UML

An example class diagram:

A Table class shown in UML notation:

class Table{

    Integer number;
    Chair[] chairs = null;
    
    Integer getNumber(){
        //...
    }

    void setNumber(Integer n){
        //...
    }
}

class Table:

  def __init__(self):
    self.number = 0
    self.chairs = []

  def get_number(self):
    # ...

  def set_number(self, n):
    # ...

Table class with visibilities shown:

This example shows an association between the Admin class and the Student class:

Logic is aware of Minefield, but Minefield is not aware of Logic

class Logic{
    Minefield minefield;
}

class Minefield{
    ...
}

class Logic:
  
  def __init__(self, minefield):
    self.minefield = minefield
    
  # ...


class Minefield:
  # ...

Here is an example of a bidirectional navigability; each class is aware of the other.

According to this object diagram the given Logic object is associated with and aware of two MineField objects.

This association represents a marriage between a Man object and a Woman object. The respective roles played by objects of these two classes are husband and wife.

Note how the variable names match closely with the association roles.

class Man{
    Woman wife;
}

class Woman{
    Man husband;
}

class Man:
  def __init__(self):
    self.wife = None # a Woman object

class Woman:
   def __init__(self):
     self.husband = None # a Man object

The role of Student objects in this association is charges (i.e. Admin is in charge of students)

class Admin{
    List<Student> charges;
}

class Admin:
  def __init__(self):
    self.charges = [] # list of Student objects

In this example, the same association is described using two different labels.

• Diagram on the left: Admin class is associated with Student class because an Admin object uses a Student object.
• Diagram on the right: Admin class is associated with Student class because a Student object is used by an Admin object.

In the diagram below, an Admin object administers (in charge of) any number of students but a Student object must always be under the charge of exactly one Admin object

In the diagram below,

• Each student must be supervised by exactly one professor. i.e. There cannot be a student who doesn't have a supervisor or has multiple supervisors.
• A professor cannot supervise more than 5 students but can have no students to supervise.
• An admin can handle any number of professors and any number of students, including none.
• A professor/student can be handled by any number of admins, including none.

Two examples of dependencies:

The diagram below depicts a multi-player Square Game being played on a board comprising of 100 squares. Each of the squares may be occupied with any number of pieces, each belonging to a certain player.

A Piece may or may not be on a Square. Note how that association can be replaced by an isOn attribute of the Piece class. The isOn attribute can either be null or hold a reference to a Square object, matching the 0..1 multiplicity of the association it replaces. The default value is null.

The association that a Board has 100 Squares can be shown in either of these two ways:

In the class diagram below, there are two enumerations in use:

In the class below, totalStudents attribute and the getTotalStudents method are class-level.

A Book consists of Chapter objects. When the Book object is destroyed, its Chapter objects are destroyed too.

Example:

Aggregation vs Composition

The distinction between composition (◆) and aggregation (◇) is rather blurred. Martin Fowler’s famous book UML Distilled advocates omitting the aggregation symbol altogether because using it adds more confusion than clarity.

Examples: The Car class inherits from the Vehicle class. The Cat and Dog classes inherit from the Pet class.

The AcademicStaff and the AdminStaff classes implement the SalariedStaff interface.

Example:

Consider the code below.

class Machine {

    Unit producePrototype() {
        Unit prototype = new Unit();
        for (int i = 0; i < 5; i++) {
            prototype.stressTest();
        }
        return prototype;
    }
}

class Unit {

    public void stressTest() {

    }
}

Here is the sequence diagram to model the interactions for the method call producePrototype() on a Machine object.

This sequence diagram shows some interactions between a human user and the Text UI of a Command Line InterfaceCLI Minesweeper game.

The player runs the newgame action on the TextUi object which results in the TextUi showing the minefield to the player. Then, the player runs the clear x y command; in response, the TextUi object shows the updated minefield.

[Common notation error] Activation bar too long: The activation bar of a method cannot start before the method call arrives and a method cannot remain active after the method had returned. In the two sequence diagrams below, the one on the left commits this error because the activation bar starts before the method Foo#xyz() is called and remains active after the method returns.

[Common notation error] Broken activation bar: The activation bar should remain unbroken from the point the method is called until the method returns. In the two sequence diagrams below, the one on the left commits this error because the activation bar for the method Foo#abc() is not contiguous, but appears as two pieces instead.

The Logic object creates a Minefield object.

Although object deletion is not that important in languages such as Java that support automatic memory management, you can still show object deletion in UML diagrams to indicate the point at which the object ceases to be used.

Note how the diagrams shows the deletion of the Minefield object

The Player calls the mark x,y command or clear x y command repeatedly until the game is won or lost.

The markCellAt(...) method of a Logic object is calling its own updateState(...) method.

In this variation, the Book#write() method is calling the Chapter#getText() method which in turn does a call back by calling the getAuthor() method of the calling object.

Minefield calls the Cell#setMine if the cell is supposed to be a mined cell, and calls the Cell:setMineCount(...) method otherwise.

Logic#markCellAt(...) calls Timer#start() only if it is the first move of the player.

Logic is calling methods CloudServer#poll() and LocalServer#poll() in parallel.

A minimal sequence diagram

An example object diagram:

Some example objects:

An example object diagram showing two associations:

Example:

Both object diagrams are derived from the same class diagram shown earlier. In other words, each of these object diagrams shows ‘an instance of’ the same class diagram.