Jedi slides 2.1 object-oriented concepts

Object-oriented Concepts TOPIC ONE Object-oriented Software Engineering

Object-oriented Software Engineering Object-oriented Software Engineering is the use of object technologies in building software.

Pressman, 1997 Object technologies is often used to encompass all aspects of an object-oriented view and includes analysis, design, and testing methods; programming languages; tools; databases; and applications that are created using object-oriented approach. Taylor, 1997, Object Technology Object technology is a set of principles guiding software construction together with languages, databases, and other tools that support those principles. Object Technology

It leads to reuse, and reuse (of program components) leads to faster software development and higher-quality programs. It leads to higher maintainability of software modules because its structure is inherently decoupled. It leads to object-oriented system that are easier to adapt and easier to scale, ie, large systems are created by assembling reusable subsystems. Benefits of Object Technology

It is a representation of an entity either physical, conceptual, or software. It allows software developers to represent real-world concepts in their software design. Object

It is an entity with a well-defined boundary and identity that encapsulates state and behavior. Object

It is one of the possible conditions that an object may exists in. It is implemented by a set of properties called attributes , along with its values and the links it may have on other objects. Object's State

It determines how an object acts and reacts. It is represented by the operations that the object can perform. Object's Behavior

Although two objects may share the same state (attributes and relationships), they are separate, independent objects with their own unique identity. Object's Identity

Abstraction Encapsulation Modularity Hierarchy Four Basic Principles of Object-orientation

Abstraction is a kind of representation that includes only the things that are important or interesting from a particular point of view. It is the process of emphasizing the commonalities while removing distinctions. It allows us to manage complexity systems by concentrating on the essential characteristics that distinguish it from all other kinds of systems. It is domain and perspective dependent. Abstraction

Sample Abstraction An applicant submits a club membership application to the club staff. A club staff schedules an applicant for the mock try-outs. A coach assigns an athlete to a squad. A squad can be a training or competing squad. Teams are formed from a squad.

Encapsulation localizes features of an entity into a single blackbox abstraction, and hides the implementation of these features behind a single interface. It is also known as information-hiding ; it allows users to use the object without knowing how the implementation fulfils the interface. It offers two kinds of protection: it protects the object's state from being corrupted and client code from changes in the object's implementation. Encapsulation

Encapsulation Illustrated Joel Santos is assigned to the Training Squad. The key is in the message interface . updateSquad(“Training”)

Modularity is the physical and logical decomposition of large and complex things into smaller and manageable components that achieve the software engineering goals. It is about breaking up a large chunk of a system into small and manageable subsystems. The subsystems can be independently developed as long as their interactions are well understood. Modularity

Modularity Illustrated Ang Bulilit Liga Squad and Team System Club Membership Maintenance System Coach Information Maintenance System Squad and Team Maintenance System

Any ranking or ordering of abstractions into a tree-like structure. Kinds of Hierarchy Aggregation Class Containment Inheritance Partition Specialization Type Hierarchy

Hierarchy Illustrated Squad Training Squad Competing Squad

It is a form of association wherein one class shares the structure and/or behavior of one or more classes. It defines a hierarchy of abstractions in which a subclass inherits from one or more superclasses. Single Inheritance Multiple Inheritance It is an is a kind of relationship. Generalization

It is a mechanism by which more-specific elements incorporate the structure and behavior of more-general elements. A class inherits attributes, operations and relationship. Inheritance Squad Name Coach MemberList listMembers() changeCoach() Training Squad Competing Squad

Inheritance In Java, all classes, including the classes that make up the Java API, are subclassed from the Object superclass. A sample class hierarchy is shown below. Superclass Any class above a specific class in the class hierarchy. Subclass Any class below a specific class in the class hierarchy.

Inheritance Superclass Any class above a specific class in the class hierarchy. Subclass Any class below a specific class in the class hierarchy.

Inheritance Benefits of Inheritance in OOP : Reusability Once a behavior (method) is defined in a superclass, that behavior is automatically inherited by all subclasses. Thus, you can encode a method only once and they can be used by all subclasses. A subclass only needs to implement the differences between itself and the parent.

Inheritance To derive a class, we use the extends keyword. In order to illustrate this, let's create a sample parent class. Suppose we have a parent class called Person. public class Person { protected String name; protected String address; /** * Default constructor */ public Person(){ System.out.println(“Inside Person:Constructor”); name = ""; address = ""; } . . . . }

Inheritance Now, we want to create another class named Student. Since a student is also a person, we decide to just extend the class Person, so that we can inherit all the properties and methods of the existing class Person. To do this, we write, public class Student extends Person { public Student(){ System.out.println(“Inside Student:Constructor”); } . . . . }

Inheritance When a Student object is instantiated, the default constructor of its superclass is invoked implicitly to do the necessary initializations. After that, the statements inside the subclass's constructor are executed.

Inheritance: To illustrate this, consider the following code, In the code, we create an object of class Student. The output of the program is, public static void main( String[] args ){ Student anna = new Student(); } I nside Person:Constructor Inside Student:Constructor

Inheritance The program flow is shown below.

The “super” keyword A subclass can also explicitly call a constructor of its immediate superclass. This is done by using the super constructor call. A super constructor call in the constructor of a subclass will result in the execution of relevant constructor from the superclass, based on the arguments passed.

The “super” keyword For example, given our previous example classes Person and Student, we show an example of a super constructor call. Given the following code for Student, public Student(){ super( "SomeName", "SomeAddress" ); System.out.println("Inside Student:Constructor"); }

The “super” keyword Few things to remember when using the super constructor call: The super() call MUST OCCUR AS THE FIRST STATEMENT IN A CONSTRUCTOR. The super() call can only be used in a constructor definition. This implies that the this() construct and the super() calls CANNOT BOTH OCCUR IN THE SAME CONSTRUCTOR.

The “super” keyword Another use of super is to refer to members of the superclass (just like the this reference ). For example, public Student() { super.name = “somename”; super.address = “some address”; }

Overriding methods If for some reason a derived class needs to have a different implementation of a certain method from that of the superclass, overriding methods could prove to be very useful. A subclass can override a method defined in its superclass by providing a new implementation for that method.

Example Suppose we have the following implementation for the getName method in the Person superclass, public class Person { : : public String getName(){ System.out.println("Parent: getName"); return name; } }

Example To override, the getName method of the superclass Person, we write in the subclass Student, Now, when we invoke the getName method of an object of the subclass Student , the overridden getName method would be called, and the output would be, public class Student extends Person{ : : public String getName(){ System.out.println("Student: getName"); return name; } : } Student: getName

Final Classes Final Classes Classes that cannot be extended To declare final classes, we write, public final ClassName{ . . . } Example: Other examples of final classes are your wrapper classes and Strings. public final class Person { . . . }

Final Methods and Classes Final Methods Methods that cannot be overridden To declare final methods, we write, public final [returnType] [methodName]([parameters]){ . . . } Static methods are automatically final.

Example public final String getName(){ return name; }

It is the ability to hide many different implementation behind a single interface. It allows the same message to be handled differently by different objects. Polymorphism <<interface>> Asset getValue(); Stock getValue() ; Bond getValue(); Mutual Fund getValue();

Polymorphism Polymorphism The ability of a reference variable to change behavior according to what object it is holding. This allows multiple objects of different subclasses to be treated as objects of a single superclass, while automatically selecting the proper methods to apply to a particular object based on the subclass it belongs to. To illustrate polymorphism, let us discuss an example.

Polymorphism Given the parent class Person and the subclass Student of the previous examples, we add another subclass of Person which is Employee. Below is the class hierarchy for that,

Polymorphism In Java, we can create a reference that is of type superclass to an object of its subclass. For example, public static main( String[] args ) { Person ref; Student studentObject = new Student(); Employee employeeObject = new Employee(); ref = studentObject; //Person reference points to a // Student object }

Polymorphism Now suppose we have a getName method in our superclass Person, and we override this method in both the subclasses Student and Employee. public class Student { public String getName(){ System.out.println(“Student Name:” + name); return name; } } public class Employee { public String getName(){ System.out.println(“Employee Name:” + name); return name; } }

Polymorphism Going back to our main method, when we try to call the getName method of the reference Person ref, the getName method of the Student object will be called. Now, if we assign ref to an Employee object, the getName method of Employee will be called.

Polymorphism public static main( String[] args ) { Person ref; Student studentObject = new Student(); Employee employeeObject = new Employee(); ref = studentObject; //Person ref. points to a // Student object //getName of Student class is called String temp=ref.getName(); System.out.println( temp ); ref = employeeObject; //Person ref. points to an // Employee object //getName of Employee class is called String temp = ref.getName(); System.out.println( temp ); }

Polymorphism Another example that illustrates polymorphism is when we try to pass references to methods. Suppose we have a static method printInformation that takes in a Person reference as parameter. public static printInformation( Person p ) { . . . . }

Polymorphism We can actually pass a reference of type Employee and type Student to the printInformation method as long as it is a subclass of the class Person. public static main( String[] args ) { Student studentObject = new Student(); Employee employeeObject = new Employee(); printInformation( studentObject ); printInformation( employeeObject ); }

It formalizes polymorphism. It defines polymorphism in a declarative way, unrelated to implementation. It is the key to the plug-n-play ability of an architecture. Interface

It is a special form of association that models a whole-part relationship between an aggregate (whole) and its parts. Aggregation Team Athletes

Summary Object Technologies Objects Object's State Object's Behavior Object's Identity Four Basic Principles of Object-orientation Abstraction Encapsulation Modularity Hierarchy

Jedi slides 2.1 object-oriented concepts

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Jedi slides 2.1 object-oriented concepts