OCL

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Introduction

  • How can we express constraints in UML ?
  • E.g. the balance attribute in a SavingsAccount class can never be less than zero and more than 100000.
  • UML only has a notion of an informal constraint in terms of a note - which can be attached to a model. However its just text.
  • To write formal and machine checkable constraints - we use the Object Constraint Language (OCL).

Assertions

  • In a program, assertions express constraints on program state that must be true at a specified point during execution.
  • In a model/diagram, they document what must be true of an implementation of a modelling element.
  • Can have optional preconditions - (must be true when operation is invoked). for e.g. an argument to a certain operation cannot be null.
  • Can have optional postconditions - (must be true when operation completes). for e.g. the operation must update the database when it completes.

Design by contract

  • A pre and post condition can be viewed as a contract between an operation and its invokers.
  • Something like - "If the client promises to invoke the operation with the precondition satisfied, then the operation guarantees that its implementing methods will deliver a final state in which the postcondition is true".
  • So the invoker doesn't care about how final state is produced
  • And the implementer doesn't need to worry about cases where the precondition is not true.

[TODO] : How about handling errors for invalid preconditions ?

  • So a contract can be many things:
    • A specification of what a method should do
    • Documentation - more precise than class interfaces [TODO : e.g. ]
    • Offers runtime checks for correctness.
    • Are a basis for testing.
    • Are a basis for formal proofs of programs correctness.

OCL

  • OCL is the constraint language for UML.
  • Used to write general constraints on UML as well as DBC.
  • Can apply to any modeling element, not only classes.
  • OCL Essential Capabilities:
    • Specifying which model element is to be constrained (the context).
    • Navigating through models to identify objects that are relevant to a constraint.
    • Asserting properties about links between objects.

OCL Context

  • Consider a BankAccountclass with an integer balance.
  • OCL constraint
context BankAccount inv:
self.balance>=0 and self.balance<=250000
  • The constraint is an invariant that applies in the context of the class BankAccount.
  • This means the balance for any BankAccount object must be within 0 and 250000.
  • self means the same thing as "this".

[TODO: check when you are using invariants, why would you use "self" ?]

Navigation Expressions

  • We can refer to object that are linked to the context object.
  • We need to start from the context object. Consider the below example:

Ocl-navigation.jpg

  • To get the set of employees working in a department :
context Department
self.staff
  • The association name has to be followed
  • Think of it something like the below code in Java, to access the Person object,
class Department {
  private Set<Person> staff;

  public Set<Person> getStaff() {
    return this.staff;
  }
}
  • If there is no association name, then the class name at the destination end of the context object can be used.
  • The class name has to be in lower case
  • Eg to get the department from the company:
context Company
self.department
  • The multiplicity of the assocation will tell us how many objects are retrieved.
  • e.g. below OCL will retrieve one object
context Department
self.company

Iterated Traversals

  • Navigations can be composed of multiple references - we can denote more paths through a diagram.
  • e.g All the people who work for a comapany
context Company
self.department.staff

self.department will give a set of departments, and then .staff is applied to each department producing a set of people. If people were not unique to a department, then a bag of people would be returned.

Names and Invariants

  • An OCL invariant need not be applicable to any instance:
  • e.g.
context bart: Person inv
bart.age == 10
  • TODO: clarify what is bart - a Class, what is Person ?
  • An invariant can be given a name:
context bart: Person inv how_old:
bart.age == 10

Basic types

  • OCL supports basic types such as boolean, integer, real, string
  • Collection types such as collection,set,bag,tuple, are also basic types.
  • Additionally, every class appearing in the UML class model can be used as a basic type in OCL.

Basic operations

  • oclIsTypeOf(t:OclType):true, if selfand t are the same type (This is sort of useless, better to use oclIsKindOf())
  • oclIsKindOf(t:OclType):true, if self and t are the same type or self is of a class derived from t (This behaves like Java's instanceof operator)
  • oclInState(s:OclState):true, if selfis in the state specified by s. s is the name of some state in the statechartfor the class.
  • oclIsNew():true if used in a postcondition and the object is created by the operation.
    • For e.g. if a new object is created as a postcondition ?
    • TODO: come up with a better example

Logic in OCL

  • Three valued: true, false and undefined.
  • e.g. illegal type conversion can result in undefined.
  • What will be the result of combining OCL's logic operators together ? i.e. what is the truth table ?
  • An expression is undefined if one of its arguments is undefined, except:
    • true OR anything is true (conventional)
    • false AND anything is false (conventional)
    • false IMPLIES anything is true. (TODO: what does this mean)

Operations on objects and collections

  • Operations and attributes defined for classes in a model can be used in an OCL expression.
  • Example:
context Person
self.age()
self.contract.grade.salary
  • Collections have some built in operations (like Java's Collection class has methods like size(), toArray() etc) which are accessed through the "->" operator.
  • e.g. sum() is a pre-defined operation on integer collection, here salary is a bag of integers - representing salarys of all staff.
  • TODO: understand this: how come there is no self defined here, how to interpret iterative traversal when collections are involved ?
context Department
staff.contract.grade.salary->sum()