@startuml
skinparam classAttributeIconSize 0
skinparam noteBackgroundColor #FFF9C4

class BankAccount {
  - accountNumber: String
  - balance: Real
  - isOverdraftEnabled: Boolean
  + withdraw(amount: Real)
  + deposit(amount: Real)
}

note right of BankAccount
  **Class Invariants:**
  context BankAccount
  
  -- Balance can never be negative unless overdraft is enabled
  inv balanceRule: 
    self.balance >= 0 or self.isOverdraftEnabled
  
  -- Overdraft limit cannot exceed 500
  inv overdraftLimit: 
    self.isOverdraftEnabled implies self.balance >= -500
end note
@enduml

@startuml
skinparam classAttributeIconSize 0
skinparam noteBackgroundColor #E8F5E9

class ShoppingCart {
  - items: Bag<Product>
  - totalAmount: Real
  + checkout(): Receipt
  + getTotal(): Real
}

note right of ShoppingCart
  **Operation Contracts:**
  
  -- Pre-condition: Cart must not be empty
  context ShoppingCart::checkout(): Receipt
  pre: self.items->notEmpty()
  
  -- Post-condition: Cart is emptied, receipt is generated
  post: self.items->isEmpty() and result <> null
  
  -- Body: Query operation definition
  context ShoppingCart::getTotal(): Real
  body: self.items->collect(price)->sum()
end note
@enduml

@startuml
skinparam classAttributeIconSize 0
skinparam noteBackgroundColor #E3F2FD

class Employee {
  - firstName: String
  - birthDate: Date
  - {derived} age: Integer
  - {initial} isActive: Boolean = true
}

note right of Employee
  **Derived & Initial Values:**
  
  -- Calculate age dynamically
  context Employee::age: Integer
  derive: self.birthDate.yearsSinceToday()
  
  -- Initial state upon creation
  context Employee::isActive: Boolean
  init: true
end note
@enduml

@startuml
skinparam stateBackgroundColor #F3E5F5

[*] --> Created
Created --> PendingApproval : submit()

state PendingApproval {
}

PendingApproval --> Approved : [total < 10000]
PendingApproval --> Rejected : [total >= 10000]

Approved --> Shipped : ship()
Shipped --> [*]

note right of PendingApproval
  **State Invariant:**
  context Order
  inv: self.status = 'Pending' implies 
       self.submittedBy->notEmpty()
end note

note right of Approved
  **Post-condition of approve():**
  context Order::approve()
  post: self.status = 'Approved' and 
        self.approvedDate <> null
end note
@enduml

@startuml
skinparam classAttributeIconSize 0
skinparam noteBackgroundColor #FFFDE7
skinparam noteBorderColor #FBC02D

class Employee {
  - SSN: String
  - firstName: String
  - birthDate: Date
  - hireDate: Date
  - salary: Float
  + age(): Integer
}

class Department {
  - number: Integer
  - name: String
  - locations: Set<String>
  - {derived} nbrEmployees: Integer
}

class Project {
  - number: Integer
  - name: String
  - location: String
}

Employee "0..*" -- "1" Department : worksFor > department
Employee "0..*" -- "0..*" Project : worksOn > project
Employee "1" -- "0..1" Department : manages > managedDept
Department "1" -- "0..*" Project : controls > projects
Employee "1" -- "0..*" Employee : supervisor > subordinates

note right of Employee
  **Complex Invariants & Navigation:**
  context Employee
  
  -- 1. Basic Attribute Rule
  inv ageCheck: self.age() >= 18
  
  -- 2. Navigating Associations (Multiplicity)
  inv maxProjects: self.worksOn->size() <= 4
  
  -- 3. Using Iterators (forAll)
  -- Supervisor must be hired before subordinates
  inv hireOrder: self.subordinates->forAll(
    sub | sub.hireDate > self.hireDate
  )
  
  -- 4. Cross-Association Rule
  -- Employee can only work on projects controlled by their dept
  inv projectControl: 
    self.worksFor.controls->includesAll(self.worksOn)
    
  -- 5. Unique Identifier (Key)
  inv uniqueSSN: Employee.allInstances()->forAll(e1, e2 | 
    e1 <> e2 implies e1.SSN <> e2.SSN
  )
end note

note bottom of Department
  **Derived Attribute:**
  context Department::nbrEmployees: Integer
  derive: self.worksFor->size()
end note

note right of Project
  **Project Invariant:**
  context Project
  
  -- Project location must be one of the department's locations
  inv validLocation: 
    self.controls.locations->includes(self.location)
end note

@enduml

@startuml
skinparam classAttributeIconSize 0

class Employee {
  - worksOn: Set<Project>
}

note right of Employee
  **Using 'let' for complex logic:**
  context Employee
  
  -- Calculate total hours and check boundaries
  inv workingHours: 
    let totalHours: Integer = 
      self.worksOn->collect(hours)->sum() 
    in totalHours >= 30 and totalHours <= 50
end note
@enduml

@startuml
skinparam classAttributeIconSize 0

class Company {
  - employees: Set<Employee>
  + hireEmployee(p: Employee)
}

note right of Company
  **Using @pre in Post-conditions:**
  context Company::hireEmployee(p: Employee)
  
  -- The new employee set is the old set + the new person
  post: self.employees = self.employees@pre->including(p)
end note
@enduml