OOP Chapter 4: Class Collaboration Part 1
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Questions and Answers

How can objects communicate with each other in an object-oriented program?

  • By sending messages through methods defined in their classes. (correct)
  • By modifying each other's code directly.
  • By manipulating shared global variables.
  • By directly accessing each other's internal data.

    • What happens during message passing in object-oriented programming?

  • One object sends a message to another, which may trigger further messages to other objects. (correct)
  • The object automatically updates all other objects in the system.
  • Messages are sent to a database without involving other objects.
  • The object sends a direct message to the processor.

    • In a chain reaction of messages, what is the typical flow?

  • A single object handles all messages by itself.
  • Each object handles its messages independently without any communication with other objects.
  • Messages are passed from one object to another, creating a sequence of interactions. (correct)
  • The first object sends a message and waits for a global response.

    • When an object is created within a method and used for communication, what happens to it after the method completes?

    <p>It is removed from memory once the method execution ends.</p> Signup and view all the answers

    What is the primary purpose of using packages or namespaces in object-oriented programming?

    <p>To organize classes into hierarchical structures and avoid naming conflicts.</p> Signup and view all the answers

    In which scenario would you use a bidirectional association?

    <p>When both classes need to send messages to each other.</p> Signup and view all the answers

    In Java, objects are always passed by value when passed as arguments to methods.

    <p>False</p> Signup and view all the answers

    A message in object-oriented programming is the same as a method.

    <p>False</p> Signup and view all the answers

    Polymorphism refers to the ability for objects of different classes to respond to the same message in different ways.

    <p>True</p> Signup and view all the answers

    A class's interface exposes all of its private methods to other classes.

    <p>False</p> Signup and view all the answers

    The internet allows objects in different computers to communicate using messages.

    <p>True</p> Signup and view all the answers

    What does it mean for a class to be 'responsible for its own data'?

    <p>It means the class manages its own state (internal data) and doesn't allow other classes to change it directly. Instead, other classes need to &quot;ask&quot; the class for permission to modify its data.</p> Signup and view all the answers

    What is the difference between class association and class dependency?

    <p>Class association is a more permanent relationship where one class knows about another and can use its methods. Class dependency is a temporary relationship, where one class relies on another only during the execution of a specific method.</p> Signup and view all the answers

    What is the purpose of dynamic linking in Java?

    <p>Dynamic linking allows Java to automatically load and connect dependent classes at runtime, without requiring them to be explicitly linked during compilation. This makes Java more flexible and allows for different versions of classes to be used at runtime.</p> Signup and view all the answers

    Study Notes

    Chapter 4: Collaboration between Classes and Methods (Part 1)

    • Dr. Sarra Namane, Department of Computer Science, Badji Mokhtar University taught 2nd Year Engineering in 2024/2025.
    • Object-Oriented Programming (OOP) focuses on how objects interact to create efficient software systems.

    Introduction

    • Message passing: Objects communicate through sending and receiving data or invoking methods.
    • Class association: Explains relationships between classes.
    • Class dependency: Shows how one class relies on another for functionality.
    • Chain reaction messaging: A sequence of interactions where one message triggers another.

    Message Passing

    • In OOP, message passing occurs when one object requests another to perform an action by calling a method.
    • Example: A coffee machine (object A) can make coffee. A cup (object B) can hold coffee. To transfer the coffee a method call is made to the cup to add the coffee.

    Message Passing (2)

    • Example code showcasing message passing using Java's CoffeeMachine and Cup classes.
    • When brewCoffee(Cup cup) method call of the CoffeeMachine class is used, a message is sent to fillCup() method of the Cup class

    Class Association

    • Class association describes how objects from different classes interact and communicate.
    • Example: A teacher (class A) assigning homework to a student (class B).
    • Teacher needs a reference to a Student object to call doHomework().

    Class Association (2)

    • Example code demonstrating class association with Teacher and Student classes in Java.
    • This shows the relationship between two classes where one class has a reference to the other.

    Class Association (reference)

    • The best way to allow two classes to communicate through message passing is by adding a reference attribute to the first class that points to the second class.
    • Example: A library has multiple books. The library class has a reference to a book, and the library knows which book it holds. So it can call the book’s getTitle() method from the book’s content.

    Class Association (reference) (2)

    • Example illustrating the process of class association between the ‘Library’ and ‘Book’ classes using Java code.

    Class Association (reference) (3)

    • Library class is associated with the Book class.
    • Library can call the getTitle() method from the Book class.
    • Library prints the title of the book it has.
    • One class can interact with another class by holding a reference to another class

    Class Association (reference) (4)

    • Library is responsible for managing books in the system.
    • Book doesn't need to know which library it belongs to, only hold information like its title, author, etc.
    • A book can be found in multiple libraries

    Class Association (reference) (5)

    • Library is a collection of books.
    • The books are part of library collection and don’t manage the library.
    • Library methods can add/remove books; a book does not control which library it belongs to.

    Class Dependency

    • Class dependency occurs when one class relies on another class during method execution.
    • Example: A car class depends on an engine to start. This dependency is temporary.

    Class Dependency (2)

    • Code example demonstrating class dependency using Car and Engine classes in Java.
    • Car temporarily depends on the Engine to start. Once the startCar method finishes, dependency ends.

    Class Dependency (3)

    • Car depends on Engine to start.
    • No long-term relationship between classes, a dependency lasts only for as long as the method is being executed.
    • The interaction between classes is short-lived.

    Class Dependency (4)

    • Temporary interaction between Printer and Document when printing a document.
    • Printer depends on the Document object during the print process; it is no longer needed once printing is finished.

    Class Dependency (5)

    • Class dependency is a temporary relationship; a short-lived relationship where one class uses another class to perform a specific task.
    • The relationship ends once execution is finished.
    • Useful when one class doesn’t need the other permanently.

    Chain Reaction of Messages

    • A chain reaction occurs when one object sends a message to another object. That object then sends a message to another object, and so on, creating a sequence of method calls.
    • Example: A customer requests support that consists of multiple steps SupportAgent receives the request. Checks IssueTracker If required, IssueTracker logs the issue.

    Chain Reaction of Messages (2)

    • Code examples illustrating a chain reaction of messages between customer service and support objects in Java.

    Chain Reaction of Messages (3)

    • Customer calls method of object to start process.
    • Support object processes the request.
    • Message passes to Issue tracker.
    • IssueTracker logs the issue.

    QCM (Question and Multiple Choice) 1

    • What is the primary purpose of using packages namespaces for object-oriented programming?
      • To organize, group classes into hierarchical structures to avoid naming conflicts.

    QCM (2)

    • In which scenario would you use a bidirectional association?
      • When both classes need to send messages to each other

    Passing Arguments by Value in Java

    • In Java, objects communicate through method calls, and when a method is called, arguments are passed by value.
    • Example code showing the pass-by-value method of object arguments.

    Passing Arguments by Value in Java (2)

    • Example code to illustrate how objects are passed as arguments. Method call to increase the pages argument does not affect the original object. This because a temporary copy was created.

    Passing Arguments by Value in Java (3)

    • Example Java code illustrating how objects are passed as arguments in printPages method.

    Passing Arguments by Value in Java (4)

    • Java creates a temporary copy of arguments' values when calling a method. Modifications inside the method do not affect the original parameter.

    Passing Arguments by Value in Java (5)

    • Java creates a temporary copy of the arguments' value; changes inside the method will not affect the original object.

    Passing an Object as an Argument in Java

    • Objects are passed by reference in Java, meaning modifications to the object within the method affect the original object.

    Passing an Object as an Argument in Java (2)

    • Example of a car class with fuel attribute and methods for adding/displaying fuel level.
    • FuelStation class with a 'refuelCar' method that accepts a Car object and an integer.

    Passing an Object as an Argument in Java (3)

    • Code examples explaining passing a car object as an argument in the refuel car method.

    Passing an Object as an Argument in Java (4)

    • Object is passed by reference. RefuelCar() method modifies the original object.

    Passing an Object as an Argument in Java (5)

    • Method changes original object, not a copy.

    Passing arguments by reference in Java

    • When objects are passed as arguments, they are passed by reference, meaning that methods that work on those objects directly modify the original object.
    • Example code showing the Book class has the changeTitle() method modify the original object.

    Passing arguments by reference in Java (3)

    • Example demonstrating passing an object by reference in Java using the Book and Library classes.

    Is Every Method a Message?

    • A message is just defining the method's name, arguments, and return type. A method is the implementation.
    • Example: Person calling car.start() only needs to know what start() does, not the implementation.

    Is Every Method a Message? (2)

    • The Person object calls a method on the Car object. The method call can be considered a message to a method.

    Same Message, Multiple Methods (Polymorphism)

    • Polymorphism: The same message can be handled by different methods in different ways.
    • Example: A Dog and Cat class, each make a sound, but they both do it differently.

    Same Message, Multiple Methods (Polymorphism) (2)

    • Code example demonstrating polymorphism where the makeSound method is handled differently in the Dog and Cat classes.

    Same Message, Multiple Methods (Polymorphism) (3)

    • The same message, makeSound(), has two different implementations, depending on whether Dog or Cat calls it.

    Interface and Encapsulation

    • Class interface is a list of method signatures.
    • Encapsulation hides the details of class implementation and only shows interface.
    • Interface increases security and modularity.

    Interface: List of Available Method Signatures

    • The objects use interface, not class. Interface can be used to show what a class can do.

    Strictly Private Methods

    • Private methods are not accessible from outside the class.

    Globalization of Messages

    • Objects can communicate across the internet to perform tasks

    Globalization of Messages (2)

    • Object communication across a network requires identification of the target object and its functionality.

    Globalization of Messages (3)

    • Distributed object mechanisms aim to simplify communication across the internet.

    Distributed Computing

    • The entire communication mechanism between objects through message sending enables distributed computer applications.

    QCM 1

    • What is a message in object-oriented programming?
      • A call to an object's method containing its name, arguments, and return type.

    QCM 2

    • In object-oriented programming, what is an interface?
      • The visible part of an object; its methods accessible to other objects .

    QCM 3

    • What is the difference between a method and a message?
      • A method is the actual implementation; a message specifies the method call.

    References

    • Lists of various text book and web resources for OOP related subjects.

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    Chapter 4: Collaboration between classes and methods (Part 1) PDF

    Description

    This quiz explores the concepts of class collaboration and message passing in Object-Oriented Programming (OOP). You'll learn about class associations, dependencies, and how objects communicate through method calls. Test your understanding of these crucial OOP principles as they apply to real-world scenarios.

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