BBM 104 Programming Assignment 3 – Object-Oriented Programming with Java




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Developing Mini Facebook system – a prototype1
Object-oriented programming (OOP) is more than just adding a few new features to programming languages. It is rather
a new way of thinking about decomposing and modeling problems with less complexity and more code reuse when

1 Covered subjects: OOP Basics, Abstraction, Encapsulation, Inheritance, Polymorphism, Abstract Class, Interfaces,
Java Collections, UML class diagrams, Javadoc
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developing programming solutions. In OOP, a program is viewed as a collection of loosely connected objects, each of
which is responsible for specific tasks. It is through the interaction of these objects that computation proceeds and the
model works as a whole.
In this assignment, you will work with the concepts of OOP in order to practice them and observe their advantages. By
the end of this assignment, you will have learnt the concepts of relationships among classes, encapsulation, abstraction,
inheritance and polymorphism, as well as how to and why use Javadoc in your Java projects.
The learning objective of this programming assignment is to let students practice the following concepts of Java
programming language in particular (and OOP in general):
 Object-oriented programming
 Abstraction
 Encapsulation
 Inheritance
 Polymorphism
 Abstract Class
 Java Interface
 Java Collections
 UML class diagrams
 Javadoc
The marking scheme is shown below. Carefully review each mark component.
Component Mark %
Creation of Base Classes with the requested class structure (the main class of your
application must be named
Successful instantiation of all initial user objects from the input file 10%
Correct execution of scenarios as requested in the input file with commands 40%
Drawing of UML class diagram saved as uml.jpg 5%
Javadoc that gives a detailed description of the classes in javadoc folder 10%
Total 100%
Note: In your solutions, your algorithm and code should be as simple as possible.
In this section you can find some useful beginner’s level information that you will need for this project. For more
information on each subject you need to do additional research and consult other resources (e.g. lecture notes, textbook,
Internet resources, etc.).
Abstraction is a general concept that denotes the progress of modeling “real things” into programming language. For
example, when you write a class named Person, you abstract a real person into a type (class). In OOP, abstraction is a
process of hiding the implementation details from the user, and only providing the functionality. In other words, the
user will have the information on what the object does instead of how it does it.
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In Java, the process of abstraction is done using interfaces, classes, abstract classes, fields, methods and variables. It is
the fundamental concept on which other things rely on such as encapsulation, inheritance and polymorphism.
2.1.1 Abstract Class
In Java, a class which contains the abstract keyword in its declaration is known as abstract class. Abstract classes may
or may not contain abstract methods, i.e., methods without body (e.g. public void get();). If a class has at least
one abstract method, then the class must be declared abstract.
Abstract classes cannot be instantiated. To use an abstract class, you have to inherit it from another class, and provide
implementations to the abstract methods in it. If you inherit an abstract class, you have to provide implementations to
all the abstract methods in it. Properties of an abstract class are inherited just like the properties of any inherited concrete
2.1.2 Abstract Methods
If you want a class to contain a particular method but you want the actual implementation of that method to be
determined by child classes, you can declare the method in the parent class as an abstract method using the abstract
keyword (e.g. public abstract int compute();). An abstract method contains a method signature, but no
method body. Instead of curly braces, an abstract method will have a semicolon (;) at the end.
Declaring a method as abstract has two consequences:
 The class containing it must be declared as abstract.
 Any class inheriting the current class must either override the abstract method or declare itself as abstract.
Finally, a child class has to implement the abstract method; otherwise, you would have a hierarchy of abstract classes
that cannot be instantiated.
An interface is a reference type in Java. It is similar to a class. Think of it as a collection of abstract methods. A class
implements an interface, thereby inheriting the abstract methods of the interface. Along with abstract methods, an
interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for
default methods and static methods.
Writing an interface is similar to writing a class. A class describes the attributes and behaviors of an object, but an
interface contains behaviors that a class implements. Unless the class that implements the interface is abstract, all the
methods of the interface need to be defined in the class.
An interface is similar to a class in the following ways:
 An interface can contain any number of methods.
 An interface is written in a file with a .java extension, with the name of the interface matching the name of the
 The byte code of an interface appears in a .class file.
 Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that
matches the package name.
An interface is different from a class in several ways, including:
 You cannot instantiate an interface.
 An interface does not contain any constructors.
 All of the methods in an interface are abstract.
 An interface cannot contain instance fields. The only fields that can appear in an interface must be declared
both static and final.
 An interface is not extended by a class; it is implemented by a class.
 An interface can extend multiple interfaces.
To declare an interface, the keyword interface is used. A class uses the implements keyword to implement an
interface. When a class implements an interface, that class agrees to implement the specific behaviors of the interface. If
a class does not implement all the behaviors of the interface, the class must declare itself as abstract.
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An interface can extend another interface in the same way that a class can extend another class. The extends keyword
is used to extend an interface, and the child interface inherits the methods of the parent interface. Unlike classes, an
interface can extend more than one parent interface. The extends keyword is used once, and the parent interfaces are
declared in a comma-separated list.
Encapsulation is one of the fundamental OOP concepts. It is a technique of wrapping (packaging) the related data
(attributes) and behavior (code – methods) together into a single unit. It also provides a way to hide and control data by
protecting it from misuse by the outside world. This is achieved by making data members private, and using public
helper methods through which we can decide the level of access we want to provide (e.g. read-only, write-only). A fully
encapsulated Java class is a class whose all variables are private.
The advantages of encapsulation include: flexibility in modifying code in case requirements change, reusability of
encapsulated code throughout multiple applications if necessary, and reducing the time of maintenance (update)
2.3.1 Visibility Modifiers
In Java, there are four access modifiers which provide various access levels: private (visible inside of the class only),
default/package (visible inside of the package), protected (visible inside the package and to all subclasses) and public
(visible to everyone).
Inheritance can be defined as the process where one class acquires the properties (methods and fields) of another class.
It is achieved with the keyword extends. The class which inherits the properties of another class is known as subclass
(derived class, child class), and the class whose properties are inherited is known as superclass (base class, parent class).
Inheritance enables managing information in a hierarchical order, code reuse, extending a class by adding new features
instead of writing a new class from scratch, and maintainability of the code (e.g. updating the code in one place –
superclass, instead of updating every single class).
Polymorphism is the ability of an object to take on many forms. In OOP, polymorphism means a type can point to
different object at different time. In other words, the actual object to which a reference type refers, can be determined at
runtime. Any Java object that can pass more than one IS-A test is considered to be polymorphic. Thus, all Java objects
are polymorphic since any object will pass the IS-A test for their own type and for the class Object.
The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class
object. A reference variable can refer to any object of its declared type or any subtype of its declared type.
Polymorphism increases code reuse, allows for flexibility in running code and code extensibility.
In Java, polymorphism is based on inheritance and overriding.
2.5.1 Method Overriding
When a class extends another class, it can use its super class’s methods. However, sometimes the subclass may need a
different behavior of a method provided by its superclass. The method implementation in the subclass overrides
(replaces) the method implementation in the superclass. In this case, the subclass method and superclass method have
the same name, parameters and return type, but different implementation. This is called method overriding.
Method overloading should not be confused with method overriding. When we need to have more than one method
with the same functionality within the same class, we don’t have to declare new methods with different names for each
one. Method overloading feature allows us to declare multiple methods with the same name, but different signatures
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(e.g. different argument list, types or order). System.out.println() is an example of method overloading in Java.
It takes float, int, double or String types as arguments.
Collections in Java is a framework that provides an architecture to store and manipulate a group of objects. A Collection
represents a single unit of objects i.e. a group. All the operations that you perform on data such as searching, sorting,
insertion, manipulation, deletion etc. can be performed by Java Collections.
Java Collection simply means a single unit of objects. Java Collection framework provides many interfaces (Set, List,
Queue, Deque etc.) and a set of standard collection classes that implement Collection interfaces (ArrayList, Vector,
LinkedList, PriorityQueue, HashSet, LinkedHashSet, TreeSet etc).
Hierarchy of Java Collection Framework:
The Unified Modeling Language (UML) is a general-purpose, developmental, modeling language in the field of
software engineering, which is intended to provide a standard way to visualize the design of a system. A UML class
diagram is a type of static structure diagram that describes the structure of a system by showing the system’s classes,
their attributes, operations (or methods), and the relationships among objects. The class diagram is the main building
block of object-oriented modeling.
In a UML diagram, classes are represented with boxes that contain three compartments:
 The top compartment contains the name of the class in bold and centered, and the first letter is capitalized.
 The middle compartment contains the attributes of the class. They are left-aligned and the first letter is
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 The bottom compartment contains the operations the class can execute. They are also left-aligned and the first
letter is lowercase.
To specify the visibility of a class member (i.e. any attribute or method), these notations must be placed before the
member’s name:
+ Public
– Private
# Protected
/ Derived (can be combined with one of the others)
~ Package
2.8.1 UML Class Relations
A relationship is a general term covering the specific types of logical connections found on class and object diagrams.
UML defines several relationships, and two of them are Association and Generalization (Inheritance). In this
assignment, you are expected to show generalization, realization, and association relationships between your classes. Generalization & Realization
Generalization defines a taxonomic relationship between two classifiers: a more general one and a more specific one
(remember inheritance). For example, : when one class groups common data and behavior of other classes, this is called
generalization. It is shown as a line with a hollow triangle that connects two classifiers. In contrast to generalization,
specialization means creating new subclasses from an existing class. If it turns out that certain attributes, associations, or
methods only apply to some of the objects of the class, a subclass can be created. The most inclusive class in a
generalization/specialization is called the superclass and is generally located at the top of the diagram. The more
specific classes are called subclasses and are generally placed below the superclass. A UML representation of
generalization is given below:
public class A2 extends B2 – B2 is a generalization for A2

Realization, on the other hand, is a specialized abstraction relationship between two sets of model elements, one
representing a specification and the other representing an implementation. A Realization dependency is shown as a
dashed line (see UML diagram fragment below) with a triangular arrowhead at the end that corresponds to the realized
This diagram fragment given below states that class A3 implements or realizes the interface defined by B3.
public class A3 implements B3 – A3 realizes behavior specified by B3
generalization link
realization link
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Association is a relationship between classifiers which is used to show that instances of classifiers could be either linked
to each other or combined logically or physically into some aggregation. UML illustration indicating association
between two classes is given below.
The arrowhead means that there is a one-way relationship. In this example it means that class A1 is associated with
class B1. In other words, class A1 uses and contains one instance of class B1, but B1 does not know about or contain any
instances of class A1. This example manifests itself as the following Java code:
public class A1 {
private B1 b1;
public B1 getB1() {
return b1;
For this assignment, you are expected to draw a basic UML diagram for your classes, which will include class names,
all attributes and methods with the specified visibility, and show all relationships between the classes. You may use any
tool for this task. For example, you can look up available UML generator tools for eclipse at
Javadoc is a documentation generator for Java, which Generates HTML pages of API documentation from Java source
files. The basic structure of writing document comments is to embed them inside /** … */. You need to submit
Javadoc for your code as well. For details refer to:
In this programming assignment, you will develop a very simplified version of Facebook, well-known social media
platform. We will call it Mini-Facebook. Note that Programming Assignments 3 and 4 are consecutive and that this
assignment is the first part and basis for the Assignment 4.
First, we need to review the requirements (specifications) of the system we want to develop. The basic components of
Mini-Facebook application include:
 users (people with a Mini-Facebook account),
 posts (classified according to a post type),
 user collection (a class that adds or removes users, lists users’ posts, and controls user log-ins).
You will be given an input file that contains information necessary for instantiating the initial users of the system.
Furthermore, another input file containing commands to test various execution scenarios will be provided, which you
will use to test your application.
The necessary components of the system are explained in detail in the following sections.
A simplified UML class diagram of our Mini-Facebook application is shown in the figure below:
association link
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3.1.1 Users
Users are people who have an account on our Mini-Facebook system. Users will not have different roles (e.g. admin or
system manager vs. other users); that is, they will all be peers. User’s attributes and behaviors are explained below:
 Every User in the system has a unique userID, a name, a unique username, a password, a date of birth,
information about school from which the user graduated, a last log-in date, a collection of friends, a collection
of blocked users, and a collection of posts.
 userIDs should be assigned as integers in order, starting from 1 for the first user that gets added to the system.
These IDs cannot be changed later. A newly added user should never get an ID that has been assigned before
to some other user, even in cases when other users get removed from the system.
 Users’ dates of birth and last log-in dates should be stored as Date variables.
 All collections must be implemented as Java Collections.
 Users must be able to
o Sign in,
o Sign out,
o Update their user profile info (name, date of birth, and school graduated),
o Change their passwords,
o Add friends to their friend lists,
o Remove friends from their friend lists,
o Add posts,
o Remove posts,
o Block users,
o Unblock users,
o List their friends,
o List all users,
o List blocked friends,
o List blocked users.
 When a user is not logged-in, he/she should be allowed only to perform a sign-in. After user log-in, other
actions should be permitted.
 Note that you are required to hide sensitive personal user info and implement setter and getter methods as
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3.1.2 Posts
Posts represent textual or multimedia content shared by users. You are expected to design your post classes in a
hierarchical structure. The necessary information about posts is as follows:
 First of all, you need to implement an interface which will specify behavior (methods) that the Post class will
 Secondly, you need to implement an abstract Post class which will implement the post interface and be a
superclass to all other post classes.
 This abstract Post class should define post attributes, implement the methods defined in the interface, and
define two additional abstract methods that will show tagged users and show post location. These methods
should be implemented in the immediate subclasses of the Post class.
 Posts are categorized as either TextPosts, ImagePosts, or VideoPosts. But be careful: all posts have a textual
part (all posts are actually text posts), and can optionally contain image or video (but not both!).
 All posts have a unique postID which should be assigned as a random UUID (immutable universally unique
identifier), a text, a date when the post originated, a location, and a collection of tagged friends.
 Location should be implemented as a separate class with the following attributes: latitude and longitude
(stored as double values).
 TextPost class should implement abstract methods from the abstract Post class. To show the tagged friends, it
is enough to print their names (if any). To show the post location, both latitude and longitude should be printed.
 ImagePosts have the image filename, and the image resolution (width and height in pixels).
 VideoPosts have the video filename, video duration, and a constant attribute that specifies the maximum
video length in minutes (the maximum allowed video duration is 10 minutes).
 Note that you are required to implement setter and getter methods as necessary.
3.1.3 User Collection
The main class in your program should delegate managing to a class named UserCollection and call it as necessary. In
Assignment 4, you will add a Graphical User Interface to your Mini-Facebook application, and this class will play the
role of an intermediary between your backend and frontend. Thus, you need to design it accordingly. UserCollection
class needs to allow for the following actions:
 Keeping track of all users,
 Adding new users to the system (by providing new user information),
 Removing existing users from the system (by providing the userID),
 User sign-in (with username and password),
 Displaying users’ posts.
3.1.4 Scenarios of Execution
i. Add New User: At the initial stage, your program should retrieve the user information from the file users.txt
and add them to the system. Besides, the system should also add new users while running. The program should
add a new customer (a functionality of the UserCollection class) if the syntax of the command is as provided
Usage Illustration:
ADDUSERAdnanadnanadnan101/01/1991Selcuk Universitesi
Adnan has been successfully added.
ii. Remove Existing User: The system should also remove an existing user (a functionality of the UserCollection
class). It takes one parameter which indicates
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Usage Illustration:
User has been successfully removed.
No such user!
iii. Show Posts: The system should also be able to display users’ posts (a functionality of the UserCollection class).
The parameter for this action is a username:
Usage Illustration:
adnan does not have any posts yet.
No such user!
adnan’s Posts
This is my 1st text post
Date: 14.04.2017
Location: 39.8833431, 32.7381663
This is my 1st text post
Date: 14.04.2017
Location: 39.8833431, 32.7381663
Friends tagged in this post: ahmet, demet, adnan
This is my 1st image post
Date: 14.04.2017
Location: 42.8833431, 32.6381533
Image: image.png
Image resolution: 135×250
Friends tagged in this post: demet, gizem
This is my 1st video post
Date: 14.04.2017
Location: 39.8833431, 32.6381533
Video: myvideo.avi
Video duration: 8 minutes
Friends tagged in this post: utku
iv. Sign-In: This is the base action for users to perform before they are allowed to interact with the system and
execute other commands. As in a real application, this evaluator takes two parameters that are a username and
a password. The UserCollection class should also provide means for a user sign-in given the same parameters.
Usage Illustration:
You have successfully signed in.
Invalid username or password! Please try again.
No such user!
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v. Sign-Out: It prevents the user from executing any scenarios other than sign-in. No argument needed to perform
this command since it is assumed only one user can log in to the system at a particular time.
Usage Illustration:
You have successfully signed out.
vi. Update Profile: User who has already logged in may change his/her personal information (name, date of birth,
and school from which graduated). It takes three arguments and these are assumed to be the updated values.
Usage Illustration:
Your user profile has been successfully updated.
Error: Please sign in and try again.
vii. Change Password: A user may want to change his/her password. The system should first check if the provided
password matches the current password.
Password mismatch! Please, try again.
Error: Please sign in and try again.
viii. Add New Friend: Friendship is the most important point in any social media platform and without which
probably it would not gain much attention. The system you will develop should allow a user who has signed
in to add an existing user as a friend. It takes only one argument, which indicates user name of the user who
will be added.
ahmet has been successfully added to your friend list.
This user is already in your friend list!
No such user!
Error: Please sign in and try again.
ix. Remove Friend: A user can also remove a user from his/her friend list. This operator takes one argument that
is a username.
ahmet has been successfully removed from your friend list.
No such friend!
Error: Please sign in and try again.
x. Add Text Post: User may create a new text post by specifying the text content, location (longitude & latitude),
and username of the friends who will be tagged to this post, each of which is separated by TAB.
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ADDPOST-TEXTThis is my 1st text post39.883343132.7381663
The post has been successfully added.
ahmet is not your friend, and will not be tagged!
The post has been successfully added.
ahmet is not your friend, and will not be tagged!
demet is not your friend, and will not be tagged!
The post has been successfully added.
Error: Please sign in and try again.
xi. Add Image Post: Not only can a user create text post, but he/she can also create an image or a video post. In
this scenario, the image path and resolution information should be provided to the system as well.
ADDPOST-IMAGEThis is my 1st image post42.883343132.6381533
The post has been successfully added.
demet is not your friend, and will not be tagged!
The post has been successfully added.
demet is not your friend, and will not be tagged!
gizem is not your friend, and will not be tagged!
The post has been successfully added.
Error: Please sign in and try again.
xii. Add Video Post: User should only provide the file path of the video.
ADDPOST-VIDEOThis is my 1st video post39.883343132.6381533
The post has been successfully added.
gizem is not your friend, and will not be tagged!
The post has been successfully added.
utku is not your friend, and will not be tagged!
gizem is not your friend, and will not be tagged!
The post has been successfully added.
Error: Your video exceeds maximum allowed duration of 10 minutes.
Error: Please sign in and try again.
xiii. Remove Post: The system should allow user to remove a post that has been created the latest. Because of the
random unique identifier, you are expected to remove the last created post rather than a particular one.
Usage Illustration:
Error: You don’t have any posts.
Your last post has been successfully removed.
Error: Please sign in and try again.
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xiv. Block User: In most of the real world social platforms, a user might want to block another user due to several
reasons (disturbing posts, etc.). In this assignment, your implementation should also allow users to block other
users given a username as the argument.
ahmet has been successfully blocked.
No such user!
Error: Please sign in and try again.
xv. Unblock User: The system should also allow a user to unblock another user who has been blocked by him/her.
ahmet has been successfully unblocked.
No such user in your blocked users list!
Error: Please sign in and try again.
xvi. List Friends: Users should be able to lists their friends, blocked or not.
You haven’t added any friends yet!
Error: Please sign in and try again.
Name: Adnan
Username: adnan
Date of Birth: 01/01/1991
School: Selcuk Universitesi
Name: Ahmet
Username: ahmet
Date of Birth: 04/25/2001
School: Meram Anadolu Lisesi
xvii. List Users: Users should be able to view all users in the system.
Error: Please sign in and try again.
Name: Adnan
Username: adnan
Date of Birth: 01/01/1991
School: Selcuk Universitesi
Name: Ahmet
Username: ahmet
Date of Birth: 04/25/2001
School: Meram Anadolu Lisesi
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xviii. Show Blocked Friends: Users should be able to lists their friends whom they have blocked.
You haven’t blocked any users yet!
You haven’t blocked any friends yet!
Error: Please sign in and try again.
Name: Adnan
User Name: adnan
Date of Birth: 01/01/1991
School: Selcuk Universitesi
xix. Show Blocked Users: Similar to the previous execution scenario, users should be able to lists all users whom
they have blocked, both friends and non-friends.
You haven’t blocked any users yet!
Error: Please sign in and try again.
Name: Adnan
User Name: adnan
Date of Birth: 01/01/1991
School: Selcuk Universitesi
Name: Ahmet
Username: ahmet
Date of Birth: 04/25/2001
School: Meram Anadolu Lisesi
You will be provided with two (tab-separated) text input files:
 Users in the system,
 Commands that you need to execute in order to test your application.
List of all users in the system will be given in the (tab-separated) users.txt file:
The format of user’s data will be given as follows:
A sample users.txt input file is given below:
Ahmet ahmet ahmet123 04/25/2001 Meram Anadolu Lisesi
Demet demet demet00 01/16/1999 Ankara Fen Lisesi
Zeki zeki zeki01 08/16/1987 Kadıkoy Lisesi
Gizem gizem gizem1 12/09/1997 Hacettepe Universitesi
Utku utku utku99 10/06/1999 Bilkent Universitesi
Hakan hakan hakan81 03/01/1981 Orta Dogu Teknik Universitesi
You are expected to create an instance for each user in this file.
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List of all commands, which will be given to test your program and which you are expected to execute correctly and in
order, will be given in the (tab-separated) commands.txt file. A detailed explanation, format and expected output for
each command is given in the ‘Scenarios of Execution’ section (Section 3.1.4) of this document.
ADDUSER Adnan adnan adnan1 01/01/1991 Selcuk Universitesi
SIGNIN cemil cemil1
SIGNIN adnan adnan1
UPDATEPROFILE Adnan 07/01/1991 Gazi University
CHPASS adnan123 adnan1234
CHPASS adnan1 adnan123
ADDPOST-TEXT This is my 1st text post 39.2 32.81 ahmet
ADDPOST-IMAGE This is my 1st image post 37.87 32.46 demet:gizem image.png 135×250
ADDPOST-VIDEO This is my 1st video post 40.87 29.24 utku:gizem myvideo.avi 8
ADDPOST-TEXT This is my 2nd text post 38.35 33.1 demet:gizem:utku
BLOCK demet
BLOCK gizem
BLOCK ahmet
Javadoc is the JDK tool that generates API documentation from documentation comments. You are expected to
document every method and class using Javadoc. Documentation comments (doc comments) are special comments in
the Java source code that are delimited by the /** … */ delimiters. These comments are processed by the Javadoc tool to
generate the API docs.
The Javadoc tool can generate output originating from four different types of “source” files. In this assignment, the
source code files we are interested in are your Java classes (.java) – these contain class, interface, field, constructor and
method comments.
A format of a doc comment is as follows:
 A doc comment is written in HTML and must precede a class, field, constructor or method declaration. It is
made up of two parts — a description followed by block tags. In this example, the block tags are @param,
@return, and @see.
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 The first sentence of each doc comment should be a summary sentence, containing a concise but complete
description of the API item. This means the first sentence of each member, class, interface or package
description. The Javadoc tool copies this first sentence to the appropriate member, class/interface or package
summary. This makes it important to write informative initial sentences that can stand on their own. E.g.
Tag conventions:
 Order of tags should be:
o @author (classes and interfaces only, required)
o @version (classes and interfaces only, required)
o @param (methods and constructors only)
o @return (methods only)
o @exception (@throws is a synonym added in Javadoc 1.2)
o @see
 Required tags:
o An @param tag is “required” (by convention) for every parameter, even when the description is
o The @return tag is required for every method that returns something other than void, even if it is
redundant with the method description. (Whenever possible, find something non-redundant (ideally,
more specific) to use for the tag comment.)
For more information on Javadoc and examples of doc comments, visit:
Important note: DO NOT use Turkish characters in your code (anywhere, not even comments)!
* Returns an Image object that can then be painted on the screen.
* The url argument must specify an absolute {@link URL}. The name
* argument is a specifier that is relative to the url argument.

* This method always returns immediately, whether or not the
* image exists. When this applet attempts to draw the image on
* the screen, the data will be loaded. The graphics primitives
* that draw the image will incrementally paint on the screen.
* @param url an absolute URL giving the base location of the image
* @param name the location of the image, relative to the url argument
* @return the image at the specified URL
* @see Image
public Image getImage(URL url, String name) {
try {
return getImage(new URL(url, name));
} catch (MalformedURLException e) {
return null;
* Class constructor.
foo() {

* Class constructor specifying number of objects to create.
foo(int n) {

BBM 104 – PA3 Page 17 of 17
 Update Profile: updateProfile method should perform this operation by invoking relevant mutator (setter)
methods within the User class.
 Add New Friend: The system should not allow users to add a friend who is already in his/her friend or does
not exist in the system.
 Remove Friend: The system should not allow user to remove a friend who does not exist or who is not his/her
friend. Also, when a friend is removed from a user’s friend list, he/she should not be removed from the system!
 Add Video Post: the maximum allowed video length is 10 minutes. If a user tries to add a video that lasts longer
than that, post should not be added, and an error message should be displayed as illustrated in Section 3.
 General constraints:
o When a user is not logged-in, he/she should be allowed only to perform a sign-in. Only after the user
signs in, other actions should be permitted.
o Users should not be able to tag other users in their posts if they are not their friends.
o It is assumed that only one user can log in to the system at a particular time, so all actions related to
users should be performed by the user who is currently logged in.
 Submissions will be accepted only electronically via
 The deadline for submission is 28.04.2017 until 23:59.
 The submission format is:
 uml.jpg
 Your main class must be named! All of your classes should be in the src folder (they may, however,
be further classified into other subfolders in case you decide to make packages). Note that only zipped folders
can be submitted.
 All work must be individual! Plagiarism check will be performed! Duplicate submissions will be graded with
0 and disciplinary action will be taken.
java Main users.txt commands.txt