Week 3 [Aug 26]

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 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:

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):
    # ...

The 'Operations' compartment and/or the 'Attributes' compartment may be omitted if such details are not important for the task at hand. '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:

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

We use arrow heads to indication the navigability of an association.

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.

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.

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.

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.

While all objects of a class has 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 message 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 variables.

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

The content below is an extract from -- Java Tutorial, with slight adaptations.

public class Bicycle {

    private int gear;
    private int speed;

    // an instance variable for the object ID
    private int id;

    // a class variable for the number of Bicycle objects instantiated
    private static int numberOfBicycles = 0;
        ...
}

public class Bicycle {

    private int gear;
    private int speed;

    private int id;

    private static int numberOfBicycles = 0;


    public Bicycle(int startSpeed, int startGear) {
        gear = startGear;
        speed = startSpeed;

        numberOfBicycles++;
        id = numberOfBicycles;
    }

    public int getID() {
        return id;
    }

    public static int getNumberOfBicycles() {
        return numberOfBicycles;
    }

    public int getGear(){
        return gear;
    }

    public void setGear(int newValue) {
        gear = newValue;
    }

    public int getSpeed() {
        return speed;
    }

    // ...

}

An Enumeration is a fixed set of values that can be considered as a data type. An enumeration is often useful when using a regular data type such as int or String would allow invalid values to be assigned to a variable.

You can define an enum type by using the enum keyword. Because they are constants, the names of an enum type's fields are in uppercase letters e.g., FLAG_SUCCESS by convention.

public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY
}

Day today = Day.MONDAY;
Day[] holidays = new Day[]{Day.SATURDAY, Day.SUNDAY};

switch (today) {
case SATURDAY:
case SUNDAY:
    System.out.println("It's the weekend");
    break;
default:
    System.out.println("It's a week day");
}

Note that while enumerations are usually a simple set of fixed values, Java enumerations can have behaviors too, as explained in this tutorial from -- Java Tutorial

Variable Arguments (Varargs) is a syntactic sugar type feature that allows writing a methods that can take a variable number of arguments.

public static void search(String ... keywords){
   // method body
}

Resources:

• Java Varargs feature (from Oracle.com)
• Java Varargs tutorial (for javaTpoint.com)

The OOP concept Inheritance allows you to define a new class based on an existing class.

• Other names for Base class: Parent class, Super class
• Other names for Derived class: Child class, Sub class, Extended class

A superclass is said to be more general than the subclass. Conversely, a subclass is said to be more specialized than the superclass.

Applying inheritance on a group of similar classes can result in the common parts among classes being extracted into more general classes.

Inheritance implies the derived class can be considered as a sub-type of the base class (and the base class is a super-type of the derived class), resulting in an is a relationship.

Inheritance does not necessarily mean a sub-type relationship exists. However, the two often go hand-in-hand. For simplicity, at this point let us assume inheritance implies a sub-type relationship.

Inheritance relationships through a chain of classes can result in inheritance hierarchies (aka inheritance trees).

Two inheritance hierarchies/trees are given below. Note that the triangle points to the parent class. Observe how the Parrot is a Bird as well as it is an Animal.

Multiple Inheritance is when a class inherits directly from multiple classes. Multiple inheritance among classes is allowed in some languages (e.g., Python, C++) but not in other languages (e.g., Java, C#).

The Honey class inherits from the Food class and the Medicine class because honey can be consumed as a food as well as a medicine (in some oriental medicine practices). Similarly, a Car is an Vehicle, an Asset and a Liability.

You can use a triangle and a solid line (not to be confused with an arrow) to indicate class inheritance.

Notation:

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

Method overloading is when there are multiple methods with the same name but different type signatures. Overloading is used to indicate that multiple operations do similar things but take different parameters.

Given below is an extract from the -- Java Tutorial, with slight adaptations.

public class Bicycle {

    public int gear;
    public int speed;

    public Bicycle(int startSpeed, int startGear) {
        gear = startGear;
        speed = startSpeed;
    }

    public void setGear(int newValue) {
        gear = newValue;
    }

    public void applyBrake(int decrement) {
        speed -= decrement;
    }

    public void speedUp(int increment) {
        speed += increment;
    }

}

public class MountainBike extends Bicycle {

    // the MountainBike subclass adds one field
    public int seatHeight;

    // the MountainBike subclass has one constructor
    public MountainBike(int startHeight, int startSpeed, int startGear) {
        super(startSpeed, startGear);
        seatHeight = startHeight;
    }

    // the MountainBike subclass adds one method
    public void setHeight(int newValue) {
        seatHeight = newValue;
    }
}

public class Superclass {

    public void printMethod() {
        System.out.println("Printed in Superclass.");
    }
}

public class Subclass extends Superclass {

    // overrides printMethod in Superclass
    public void printMethod() {
        super.printMethod();
        System.out.println("Printed in Subclass");
    }
    public static void main(String[] args) {
        Subclass s = new Subclass();
        s.printMethod();
    }
}

Printed in Superclass.
Printed in Subclass

public MountainBike(int startHeight, int startSpeed, int startGear) {
    super(startSpeed, startGear);
    seatHeight = startHeight;
}

Well-written applications include error-handling code that allows them to recover gracefully from unexpected errors. When an error occurs, the application may need to request user intervention, or it may be able to recover on its own. In extreme cases, the application may log the user off or shut down the system. --_Microsoft

Exceptions are used to deal with 'unusual' but not entirely unexpected situations that the program might encounter at run time.

Given below is an extract from the -- Java Tutorial, with some adaptations.

Most languages allow code that encountered an "exceptional" situation to encapsulate details of the situation in an Exception object and throw/raise that object so that another piece of code can catch it and deal with it. This is especially useful when the code that encountered the unusual situation does not know how to deal with it.

The extract below from the -- Java Tutorial (with slight adaptations) explains how exceptions are typically handled.

Advantages of exception handling in this way:

• The ability to propagate error information through the call stack.
• The separation of code that deals with 'unusual' situations from the code that does the 'usual' work.

The content below uses extracts from the -- Java Tutorial, with some adaptations.

A program can catch exceptions by using a combination of the try, catch blocks.

• The try block identifies a block of code in which an exception can occur.
• The catch block identifies a block of code, known as an exception handler, that can handle a particular type of exception.

public void writeList() {
    print("starting method");
    try {
        print("starting process");
        process();
        print("finishing process");

    } catch (IndexOutOfBoundsException e) {
        print("caught IOOBE");

    } catch (IOException e) {
        print("caught IOE");

    }
    print("finishing method");
}

You can use a finally block to specify code that is guaranteed to execute with or without the exception. This is the right place to close files, recover resources, and otherwise clean up after the code enclosed in the try block.

The writeList() method below has a finally block:

public void writeList() {
    print("starting method");
    try {
        print("starting process");
        process();
        print("finishing process");

    } catch (IndexOutOfBoundsException e) {
        print("caught IOOBE");

    } catch (IOException e) {
        print("caught IOE");

    } finally {
        // clean up
        print("cleaning up");
    }
    print("finishing method");
}

Some possible outputs:

• The try statement should contain at least one catch block or a finally block and may have multiple catch blocks.

• The class of the exception object indicates the type of exception thrown. The exception object can contain further information about the error, including an error message.

You can use the throw statement to throw an exception. The throw statement requires a throwable object as the argument.

if (size == 0) {
    throw new EmptyStackException();
}

In Java, Checked exceptions are subject to the Catch or Specify Requirement: code that might throw checked exceptions must be enclosed by either of the following:

• A try statement that catches the exception. The try must provide a handler for the exception.
• A method that specifies that it can throw the exception. The method must provide a throws clause that lists the exception.

Unchecked exceptions are not required to follow to the Catch or Specify Requirement but you can apply the requirement to them too.

public void writeList() throws IOException, IndexOutOfBoundsException {
    print("starting method");
    process();
    print("finishing method");
}

Some possible outputs:

Java comes with a collection of built-in exception classes that you can use. When they are not enough, it is possible to create your own exception classes.

In general, use exceptions only for 'unusual' conditions. Use normal return statements to pass control to the caller for conditions that are 'normal'.