Week 9 [Mon, Mar 13th] - SE Topics

UML class diagrams describe the structure (but not the behavior) of an OOP solution. These are possibly the most often used diagrams in the industry and are an indispensable tool for an OO programmer.

An example class diagram:

The basic UML notations used to represent a class:

A Table class shown in UML notation:

The equivalent code

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The 'Operations' compartment and/or the 'Attributes' compartment may be omitted if such details are not important for the task at hand. Similarly, some attributes/operations can be omitted if not relevant. 'Attributes' always appear above the 'Operations' compartment. All operations should be in one compartment rather than each operation in a separate compartment. Same goes for attributes.

The visibility of attributes and operations is used to indicate the level of access allowed for each attribute or operation. The types of visibility and their exact meanings depend on the programming language used. Here are some common visibilities and how they are indicated in a class diagram:

• + : public
• - : private
• # : protected
• ~ : package private

Table class with visibilities shown:

The equivalent code

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Generic classes can be shown as given below. The notation format is shown on the left, followed by two examples.

While all objects of a class have the same attributes, each object has its own copy of the attribute value.

However, some attributes are not suitable to be maintained by individual objects. Instead, they should be maintained centrally, shared by all objects of the class. They are like ‘global variables’ but attached to a specific class. Such variables whose value is shared by all instances of a class are called class-level attributes.

Similarly, when a normal method is being called, a message is being sent to the receiving object and the result may depend on the receiving object.

However, there can be methods related to a specific class but not suitable for sending messages to a specific object of that class. Such methods that are called using the class instead of a specific instance are called class-level methods.

Class-level attributes and methods are collectively called class-level members (also called static members sometimes because some programming languages use the keyword static to identify class-level members). They are to be accessed using the class name rather than an instance of the class.

In UML class diagrams, underlines denote class-level attributes and methods.

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

Objects in an OO solution need to be connected to each other to form a network so that they can interact with each other. Such connections between objects are called associations.

Associations in an object structure can change over time.

Associations among objects can be generalized as associations between the corresponding classes too.

Implementing associations

You use instance level variables to implement associations.

You should use a solid line to show an association between two classes.

Association labels describe the meaning of the association. The arrow head indicates the direction in which the label is to be read.

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.

Association Role labels are used to indicate the role played by the classes in the association.

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

    

    


    
    

Multiplicity is the aspect of an OOP solution that dictates how many objects take part in each association.

Implementing multiplicity

A normal instance-level variable gives us a 0..1 multiplicity (also called optional associations) because a variable can hold a reference to a single object or null.

class Logic {
    Minefield minefield;
    // ...
}

class Minefield {
    //...
}

    

    


    
    

class Logic:
  
  def __init__(self):
    self.minefield = None
    
  # ...


class Minefield:
  # ...

    

    


    
    

A variable can be used to implement a 1 multiplicity too (also called compulsory associations).

class Logic {
    ConfigGenerator cg = new ConfigGenerator();
    ...
}

    

    


    
    

class Logic:

  def __init__(self):
    self.config_gen = ConfigGenerator()

    

    


    
    

Bidirectional associations require matching variables in both classes.

class Foo {
    Bar bar;
    //...
}

class Bar {
    Foo foo;
    //...
}


    

    


    
    

class Foo:
  
  def __init__(self, bar):
    self.bar = bar;


class Bar:
  
  def __init__(self, foo):
    self.foo = foo;
    

    

    


    
    

To implement other multiplicities, choose a suitable data structure such as Arrays, ArrayLists, HashMaps, Sets, etc.

class Minefield {
    Cell[][] cell;
    ...
}

    

    


    
    

class Minefield:
  
  def __init__(self):
    self.cells = {1:[], 2:[], 3:[]}

    

    


    
    

Commonly used multiplicities:

• 0..1 : optional, can be linked to 0 or 1 objects.
• 1 : compulsory, must be linked to one object at all times.
• * : can be linked to 0 or more objects.
• n..m : the number of linked objects must be within n to m inclusive.

In the diagram below, an Admin object administers (is 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.

When two classes are linked by an association, it does not necessarily mean the two objects taking part in an instance of the association knows about (i.e., has a reference to) each other. The concept of which object in the association knows about the other object is called navigability.

Navigability can be unidirectional or bidirectional. Suppose there is an association between the classes Box and Rope, and the Box object b and the Rope object r is taking part in one instance of that association.

• Unidirectional: If the navigability is from Box to Rope, b will have a reference to r but r will not have a reference to b. Similarly, if the navigability is in the other direction, r will have a reference to b but b will not have a reference to r.
• Bidirectional: b will have a reference to r and r will have a reference to b i.e., the two objects will be pointing to each other for the same single instance of the association.

Note that two unidirectional associations in opposite directions do not add up to a single bidirectional association.

class Person {
    Cat pet;
    //...
}

class Cat{
    Person owner;
    //...
}

    

    


    
    

class Person:

  def __init__(self):
    self.pet = None  # a Cat object


class Cat:

  def __init__(self):
    self.owner = None  # a Person object

    

    


    
    

class Person {
    Cat pet;
    //...
}

class Cat{
    Person breeder;
    //...
}

    

    


    
    

class Person:

  def __init__(self):
    self.pet = None  # a Cat object


class Cat:

  def __init__(self):
    self.breeder = None  # a Person object

    

    


    
    

Use arrowheads to indicate the navigability of an association.

In this example, the navigability is unidirectional, and is from the Logic class to the Minefield class. That means if a Logic object L is associated with a Minefield object M, L has a reference to M but M doesn't have a reference to L.

class Logic {
    Minefield minefield;
    // ...
}

class Minefield {
    //...
}

    

    


    
    

class Logic:
  
  def __init__(self):
    self.minefield = None
    
  # ...


class Minefield:
  # ...

    

    


    
    

Here is an example of a bidirectional navigability; i.e., if a Dog object d is associated with a Man object m, d has a reference to m and m has a reference to d.

class Dog {
    Man man;
    // ...
}

class Man {
    Dog dog;
    // ...
}

    

    


    
    

class Dog:
  
  def __init__(self):
    self.man = None
    
  # ...


class Man:
  def __init__(self):
    self.dog = None

  # ...

    

    


    
    

Navigability can be shown in class diagrams as well as object diagrams.

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

An association can be shown as an attribute instead of a line.

Association multiplicities and the default value can be shown as part of the attribute using the following notation. Both are optional.

name: type [multiplicity] = default value

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:

Show each association as either an attribute or a line but not both. A line is preferred as it is easier to spot.

An object diagram shows an object structure at a given point of time.

An example object diagram:

Notation:

Notes:

• The class name and object name e.g. car1:Car are underlined.
• objectName:ClassName is meant to say 'an instance of ClassName identified as objectName'.
• Unlike classes, there is no compartment for methods.
• Attributes compartment can be omitted if it is not relevant to the task at hand.
• Object name can be omitted too e.g. :Car which is meant to say 'an unnamed instance of a Car object'.

Some example objects:

A solid line indicates an association between two objects.

Compared to the notation for class diagrams, object diagrams differ in the following ways:

• Show objects instead of classes:
  • Instance name may be shown
  • There is a : before the class name
  • Instance and class names are underlined
• Methods are omitted
• Multiplicities are omitted. Reason: an association line in an object diagram represents a connection to exactly one object (i.e., the multiplicity is always 1).

Furthermore, multiple object diagrams can correspond to a single class diagram.

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.

When the class diagram has an inheritance relationship, the object diagram should show either an object of the parent class or the child class, but not both.

Suppose Employee is a child class of the Person class. The class diagram will be as follows:

Now, how do you show an Employee object named jake?

• This is not correct, as there should be only one object.

• This is OK.

• This is OK, as jake is a Person too. That is, we can show the parent class instead of the child class if the child class doesn't matter to the purpose of the diagram (i.e., the reader of this diagram will not need to know that jake is in fact an Employee).

Association labels/roles can be omitted unless they add value (e.g., showing them is useful if there are multiple associations between the two classes in concern -- otherwise you wouldn't know which association the object diagram is showing)

Consider this class diagram and the object diagram:

We can clearly see that both Adam and Eve lives in hall h1 (i.e., OK to omit the association label lives in) but we can't see if History is Adam's major or his minor (i.e., the diagram should have included either an association label or a role there). In contrast, we can see Eve is an English major.

UML notes can augment UML diagrams with additional information. These notes can be shown connected to a particular element in the diagram or can be shown without a connection. The diagram below shows examples of both.

Example: