A Boolean attribute stores a true/false decision in your MDriven model and is for anyone writing OCL expressions, constraints, or derived values.

Boolean types

A Boolean has two possible values: true and false. In OCL, you can declare it as either non-nullable or nullable.

Declaration	Meaning	Use it when	Example
Boolean	A non-nullable Boolean. Its value is always true or false.	The attribute represents a normal yes/no decision.	IsActive : Boolean
Boolean?	A nullable Boolean. Its value can be true, false, or null.	The absence of an answer has a meaning that is different from both true and false.	ApprovalReceived : Boolean?

Choose non-nullable Boolean attributes for stored decisions

For an attribute on a class that is persisted to the database, use Boolean in almost all cases.

A nullable attribute defaults to null. This adds a third state that expressions and user-interface logic must handle. It is easy to treat false and null as the same value by mistake.

For example, model whether an order is shipped as:

IsShipped : Boolean

Use a nullable Boolean only when all three states are required. For example, an optional approval can mean:

ApprovalReceived	Meaning
true	Approval was received.
false	Approval was declined.
null	No approval decision has been recorded.

Use Boolean expressions directly

When an expression already evaluates to a Boolean, use that expression directly. Do not compare it to true or false.

For an IsActive Boolean attribute, write:

customer.IsActive
not customer.IsActive

Do not write the following when you only need to test the Boolean value:

customer.IsActive = true
customer.IsActive = false

The equality operator, =, compares two values. Use it when you are actually comparing values, such as checking whether a status has a specific value. See Documentation:Mathematical symbols for comparison operators and Documentation:OCL Boolean Operators for logical operators.

Do not test a Boolean with collection operations

A Boolean is a single value, not a collection. Do not use collection operations such as notEmpty to determine whether it is true.

The following expression is incorrect:

customer.IsActive->notEmpty

It always returns true: the Boolean is converted to a collection containing one Boolean value, and that collection is not empty. It does not test whether IsActive is true.

Write the Boolean expression itself instead:

customer.IsActive

To test the opposite condition, use:

not customer.IsActive

Example: deriving a decision

Assume an Order has a non-nullable IsPaid : Boolean attribute and an IsShipped : Boolean attribute. An expression that identifies paid orders that are not shipped can be written as:

self.IsPaid and not self.IsShipped

Each attribute is used as a Boolean expression. The and operator combines the two conditions.

See also

• Documentation:OCL Boolean Operators
• Documentation:Mathematical symbols
• Documentation:String

[[Category:Attributes and data types [Beginner]]]