OOP - Concepts

Introduction

Introduction

🏆 Can describe OOP at a higher level

Object-Oriented Programming (OOP) is a programming paradigm. A programming paradigm guides programmers to analyze programming problems, and structure programming solutions, in a specific way.

Programming languages have traditionally divided the world into two parts—data and operations on data. Data is static and immutable, except as the operations may change it. The procedures and functions that operate on data have no lasting state of their own; they’re useful only in their ability to affect data.

This division is, of course, grounded in the way computers work, so it’s not one that you can easily ignore or push aside. Like the equally pervasive distinctions between matter and energy and between nouns and verbs, it forms the background against which we work. At some point, all programmers—even object-oriented programmers—must lay out the data structures that their programs will use and define the functions that will act on the data.

With a procedural programming language like C, that’s about all there is to it. The language may offer various kinds of support for organizing data and functions, but it won’t divide the world any differently. Functions and data structures are the basic elements of design.

Object-oriented programming doesn’t so much dispute this view of the world as restructure it at a higher level. It groups operations and data into modular units called objects and lets you combine objects into structured networks to form a complete program. In an object-oriented programming language, objects and object interactions are the basic elements of design.

-- Object-Oriented Programming with Objective-C, Apple

Some other examples of programming paradigms are:

Paradigm Programming Languages
Procedural Programming paradigm C
Functional Programming paradigm F#, Haskel, Scala
Logic Programming paradigm Prolog

Some programming languages support multiple paradigms.

📦 Java is primarily an OOP language but it supports limited forms of functional programming and it can be used to (although not recommended) write procedural code.  e.g. se-edu/addressbook-level1

📦 JavaScript and Python support functional, procedural, and OOP programming.

A) Choose the correct statements

  • a. OO is a programming paradigm
  • b. OO guides us in how to structure the solution
  • c. OO is mainly an abstraction mechanism
  • d. OO is a programming language
  • e. OO is modeled after how the objects in real world work

B) Choose the correct statements

  • a. Java and C++ are OO languages
  • b. C language follows the Functional Programming paradigm
  • c. Java can be used to write procedural code
  • d. Prolog follows the Logic Programming paradigm

A) (a)(b)(c)(e)

Explanation: While many languages support the OO paradigm, OO is not a language itself.

B) Choose the correct statement

(a)(b)(c)(d)

Explanation: C follows the procedural paradigm. Yes, we can write procedural code using OO languages e.g., AddressBook-level1.

OO is a higher level mechanism than the procedural paradigm.

True.

Explanation: Procedural languages work at simple data structures (e.g., integers, arrays) and functions level. Because an object is an abstraction over data+related functions, OO works at a higher level.

Objects

Basic

🏆 Can describe how OOP relates to the real world

Every object has both state (data) and behavior (operations on data). In that, they’re not much different from ordinary physical objects. It’s easy to see how a mechanical device, such as a pocket watch or a piano, embodies both state and behavior. But almost anything that’s designed to do a job does, too. Even simple things with no moving parts such as an ordinary bottle combine state (how full the bottle is, whether or not it’s open, how warm its contents are) with behavior (the ability to dispense its contents at various flow rates, to be opened or closed, to withstand high or low temperatures).

It’s this resemblance to real things that gives objects much of their power and appeal. They can not only model components of real systems, but equally as well fulfill assigned roles as components in software systems.

-- Object-Oriented Programming with Objective-C, Apple

Object Oriented Programming (OOP) views the world as a network of interacting objects.

📦 A real world scenario viewed as a network of interacting objects:

You are asked to find out the average age of a group of people Adam, Beth, Charlie, and Daisy. You take a piece of paper and pen, go to each person, ask for their age, and note it down. After collecting the age of all four, you enter it into a calculator to find the total. And then, use the same calculator to divide the total by four, to get the average age. This can be viewed as the objects You, Pen, Paper, Calculator, Adam, Beth, Charlie, and Daisy interacting to accomplish to achieve the end result of calculating the average age of the four persons. These objects can be considered as connected in a certain network of certain structure.

OOP solutions try to create a similar object network inside the computer’s memory – a sort of a virtual simulation of the corresponding real world scenario – so that a similar result can be achieved programmatically.

OOP does not demand that the virtual world object network follow the real world exactly.

📦 Our previous example can be tweaked a bit as follows:

  • Use an object called Main to represent your role in the scenario.
  • As there is no physical writing involved, we can replace the Pen and Paper with an object called AgeList that is able to keep a list of ages.

Every object has both state (data) and behavior (operations on data).

Object Real World? Virtual World? Example of State (i.e. Data) Examples of Behavior (i.e. Operations)
Adam 🗸 🗸 Name, Date of Birth Calculate age based on birthday
Pen 🗸 - Ink color, Amount of ink remaining Write
AgeList - 🗸 Recorded ages Give the number of entries, Accept an entry to record
Calculator 🗸 🗸 Numbers already entered Calculate the sum, divide
You/Main 🗸 🗸 Average age, Sum of ages Use other objects to calculate

Every object has an interface and an implementation.

Every real world object has an interface that other objects can interact with and an implementation that supports the interface but may not be accessible to the other object.

📦 The interface and implementation of some real-world objects in our example:

  • Calculator: the buttons and the display are part of the interface; circuits are part of the implementation.
  • Adam: In the context of our 'calculate average age' example, the interface of Adam consists of requests that adam will respond to, e.g. "Give age to the nearest year, as at Jan 1st of this year" "State your name"; the implementation includes the mental calculation Adam uses to calculate the age which is not visible to other objects.

Similarly, every object in the virtual world has an interface and an implementation.

📦 The interface and implementation of some virtual-world objects in our example:

  • Adam: the interface might have a method getAge(Date asAt); the implementation of that method is not visible to other objects.

Objects interact by sending messages.

Both real world and virtual world object interactions can be viewed as objects sending message to each other. The message can result in the sender object receiving a response and/or the receiving object’s state being changed. Furthermore, the result can vary based on which object received the message, even if the message is identical (see rows 1 and 2 in the example below).

Examples:

World Sender Receiver Message Response State Change
Real You Adam "What is your name?" "Adam" -
Real as above Beth as above "Beth" -
Real You Pen Put nib on paper and apply pressure Makes a mark on your paper Ink level goes down
Virtual Main Calculator (current total is 50) add(int i): int i = 23 73 total = total + 23

Consider the following real-world scenario.

Tom read a Software Engineering textbook (he has been assigned to read the book) and highlighted some of the text in it.

Explain the following statements about OOP using the above scenario as an example.

  1. Object Oriented Programming (OOP) views the world as a network of interacting objects.
  2. Every object has both state (data) and behavior (operations on data).
  3. Every object has an interface and an implementation.
  4. Objects interact by sending messages.
  5. OOP does not demand that the virtual world object network follow the real world exactly.

[1] Object Oriented Programming (OOP) views the world as a network of interacting objects.

Interacting objects in the scenario: Tom, SE Text Book (Book for short), Text, (possibly) Highlighter

💡 objects usually match nouns in the description

[2]Every object has both state (data) and behavior (operations on data).

Object Examples of state Examples of behavior
Tom memory of the text read read
Book title show text
Text font size get highlighted

[3] Every object has an interface and an implementation.

  • Interface of an object consists of how other objects interact with it i.e., what other objects can do to that object
  • Implementation consist of internals of the object that facilitate the interactions but not visible to other objects.
Object Examples of interface Examples of implementation
Tom receive reading assignment understand/memorize the text read, remember the reading assignment
Book show text, turn page how pages are bound to the spine
Text read how characters/words are connected together or fixed to the book

[4] Objects interact by sending messages.

Examples:

  • Tom sends message turn page to the Book
  • Tom sends message show text to the Book. When the Book shows the Text, Tom sends the message read to the Text which returns the text content to Tom.
  • Tom sends message highlight to the Highlighter while specifying which Text to highlight. Then the Highlighter sends the message highlight to the specified Text.

[5] OOP does not demand that the virtual world object network follow the real world exactly.

Examples:

  • A virtual world simulation of the above scenario can omit the Highlighter object. Instead, we can teach Text to highlight themselves when requested.

Objects as Abstractions

🏆 Can explain the abstraction aspect of OOP

The concept of Objects in OOP is an abstraction mechanism because it allows us to abstract away the lower level details and work with bigger granularity entities i.e. ignore details data formats and the method implementation details and work at the level of objects.

📦 We can deal with a Person object that represents the person Adam and query the object for Adam's age instead of dealing with details such as Adam’s date of birth (DoB), in what format the DoB is stored, the algorithm used to calculate the age from the DoB, etc.

Encapsulation Of Objects

🏆 Can explain the encapsulation aspect of OOP

Encapsulation protects an implementation from unintended actions and from inadvertent access.
-- Object-Oriented Programming with Objective-C, Apple

An object is an encapsulation of some data and related behavior in two aspects:

1. The packaging aspect: An object packages data and related behavior together into one self-contained unit.

2. The information hiding aspect: The data in an object is hidden from the outside world and are only accessible using the object's interface.

Choose the correct statement

  • a. An object is an encapsulation because it packages data and behavior into one bundle.
  • b. An object is an encapsulation because it lets us think in terms of higher level concepts such as Students rather than student-related functions and data separately.

Don't confuse encapsulation with abstraction.

Choose the correct statement

(a)

Explanation: The second statement should be: An object is an abstraction encapsulation because it lets ...

Classes

Basic

🏆 Can explain the relationship between classes and objects

Writing an OOP program is essentially writing instructions that the computer uses to,

  1. create the virtual world of object network, and
  2. provide it the inputs to produce the outcome we want.

A class contains instructions for creating a specific kind of objects. It turns out sometimes multiple objects have the same behavior because they are of the same kind. Instructions for creating a one kind (or ‘class’) of objects can be done in one go and use that same instructions to instantiate (i.e. create) objects of that kind. We call such instructions a Class.

📦 Classes and objects in an example scenario

When writing an OOP program to calculate the average age of Adam, Beth, Charlie, and Daisy, instructions for creating objects Adam, Beth, Charlie, and Daisy will be very similar because they are all of the same kind : they all represent ‘persons’ with the same interface, the same kind of data (i.e. name, DoB, etc.), and the same kind of behavior (i.e. getAge(Date), getName(), etc.). Therefore, we can have a class called Person containing instructions on how to create Person objects and use that class to instantiate objects Adam, Beth, Charlie, and Daisy. Similarly, we need classes AgeList, Calculator, and Main classes to instantiate one each of AgeList, Calculator, and Main objects.

Let us use the UML notation to illustrate classes and objects under discussion.

Person class and some example instances of the Person class.

Consider the following scenario. If you were to simulate this in an OOP program, what are the classes and the objects you would use? Use UML notation to give your answer.

John gave a checque to the Cashier pay for the LoTR and GoT books he bought.
Class Objects
Customer John
Book LoTR GoT
Cheque checqueJohnGave
Cashier bookshopCashier

Class Level Members

🏆 Can explain class-level members

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

Example:

All Person objects have the Name attribute but the value of that attribute varies between Person objects.

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.

Example:

The attribute totalPersons should be maintained centrally and shared by all Person objects rather than copied at each Person object.

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.

Example:

Sending the getName() message to Adam object results in the response "Adam" while sending the same message to the Beth object gets the response "Beth".

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.

Example:

The method getTotalPersons() is not suitable to send to a specific Person object because a specific object of the Person class should not know about the total number of Person objects.

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.

📦 A Student class with a class-level attribute and a method:

Which of these are suitable as class-level variables?

  • a. system: multi-player Pac Man game, Class: Player, variable: totalScore
  • b. system: eLearning system, class: Course, variable: totalStudents
  • c. system: ToDo manager, class: Task, variable: totalPendingTasks
  • d. system: any, class: ArrayList, variable: total (i.e., total items in a given ArrayList object)

(c)

Explanation: totalPendingTasks should not be managed by individual Task objects and therefore suitable to be maintained as a class-level variable. The other variables should be managed at instance level as their value varies from instance to instance. e.g., totalStudents for one Course object will differ from totalStudents of another.

Enumerations

🏆 Can explain the meaning of enumerations

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. e.g. if a variable can only take values 0 and 1, declaring it as an int would allow invalid values such as 2 to be assigned to it. But if you define an enumeration called Binary that has values 0 and 1 only, a variable of type Binary will never be assigned an invalid value such as 2 because the compiler is able to catch the error.

📦 Priority can be considered an enumeration because it has only three values.

Priority: HIGH, MEDIUM, LOW

Show (in UML notation) an enumeration called WeekDay to use when the value can only be Monday ... Friday.

Associations

Basic

🏆 Can explain associations

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.

📦 An object diagram example showing associations among objects. For example, there is an association between the AgeList object and the Main object.

Associations in an object structure can change over time.

📦 In this example, the associations in the object structure have changed (from left to right) due to a new Person object being connected to the Main object.

  →

Associations among objects are reflected correspondingly in the class diagram too, as it is the class diagram that dictates the nature of the associations allowed in the object structures.

📦 An example class diagram showing associations between classes.

🏆 Can explain the meaning of navigability

When two classes are linked by an association, it does not necessarily mean both classes know about each other. The concept of which class in the association knows about the other class is called navigability.

(a)

Explanation: The diagram indicates that Unit object should know about the Item object. In the implementation, the Unit object will hold a Unit object in one of its variables.

Multiplicity

🏆 Can explain the meaning of multiplicity

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

📦 This class diagram does not tell us multiplicities. For exaple, how many Calculator objects can be associated with one Main object? How many Main objects can be associated with one Calculator object? and so on.

  • a. A Unit object can be linked to any number of Item objects
  • b. An Item object must be linked 1,2,3 or 4 Unit objects.
  • c. A Unit object must be linked 1,2,3, or 4 Item objects
  • d. A Item object can be 0 or more Unit objects.

(c)(d)

Dependencies

🏆 Can explain dependencies among classes

Dependencies are objects that are not directly linked in the object network can still interact with each other. These are a weaker form of associations we call dependencies.

Composition

🏆 Can explain the meaning of composition

A composition is an association that represents a strong whole-part relationship. When the whole is destroyed, parts are destroyed too.

📦 A Board (used for playing board games) consists of Square objects.

Composition also implies that there cannot be cyclical links.

📦 In this ‘sub-folder’ association, a Folder cannot be a sub-folder of itself. If the diamond is removed, it is no longer a composition relationship and technically, allows a folder to be sub-folder of itself.

Aggregation

🏆 Can explain the meaning of aggregations

Aggregation represents a container-contained relationship. It is a weaker relationship than composition.

📦 Club acts as a container for Person objects. Person objects can survive without a Club object.

Aggregation vs Composition

The distinction between composition (◆) and aggregation (◇) is rather blurred. Martin Fowler’s famous book UML Distilled advocates omitting the aggregation symbol altogether because using it adds more confusion than clarity.

Which one of these is recommended not to use in UML diagrams because it adds more confusion than clarity?

(b)

Association Classes

🏆 Can explain the meaning of association classes

An association class represents additional information about an association. It is a normal class but plays a special role from a design point of view.

📦 A Man class and a Woman class is linked with a ‘married to’ association and there is a need to store the date of marriage. However, that data is related to the association rather than specifically owned by either the Man object or the Woman object. In such situations, an additional association class can be introduced, e.g. a Marriage class, to store such information.

Inheritance

What

🏆 Can explain the meaning of inheritance

The OOP concept Inheritance allows you to define a new class based on an existing class. For example, you can use inheritance to define an EvaluationReport class based on an existing Report class so that the EvaluationReport class does not have to duplicate code that is already implemented in the Report class.

📦 Example: The EvaluationReport inherits the wordCount attribute and the print() method from the base class Report.

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

A super class is said to be more general than the sub class. Conversely, a sub class is said to be more specialized than the super class.

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

📦 Man and Woman behaves the same way for the 'owes money' association. However, the two classes cannot be simply replaced with a more general class Person because of the need to distinguish between Man and Woman for the ‘marriage’ association. A solution is to add the Person class as a super class and let Man and Woman inherit from Person.

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.

📦 In this class diagrams of a Snakes and Ladders board game,

  • SnakeHeadSquare is a Square  (a SnakeHeadSquare is a square in which the head of a snake appears)
  • SnakeTailSquare is a Square

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

📦 Two inheritance hierarchies/trees are given below. Note that Parrot is a Bird as well as it is an Animal.

Multiple Inheritance is when a class inherits directly from multiple classes. Multiple inheritance is allowed among C++ classes but not among Java classes.

📦 The TA class inherits from the Staff class and the Student.

Overriding

🏆 Can explain method overriding

Method overriding is when a sub-class changes the behavior inherited from the parent class by re-implementing the method. Overridden methods have the same name, same type signature, and same return type.

📦 In the diagram below,

  • Report#print() method is overridden by EvaluationReport#print() method.
  • Report#write(String) method is overridden by EvaluationReport#write(String) method.
  • Report#read():String method is NOT overridden by EvaluationReport#read(int):String method.  Reason: the two methods have different signatures; EvaluationReport#read(int):String overloads (rather than overrides) the Report#read():String method.

Design → Object Oriented Programming →

Overloading

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.

Type Signature: The type signature of an operation is the type sequence of the parameters. The return type and parameter names are not part of the type signature. However, the parameter order is significant.

Method Type Signature
int add(int X, int Y) (int, int)
void add(int A, int B) (int, int)
void m(int X, double Y) (int, double)
void m(double X, int Y) (double, int)

📦 In the class below, the calculate method is overloaded because the two methods have the same name but different type signatures (String) and (int[])

class RatingCalculator{
    void calculate(String matric) { ... }
    void calculate(int[] averages) { ... }
}

Overloading

🏆 Can explain method overloading

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.

Type Signature: The type signature of an operation is the type sequence of the parameters. The return type and parameter names are not part of the type signature. However, the parameter order is significant.

Method Type Signature
int add(int X, int Y) (int, int)
void add(int A, int B) (int, int)
void m(int X, double Y) (int, double)
void m(double X, int Y) (double, int)

📦 In the class below, the calculate method is overloaded because the two methods have the same name but different type signatures (String) and (int[])

class RatingCalculator{
    void calculate(String matric) { ... }
    void calculate(int[] averages) { ... }
}

Interfaces

🏆 Can explain interfaces

An interface is a behavior specification i.e. a collection of method specifications. If a class implements the interface, it means the class is able to support the behaviors specified by the said interface.

There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a "contract" that spells out how their software interacts. Each group should be able to write their code without any knowledge of how the other group's code is written. Generally speaking, interfaces are such contracts. --Oracle Docs on Java

📦 SalariedStaff is an interface that contains two methods setSalary(int) and getSalary(). AcademicStaff implements the SalariedStaff interface.  That means an AcademicStaff object is able to support the behaviors setSalary(int) and getSalary(). That's why the same two methods also appear under the methods implemented by the AcademicStaff class (in blue)

A class implementing an interface results in an is-a relationship, just like in class inheritance.

📦 In the example above, AcademicStaff is a SalariedStaff. An AcademicStaff object can be used anywhere a SalariedStaff object is expected e.g. SalariedStaff ss = new AcademicStaff().

 Similarly, AdminStaff is a SalariedStaff too.

Abstract Classes

🏆 Can implement abstract classes

Abstract Class:

An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed. -- Oracle's Java Documentation

An abstract method is simply the method interface without the implementation.

📦 The Account class has an abstract method (addInterest()).

Dynamic and Static Binding

🏆 Can explain dynamic and static binding

Dynamic Binding ( aka late binding) : a mechanism where method calls in code are resolved at runtime, rather than at compile time.

Overridden methods are resolved using dynamic binding, and therefore resolves to the implementation in the actual type of the object.

Implementation → Object Oriented Programming →

Implementing Overriding

To override a method inherited from an ancestor class, simply re-implement the method in the target class.

📦 A simple example where the Report#print() method is overridden by EvaluationReport#print() method:


class Report{

    void print(){
        System.out.println("Printing report");
    }

}

class EvaluationReport extends Report{

    @Override  // this annotation is optional
    void print(){
        System.out.println("Printing evaluation report");
    }

}

class ReportMain{

    public static void main(String[] args){
        Report report = new Report();
        report.print(); // prints "Printing report"

        EvaluationReport evaluationReport = new EvaluationReport();
        evaluationReport.print(); // prints "Printing evaluation report"
    }
}

Which of these methods override another method?

  • a
  • b
  • c
  • d
  • e

d

Explanation: Method overriding requires a method in a child class to use the same method name and same parameter sequence used by one of its ancestors

📦 Consider the code below. The declared type of s is Staff and it appears as if the adjustSalary(int) operation of the Staff class is invoked.

void adjustSalary(int byPercent) {
    for (Staff s: staff) {
        s.adjustSalary(byPercent);
    }
}

However, at runtime the adjustSalary(int) operation of the actual object will be called (i.e. adjustSalary(int) operation of Admin or Academic). If the actual object does not override that operation, the operation defined in the immediate superclass (in this case, Staff class) will be called.

Static binding (aka early binding): When a method call is resolved at compile time.

In contrast, overloaded methods are resolved using static binding.

Implementation → Object Oriented Programming →

Implementing Overloading

An operation can be overloaded inside the same class or in sub/super classes.

📦 The constructor of the Account class below is overloaded because there are two constructors with different signatures: () and (String, String, double). Furthermore, the save method in the Account class is overloaded in the child class SavingAccount.

class Account {
    Account () {
        ...
    }
    
    Account (String name, String number, double balance) {
        ...
    }
    
    void save(int amount){
        ...
    }
}

class SavingAccount extends Account{
    
    void save(Double amount){
        ...
    }
}

📦 Note how the constructor is overloaded in the class below. The method call new Account() is bound to the first constructor at compile time.

class Account {
    Account () {
        // Signature: ()
        ...
    }
    Account (String name, String number, double balance) {
        // Signature: (String, String, double)
        ...
    }
}

📦 Similarly, the calcuateGrade method is overloaded in the code below and a method call calculateGrade("A1213232") is bound to the second implementation, at compile time.

void calculateGrade (int[] averages) { ... }
void calculateGrade (String matric) { ... }

Substitutability

🏆 Can explain substitutability

Every instance of a subclass is an instance of the superclass, but not vice-versa. As a result, inheritance allows substitutability : the ability to substitute a child class object where a parent class object is expected.

📦 an Academic is an instance of a Staff, but a Staff is not necessarily an instance of an Academic. i.e. wherever an object of the superclass is expected, it can be substituted by an object of any of its subclasses.

The following code is valid because an AcademicStaff object is substitutable as a Staff object.

Staff staff = new AcademicStaff (); // OK

But the following code is not valid  because staff is declared as a Staff type and therefore its value may or may not be of type AcademicStaff, which is the type expected by variable academicStaff.

Staff staff;
...
AcademicStaff academicStaff = staff; // Not OK

Polymorphism

Introduction

🏆 Can explain OOP polymorphism

Polymorphism:

The ability of different objects to respond, each in its own way, to identical messages is called polymorphism. -- Object-Oriented Programming with Objective-C, Apple

Take the example of writing a payroll application for a university to facilitate payroll processing of university staff. Suppose an adjustSalary(int) operation adjusts the salaries of all staff members. This operation will be executed whenever the university initiates a salary adjustment for its staff. However, the adjustment formula is different for different staff categories, say admin and academic. Here is one possible way of designing the classes in the Payroll system.

📦 Calling adjustSalary() method for each Staff type:

Here is the implementation of the adjustSalary(int) operation from the above design.

class Payroll1 {
    ArrayList< Admin > admins;
    ArrayList< Academic > academics;
    // ...

    void adjustSalary(int byPercent) {
        for (Admin ad: admins) {
            ad.adjustSalary(byPercent);
        }
        for (Academic ac: academics) {
            ac.adjustSalary(byPercent);
        }
    }
}

Note how processing is similar for the two staff types. It is as if the type of staff members is irrelevant to how they are processed inside this operation! If that is the case, can the staff type be "abstracted away" from this method? Here is such an implementation of adjustSalary(int):

class Payroll2 {
    ArrayList< Staff > staff;
    // ...

    void adjustSalary(int byPercent) {
        for (Staff s: staff) {
            s.adjustSalary(byPercent);
        }
    }
}

Notice the following:

  • Only one data structure ArrayList< Staff >. It contains both Admin and Academic objects but treats them as Staff objects
  • Only one loop
  • Outcome of the s.adjustSalary(byPercent) method call depends on whether s is an Academic or Admin object

The above code is better in several ways:

  • It is shorter.
  • It is simpler.
  • It is more flexible (this code will remain the same even if more staff types are added).

This does not mean we are getting rid of the Academic and Admin classes completely and replacing them with a more general class called Staff. Rather, this part of the code “treats” both Admin and Academic objects as one type called Staff.

For example, ArrayList staff contains both Admin and Academic objects although it treats all of them as Staff objects. However, when the adjustSalary(int) operation of these objects is called, the resulting salary adjustment will be different for Admin objects and Academic objects. Therefore, different types of objects are treated as a single general type, but yet each type of object exhibits a different kind of behavior. This is called polymorphism (literally, it means “ability to take many forms”). In this example, an object that is perceived as type Staff can be an Admin object or an Academic object.

Mechanism

🏆 Can explain how substitutability operation overriding, and dynamic binding relates to polymorphism

Three concepts combine to achieve polymorphism: substitutability, operation overriding, and dynamic binding.

  • Substitutability: Because of substitutability, you can write code that expects object of a parent class and yet use that code with objects of child classes. That is how polymorphism is able to treat objects of different types as one type.
  • Overriding: To get polymorphic behavior from an operation, the operation in the superclass needs to be overridden in each of the subclasses. That is how overriding allows objects of different sub classes to display different behaviors in response to the same method call.
  • Dynamic binding: Calls to overridden methods are bound to the implementation of the actual object's class dynamically during the runtime. That is how the polymorphic code can call the method of the parent class and yet execute the implementation of the child class.

Which one of these is least related to how OO programs achieve polymorphism?

(c)

Explanation: Operation overriding is the one that is related, not operation overloading. Interfaces and abstract classes, although not required, can be used in achieving polymorphism.

Miscellaneous

Miscellaneous

🏆 Can answer frequently asked OOP questions

What’s the difference between a Class, an Abstract Class, and an Interface?

  • An interface is a behavior specification with no implementation.
  • A class is a behavior specification + implementation.
  • An abstract class is a behavior specification + a possibly incomplete implementation.

How does overriding differ from overloading?

Overloading is used to indicate that multiple operations do similar things but take different parameters. Overloaded methods have the same method name but different method signatures and possibly different return types.

Overriding is when a sub-class redefines an operation using the same method name and the same type signature. Overridden methods have the same name, same method signature, and same return type.

Review

Review

🏆 Can combine some OOP concepts

...

In the context of OOP, what is the relationship between abstraction and encapsulation?

Explanation: We can have abstraction without encapsulation. For example, we can think in terms of student entities when we code in C (a procedural language). But when we use a language that has encapsulation too (e.g., in Java/C++/C#), we can package all student related data and functions into one construct (i.e. a class). This makes the ‘student’ abstraction stronger because now we can not only think in terms of students, the program can perform actions using objects that represent student entities.

Which of these do not belong to four main OO principles?