dont 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

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.

12
References

• Ken Arnold et al The Java Programming Language,
  4th edition, Addison Wesley 2006
• 12.8 Assertions, page 296-303
• Sun Microsystems Programming with Assertions
• http//java.sun.com/j2se/1.4.2/docs/guide/lang/ass
  ert.html
• Qusay H. Mahmoud Using Assertions in Java
  Technology, Sun Microsystems 2005
• http//java.sun.com/developer/technicalArticles/Ja
  vaLP/assertions/