OCL

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:

• 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.