ICS 121: Object Constraint Language

OCL is the Object Constraint Language. It is a standard notation that is closely related to UML. It can make UML models more precise.
OCL constraints are logical conditions for:
- Operations: pre- and post-conditions
- Attributes: conditions on the value of that attribute
- Association: conditon on membership in that association
- Class: conditions on all attributes and associations of that class

Use OCL for Design by Contract in UML
OCL can express constraints that cannot be handled with just multiplicity and attribute types
OCL conditions in a UML model help make the meaning of the model more clear.
UML tools can assert statements from OCL conditions.

Each constraint applies to some UML model element called its "context". If you draw the constraint directly on a UML diagram, the context should be clear.
You can reference attributes and properties of the context. You can follow associations by using dot-notation.
Since associations are sets, most values in OCL expressions are sets.
OCL provides basic math operators: >, <, =, <>, +, -, *, /, mod, abs, ...
OCL provides basic logical operatiors: and, or, not, xor, implies
OCL provides basic set operatiors: size, includes, includesAll, union, intersection, sum, ...
OCL provides first-order-predicate logic operators:
- exists: true if there is one element of the set that satisfied the condition
- forAll: true if all elements of the set satisfy the condition
For more details see the OCL specification

context Student::add(c: Course)
- pre: not enrolled->includes(c) and transcript->includesAll(c.prereqs())
- post: enrolled->includes(c)
context Course
- inv: enrolled->size() <= capacity
- inv: (not waitlist->isEmpty()) implies (enrolled->size() = capacity)
- inv: allInstances()->isUnique(courseID)
- inv: honors implies enrolled->forAll(s | s.isHonors() )