Object Constraint Language (OCL) Cheatsheet

# Object Constraint Language (OCL) Cheatsheet

1. Introduction to OCL

The Object Constraint Language (OCL) is a formal language used to describe constraints, business rules, and logic that cannot be expressed using UML diagrams alone. OCL is declarative, meaning it describes what must be true, rather than how to make it true.

Common Use Cases:

Defining preconditions, postconditions, and invariants
Defining derived attributes
Specifying query operations

2. OCL Syntax Basics

OCL expressions operate on UML model elements such as classes, attributes, and associations.

General Syntax:

context <Classifier>
inv [<name>] : <OCL expression>

context: Specifies the UML element (class, operation, or attribute) being constrained.
inv: Declares an invariant (can also use pre for preconditions or post for postconditions).

Example:

context Person
inv: self.age >= 0

This constraint ensures that the age of a Person is always non-negative.

3. Types in OCL

OCL has its own data types, similar to programming languages.

Data Type	Description	Example
Boolean	True or False	true, false
Integer	Whole numbers	0, -1, 42
Real	Floating point numbers	3.14, -0.5
String	Sequence of characters	'Hello'
Collection	Set, Bag, Sequence, or OrderedSet	Set{1, 2, 3}

4. OCL Operators

Arithmetic Operators

Operator	Description	Example
+	Addition	5 + 3 = 8
-	Subtraction	5 - 3 = 2
*	Multiplication	5 * 3 = 15
/	Division	5 / 2 = 2.5
div	Integer division	5 div 2 = 2
mod	Modulo	5 mod 2 = 1

Comparison Operators

Operator	Description	Example
=	Equality	5 = 5 = true
<>	Inequality	5 <> 3 = true
<	Less than	3 < 5 = true
<=	Less than or equal	3 <= 3 = true
>	Greater than	5 > 3 = true
>=	Greater or equal	5 >= 5 = true

Logical Operators

Operator	Description	Example
and	Logical AND	true and false = false
or	Logical OR	true or false = true
not	Logical NOT	not true = false
implies	Logical implication	true implies false = false

5. Collections in OCL

OCL supports four main collection types: Set, Bag, Sequence, and OrderedSet.

Collection Type	Description
Set	Unique elements, unordered
Bag	Non-unique elements, unordered
Sequence	Ordered, non-unique elements
OrderedSet	Ordered, unique elements

Collection Operations

Operation	Description	Example
size()	Number of elements	Set{1, 2, 3}.size() = 3
includes(x)	Check if x is in the collection	Set{1, 2, 3}.includes(2) = true
excludes(x)	Check if x is not in the collection	Set{1, 2, 3}.excludes(4) = true
count(x)	Number of occurrences of x	Bag{1, 1, 2}.count(1) = 2
sum()	Sum of elements (if numeric)	Set{1, 2, 3}.sum() = 6
select(x	x > 2)	Filter elements based on condition
collect(x	x.attr)	Flattens nested collections

6. Constraints

Invariants

An invariant ensures that an attribute or relationship always satisfies a condition.

context Person
inv: self.age >= 0

This invariant ensures the age attribute of a Person is non-negative.

Preconditions and Postconditions

Preconditions and postconditions are constraints on operations.

Precondition: A constraint that must be true before an operation is executed.

context Account::withdraw(amount: Real)
pre: amount > 0

This precondition ensures the amount to withdraw is positive.

Postcondition: A constraint that must be true after an operation is executed.

context Account::withdraw(amount: Real)
post: self.balance = self.balance@pre - amount

The @pre indicates the value before execution, and the postcondition ensures the balance is reduced accordingly.

7. Defining Attributes and Operations

Derived Attributes

A derived attribute is computed from other attributes.

context Person
derive: self.fullName = self.firstName + ' ' + self.lastName

This constraint defines the fullName as a concatenation of firstName and lastName.

Query Operations

OCL can define query operations that return values.

context Person
def: getFullName() : String = self.firstName + ' ' + self.lastName

This query operation returns the fullName of a Person.

8. Advanced Features

Iterators (select, reject, forAll, exists, collect)

select: Filter elements from a collection.
reject: Remove elements from a collection.
forAll: Ensures a condition is true for all elements.
exists: Checks if at least one element satisfies the condition.
collect: Projects each element to another value.