Design By Contract

1
Design By Contract

• Formal Methods se304/cs407
• Thanks to Gary Leavens and Yoonsik Cheon

2
Recap

• Pre-condition
• Benefits supplier
• Obligation of client
• Usually based on arguments to method
• Must be true before method begins
• Keyword requires

3
Recap

• Post-condition (normal and exceptional)
• Benefits client
• Obligation of supplier
• Usually something to do with return value of
  method
• Must be true after method is completed
• Keyword ensures or signals

4
Advantages

• Assign blame
• Avoid certain subtle errors
• binarySearch(int list, int a)
• Automatic documentation, test generation
• Encourages thorough, formal thought processes

5
New DBC concept - invariant

• Always true of object in stable states
• After constructor
• Before and after method calls
• Allows you to define
• Acceptable states for object
• What must remain consistant
• /_at_ public invariant !name.equals()
• _at_ weight gt 0 /

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Example from lab

• What would be the invariants for Student?
• Class Student
• private String name
• private String subject1, subject2
• //_at_ public invariant name ! null
• //_at_ public invariant !subject1.equals(subject2)

7
Quantifiers

• Sometimes you need more complexity
• What if you wanted to say
• All the Students in the Admin object must do
  Maths
• The minimum marks of all this Students subjects
  must be greater than or equal to 40
• ?

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Quantifiers

• For all ?
• \forall
• There exists ?
• \exists
• Sum, Product
• \sum, \product
• Minimum, Maximum
• \min, \max
• Number of
• \num_of

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Quantifiers - syntax

• /_at_ invariant/requires/ensures
• _at_ (quantifier declarations
• _at_ restrictions
• _at_ spec_expression) /

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Quantifiers - example

• All the Students in the Admin object must do
  maths
• /_at_ invariant (\forall Student s
• _at_ this.contains(s)
• _at_ s.doesSubject(Maths) ) /

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Quantifiers - example

• The minimum marks for a Student must be greater
  than or equal to 40

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Student example

• class Student
• private String name
• private Subject subjects
• Class Subject
• int getMarks()

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Quantifiers - example

• /_at_ requires ( list is sorted in
• _at_ ascending order ) /
• binarySearch(int list, int a)

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What do you need to know about DBC?

• For the final exam
• You will have to be able to add JML to Java
• And understand JML given to you
• Pre-conditions
• Post-conditions
• Invariants
• Quantifiers