UML and OCL Constraints

Why Do We Need OCL?

• A UML diagram, such as a class diagram, is not refined enough to provide all the relevant aspects of a specification.
• There is a need to describe additional constraints about the objects in the model, which are often described in natural language and can result in ambiguities.
• Formal languages, including OCL, have been developed to write unambiguous constraints.

OCL Overview

• OCL is a textual language of typed expressions based on mathematical sets and logic.
• It was developed by IBM in 1995 and emphasizes precision and simplicity.
• OCL does not use special mathematical symbols.

What is a Constraint?

• A constraint specifies a restriction on one or more values of (part of) an object-oriented model or system.
• A constraint is a valid OCL expression of type Boolean.
• Examples of simple constraints on attributes of a class:
  • age >= 18 and age < 66
  • title = if isMale then 'Mr.' else 'Ms.' endif
  • name.size < 100

OCL Characteristics

• Declarative: states what should be done, not how, and is implementation-independent.
• Expressions have no side effects: evaluation does not change the system state.
• Strongly typed: each OCL expression has a type and evaluates to a value or to an object within the system.

OCL Statement Parts

• OCL statements consist of four parts:
  • Context: defines the limited situation in which the statement is valid
  • Property: represents some characteristics of the context
  • Operation: manipulates or qualifies a property
  • Keywords: used to specify conditional expressions

Places to Use OCL in UML Models

• Invariants: constraints on a class or type that must always hold
• Preconditions: constraints that must hold before the execution of an operation
• Postconditions: constraints that must hold after the execution of an operation
• OCL is not limited to class diagrams

Types in OCL

• Predefined types: Basic types (Integer, Real, String, Boolean), Collection types (Set, Sequence)
• User-defined model types: Enumeration, Classes

Context of an OCL Expression

• Every OCL expression is bound to a specific context, where self can be used as a reference to this context.
• Example: context Customer inv: self.age >= 18 and self.age < 66

Standard Operations for Real and Integer Types

• Operations include: equals, not equals, less, more, less or equal, plus, minus, multiply, divide, modulus, integer division, absolute value, maximum, minimum, round, floor

Standard Operations for the Boolean Type

• Operations include: not, and, or, implies, xor, etc.