pre- and postconditions, using assertions and exceptions 1 pre- and postconditions using assertions...
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Pre- and postconditions, Using assertions and exceptions
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Pre- and postconditions
Using assertions and exceptions
Pre- and postconditions, Using assertions and exceptions
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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
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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
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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 ≥ 0 allows 0
– B• result is true or false
– Y• result is always true
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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
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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.
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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’– }
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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.
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Enabling assertions in NetBeans
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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
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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.
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References
• Sun Microsystems Programming with Assertions – http://java.sun.com/j2se/1.4.2/docs/guide/lang
/assert.html
• Qusay H. Mahmoud Using Assertions in Java Technology, Sun Microsystems 2005– http://java.sun.com/developer/technicalArticle
s/JavaLP/assertions/