Object-Oriented Programming and Java

Object-Oriented Programming and Java
Instructor: M.Sc. Erkki Mattila, lecturer
Office: C136
Office hours: according to the weekly schedule published on the net
Mobile tel.: [phone]
E-mail: [e-mail]
OOP, Rovaniemi University of Applied Sciences

Object-Oriented Programming and Java
COURSE CODE
504D24A
DURATION
3 CU (+5 CU = 8 CU)
TEACHING
40 hours of classes, 40 hours of self-supervised work (first part)
OOP, Rovaniemi University of Applied Sciences

Object-Oriented Programming and Java
OBJECTIVES
The main objective of the first part of the course is to introduce the basic concepts and principles of object-oriented programming
The second part introduces the student to Java programming (Java SE)
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Object-Oriented Programming and Java
COURSE MATERIAL
Lecture notes, available at O:\Opettajat\Erkki Mattila\Object-oriented Programming and Java 504D24A - Part 1
Budd T. 2002. An Introduction to Object-Oriented Programming, 3rd Edition. Addison-Wesley Longman
Sebesta R. W. 2008. Concepts of Programming Languages, 8th Edition. Pearson Education. Addison-Wesley
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Object-Oriented Programming and Java
ASSESMENT
Mid-term exam, which will be graded on a scale from 1 to 5 and F. 30 percent of the maximum points are required for grade 1.
The final exam will be held after the second 5 CU part of the course. The course grade will be calculated as a weighted average of the mid-term and final exam grades
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Student level assesment
Which programming courses have you participated earlier?
How familiar are you with the concepts and principles of object-oriented programming? You may also answer on a scale from 0 to 5, zero being not at all and five being master level?
Do you have prior experience of object-oriented programming languages? If so, which programming languages/tools have you used?
Are you familiar with UML (0-5)?
What do you expect/hope to get from this course?
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Course Contents
Part I: Concepts and Principles of Object-Oriented Programming
Part II: Object-oriented Programming
Closer look at OOP
Part III: Introduction to Java Programming
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Part I Contents
Abstraction
The concept of abstraction, data and process abstraction, abstract data types
Concepts of Object-Oriented Programming
Class, object, attribute, method, etc.
Principles of Object-oriented Programming
Data abstraction (encapsulation and information hiding), inheritance, and polymorphism
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1. Abstraction

Object-oriented Programming Paradigm
OOP is a common programming paradigm. A programming paradigm is a way to conceptualize how to structure a program to solve a problem
Other programming paradigms include functional programming, logic programming, imperative programming and declarative programming
Common OO programming languages nowadays are Java, C++ ja C#
Common OO modelling languages are UML, OMT ja OMT++. UML is nowadays the de facto standard
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C Data Types Revisited
Which built-in, primitive data types the C programming language has?
What are structure types (struct) in C language?
What is the relationship between functions and data in C language?
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Definition: Abstract Data Type
An abstract data type is a data type that satisfies the following conditions:
The declarations of the type and (the protocols of) the operations of the type are contained in a single syntactic unit (encapsulation)
Other program units are allowed to create variables of the type
The representation of objects of the type is hidden from other program units. The only direct operations possible on those objects are those provided in the type’s definition (information hiding)
Sebesta R.W. 2008. Concepts of Programming Languages , 8th Edtion. Addison Wesley
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Abstract Data Types (ADT)
ADTs are defined by the user (user-defined type)
ADTs should provide the same characteristics provided by language-defined types, such as an integer or a floating point type:
A type’s definition that allows program units to declare variables of the type, but hides the representation of the objects of the type
A set of operations for manipulating objects of the type
In OOP abstract data types are implemented as classes
An instance of an abstract data type is called an object
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Scope
A class consists of data fields (attributes) and operations, which manipulate the data
The data fields/attributes can be accessed anywhere inside the class itself; i.e. all operations of the class have direct access to class’s data
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Group Work
Define a new abstract data type
Name of the type
Attributes (Data fields)
Operations
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Relation of Abstract Data Types to Object-oriented Programming
Object-oriented programming (OOP) is an outgrowth of the use of ADTs
Data abstraction is one the most important components of OOP
In OOP languages abstract data types are implemented as classes!
An instance of a class is called an object!
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2. Concepts of Object-oriented Programming

Class
Classes are reusable software components that model items in the real world
A class encapsulates the data and procedural abstractions (operations) that are required to describe the content and behaviour of some real world entity
A program written in pure OOP language consists of classes – there can be no code outside of classes
A class is a compile-time concept
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attributes:
class name
methods:

Object
An object is an instance of a class
Objects are created during program execution – object is a run-time concept
Multiple objects can be instantiated of the same class
Each object has its own values for the instance variables (attributes) of its class
These values define the state of the object
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Classes and Objects
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An object is an instance of a class. It gives specific values to the fields of the class

Attributes
People have features including date of birth, name, height and eye colour
Physical objects have features such as shape, weight, colour, etc
Similarly classes have attributes (=fields, member variables)
The values assigned to an object’s attributes make that object unique; they define the state of an object
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Methods
A class encapsulates data and the algorithms that process that data. These algorithms are called methods, operations, functions, routines or services
Each of the methods provide a representation of one of the behaviours of the object
Behaviour = method implementation
Whenever an object receives a message, it initiates some behaviour by executing a method
Message = method call
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Messages (Method Calls)
Messages are the means by which objects interact. A message stimulates some behaviour to occur in the receiving object. The behaviour is accomplished when a method is called and executed
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Sender object
Receiver object
sender.method(parameters)
receiver.method(parameters)
Pressman R., p.533

Constructor
A specialized method used to instantiate an object
The constructor function has the same name as the class
It is never called directly, but the run-time system calls it when a new object is created
Constructors are usually overloaded; a class contains multiple constructors, which have a different set of parameters
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A Class Definition in Java Language
public class Shape
{
private static int a_numberOfShapes=0;
private Color a_color;
public Shape()
{
Shape.a_numberOfShapes++;
a_color = Color.BLACK;
}
public Shape(Color c)
{
Shape.a_numberOfShapes++;
a_color = c;
}
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A Class Definition in Java Language
public class Circle extends Shape // Class header
{
private int radius ; // Member variable
public Circle() // Constructor
{
radius = 0;
}
public Circle(int r, Color c) {
super( c ); // Calling superclass contructor
radius = r;
}
public long calculateArea() // Member function
{
return Math.round(Math.PI*radius*radius);
}
}
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Group Work
Your task is to design an ATM (Automatic Bank Teller Machine) system
Which classes (and objects) should the system contain?
Which attributes and operations the classes should have?
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Class Relationships
Generalization (Inheritance)
Aggregation
Special case: composition
Association
Dependency and Realization relationships will be coverered in the Design Methods course

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Inheritance (“is a”) Relationship
Classes may be arranged in a class hierarchy where one class (a superclass) is a generalisation of one or more other classes (subclasses)
Generalization: creating a common supertype for a group of classes
Dog, Cat, Horse => supertype Animal
Specialization: creating extended versions of existing classes or objects. This is also called subtyping. Subclass is a special case of the superclass
Animal => subtypes Dog, Cat, Horse
A subclass inherits the attributes and operations from its superclass and may add new methods or attributes of its own
For instance Animal class contains the features common to all animals

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Generalization (Inheritance)

Group work
Define a rectangle and a square as classes.
Can they be modelled with a single class?
If they are modelled as separate classes, is one a superclass of the other? Why?
Start by defining the Rectangle class. Declare attributes and constructor methods. Initilize attributes in contructors.
Remember that constructors can be overloaded
Remember that superclass constructor method can be called in the subclass using the keyword super
Add a method for calculating the area of the rectangle
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Aggregation (“part of“) Relationship
Shows how classes that are collections are composed of other classes.
Models the notion that one object uses another object without "owning" it and thus is not responsible for its creation or destruction.
Similar to the part-of relationship in semantic data models.

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Composition (“has a “) Relationship
Composition is a special form of aggregation describing the situation where an object contains a number of other objects and when the containing object is deleted, all the instances of the objects that are contained disappear
The contained (member) object cannot exist without its owning object and can belong to one owner only at any given time

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Composition vs. Aggregation
Composition is a stricter relationship than aggregation:
Member objects cannot exist without the containing object.
A member object can belong to only one containing object at a time.
Example of composition: a minister cannot exist without a government, and a minister can be a part of only one government at a time.

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Association (“uses”) Relationship
One entity uses another entity as part of its behavior
Used when the relationship is permanent
In the code level one class has a member variable of another class type

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Attributes and Associations
Attributes and associations are exchangeable!
When the relationship exists between classes in your own class model, use an association in the UML class diagram
When the relationship exists between a class in your own class model and a class from a class library, use an attiribute
is the same as:

Group Work
Draw an UML class diagram containing the following classes: Car, Bicycle, Motorcycle, Steamship, Sailboat, Sail, Train, Wheel (tyre), Engine, Motorship, Watercraft, Vehicle (means of transportation), Landvehicle, Road, Railway, Waterway, Seat (bench).
Put classes to a class diagram and add some attributes to them. Do the classes have some common attributes?
Add appropriate relationships between the classes
Start by defining the class hierarchy
Add common features as high as possible in the inheritance hierarchy
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3. Principles of Object-oriented Programming

Encapsulation
A class encapsulates together data (attributes), methods, constants, and other related information
Encapsulation means that all of this information is packaged under one name and can be reused as one specification or program component
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Encapsulation: Example
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G. Booch, “Object-oriented analysis and design with applications”

Benefits of Encapsulation
Data structures and the methods that manipulate them are merged in a single named entity – the class
Facilitates component reuse
Interfaces among encapsulated objects are simplified. An object that sends a message need not be concerned with the details of internal data structures
Greatly reduces programmer’s memory load
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Data Abstraction and Information Hiding
DA isolates how a compound data object is used from the details of how it is constructed from more primitive data objects
Consider a class Stack and its operations push and pop
The client cares about what services a class offers, not about class’s internal data structures or how the services are implemented
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Data Abstraction and Information Hiding
Classes normally hide the details of their implementation from their clients. This is called information hiding
Although programmers might know the details of a class’s implementation, they should not write code that depends on these details as the details may later change
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Data Abstraction and Information Hiding
From the definition of an abstract data type:
The declarations of the type and the protocols of the operations on objects of the type, which provide type’s interface, are contained in a single syntactic unit
Data Abstraction (type’s interface) and Encapsulation (single syntactic unit)
The representation of objects of the type is hidden from other program units. The only direct operations possible on those objects are those provided in the type’s definition
Information hiding
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Information Hiding
A type's internal form is hidden behind a set of access functions
Values of the type are created, inspected and modified only by calls to the access functions
This allows the implementation of the type to be changed without requiring any changes outside the module in which it is defined
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Benefits of Information Hiding
The internal implementation details of data and procedures are hidden from the outside world. This reduces the propagation of side effects when changes occur
Preserving the integrity of objects
Illegal attribute values not allowed
More on information hiding and using access specifiers to implement it in the second slide set
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Inheritance
Inheritance is a mechanism that enables the responsibilities of one object to be propagated to other objects
In a class hierarchy the attributes and methods of the superclass are inherited by subclasses that may each add additional “private” attributes and methods
Inheritance can also be called as
Specialization: creating extended versions of existing classes or objects. This is also called subtyping. Subclass is a special case of the superclass
Generalization: creating a common supertype for a group of classes
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Class Hierarchy
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Class Hierarchy
Direct superclass
Inherited explicitly (one level up hierarchy)
Indirect superclass
Inherited two or more levels up hierarchy
Single inheritance
Inherits from one direct superclass
Multiple inheritance
Inherits from multiple direct superclasses
C++ supports multiple inheritance, Java does not
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Benefits of Inheritance
Inheritance occurs throughout all levels of a class hierarchy. Changes at the higher level are immediately propagated through a system
Reuse of components is accomplished directly
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Group Work
Consider the following classes:
Building (properties: address, in use, build year, value,…)
Apartment (properties: number, size,…)
Office (business premises, properies: number, size,…)
Resident (add properties)
Company (properties: founding year, turnover,…)
Public Company (publicly listed company, which has its own premises. Add properties)
Private Company (unlisted company, which has its own premises. Add properties)
Home Business / Company (operates in someone’s home. Add properties)
Employee (add properties)
Define the properties of the classes. Define the public interface of the classes. Define the relationships among classes.

Additional Material:
Forms of Inheritance
Specialization
The child class is a special case of the parent class
Specification
The parent class defines behaviour that is implemented in the child class, but not in the parent class
Used to guarantee that classes maintain a certain common interface – that is, they implement the same methods
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Additional Material:
Forms of Inheritance
Construction
The child class makes use of the behaviour provided by the parent class, but is not a subtype of the parent class
Generalization
The child class modifies or overrides some of the methods of the parent class to create a more general kind of object
Bad style. Use only if you cannot modify the parent class
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Additional Material:
Forms of Inheritance
Extension
The child class adds new functionality, but does not change any inherited behaviour
Variance
The child class and parent class are variants of each other, and the class-subclass relationship is arbitrary
Bad style. Declare a common abstract superclass instead
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Additional Material:
Forms of Inheritance
Combination
The child class inherits features from more than one direct parent class
Known as multiple inheritance
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Benefits of OOP
Characteristics of OOP software:
Natural
Instead of programming in terms of an array or region of memory, you can program using the terminology of your particular problem
Reliable
The modular nature of objects allows you to make changes to one part of your program without affecting other parts
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Benefits of OOP
Reusable
OOP introduces inheritance to allow you to extend existing classes and polymorphism allows you to write generic code
Maintainable
To fix a bug, you only need to correct it in one place
Extendable
OOP presents techniques to extend the code: inheritance, polymorphism and overriding
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