introduction to oop with java - abu khleif · 2017-09-30 · introduction to oop with java...

Introduction to OOP with Java

Instructor: AbuKhleif, Mohammad Noor

Sep 2017

www.abukhleif.com

http://www.abukhleif.com/

Inheritance and Polymorphism – Part 1Instructor: AbuKhleif, Mohammad Noor

Sep 2017

www.abukhleif.com

Lecture 11:


Instructor

• AbuKhleif, ‘Mohammad Noor’• Computer Engineer (JU 2012-2017)• Software Automation Engineer @ Atypon – John Wiley and

Sons Company - Jordan Branch

• Reach me at:• www.abukhleif.com• [email protected]• facebook.com/moh.noor94• twitter.com/moh_noor94

Introduction to OOP with Java - AbuKhleiF www.abukhleif.com

3


mailto:[email protected]

http://facebook.com/moh.noor94

http://twitter.com/moh_noor94

Course

• Java SE Basics• Object Oriented Programming• Course Page:

www.abukhleif.com/courses/java-101-sep-2017• Or, go to: www.abukhleif.com Courses Java 101 Course

– Sep 2017• Course Facebook Group:

www.facebook.com/groups/AKF2017Java


4

http://www.abukhleif.com/courses/java-101-sep-2017


https://www.facebook.com/groups/AKF2017Java/

Motivations

• Suppose you will define classes to model circles, rectangles, and triangles. These classes have many common features.

• What is the best way to design these classes so to avoid redundancy?

• The answer is to use inheritance.


5

Let’s Start!


6

What is Inheritance?

• Classes are used to model objects of the same type.

• Different classes may have some common properties and behaviors.

• Inheritance allows you to:• Define a generalized class that includes the common properties

and behavior.

• Define specialized classes that extend the generalized class.

• Inherit the properties and methods from the general class.

• Add new properties and methods.


7

Super classes and Sub classes


8

• A class (A) that is extended from another class (B) is called a subclass. (B) is called a superclass.

• A subclass:• Inherits accessible data fields and methods from its superclass.

• May also add new data fields and methods.

Example

Introduction to OOP with Java - AbuKhleiF www.abukhleif.com 9

GeometricObject

-color: String

-filled: boolean

-dateCreated: java.util.Date

+GeometricObject()

+GeometricObject(color: String,

filled: boolean)

+getColor(): String

+setColor(color: String): void

+isFilled(): boolean

+setFilled(filled: boolean): void

+getDateCreated(): java.util.Date

+toString(): String

The color of the object (default: white).

Indicates whether the object is filled with a color (default: false).

The date when the object was created.

Creates a GeometricObject.

Creates a GeometricObject with the specified color and filled

values.

Returns the color.

Sets a new color.

Returns the filled property.

Sets a new filled property.

Returns the dateCreated.

Returns a string representation of this object.

Circle

-radius: double

+Circle()

+Circle(radius: double)

+Circle(radius: double, color: String,

filled: boolean)

+getRadius(): double

+setRadius(radius: double): void

+getArea(): double

+getPerimeter(): double

+getDiameter(): double

+printCircle(): void

Rectangle

-width: double

-height: double

+Rectangle()

+Rectangle(width: double, height: double)

+Rectangle(width: double, height: double

color: String, filled: boolean)

+getWidth(): double

+setWidth(width: double): void

+getHeight(): double

+setHeight(height: double): void

+getArea(): double

+getPerimeter(): double

Example


public class GeometricObject {

private String color = "White";

private boolean filled;

public String getColor() {

return color;

}

public void setColor(String color) {

this.color = color;

}

public boolean isFilled() {

return filled;

}

public void setFilled(boolean filled) {

this.filled = filled;

}

public void printObjectDetails() {

System.out.println("Color is " + color + "Is filled? " + filled);

}

}

Example


public class Circle extends GeometricObject {

private double radius = 1;

public double getRadius() {

return radius;

}

public void setRadius(double radius) {

this.radius = radius;

}

public double getArea() {

return radius * radius * Math.PI;

}

public double getPerimeter() {

return 2 * radius * Math.PI;

}

public void printCircleDetails() {

System.out.println("Circle color is " + getColor() + " , circle

filled ? "+ isFilled()+ ", circle radius is"+ radius);

}

}

Example


public class Rectangle extends GeometricObject {

private double width = 1;

private double height = 1;

public double getWidth() {

return width;

}

public void setWidth(double width) {

this.width = width;

}

public double getHeight() {

return height;

}

public void setHeight(double height) {

this.height = height;

}

public double getArea() {

return width * height;

}

public double getPerimeter() {

return 2 * (width + height);

}

public void printRectangleDetails() {

System.out.println("Rectangle color is " + getColor() + " , rectangle filled?

"+ isFilled()+ ", rectangle width is"+ width + ", rectangle height is" +

height);

}

}

Example


public class TestCircleRectangle {

public static void main(String[] args) {

Circle c = new Circle();

c.printObjectDetails();

c.setColor("Black");

c.setFilled(true);

c.setRadius(5);

c.printCircleDetails();

Rectangle r = new Rectangle();

r.setColor("Red");

r.setFilled(true);

r.setWidth(3);

r.setHeight(5);

r.printRectangleDetails();

}

}

These methods are inherited from the GeometricObject class.

Important Notes

• Contrary to conventional interpretation, a subclass is not a subset of its superclass.• In fact, a subclass usually contains more information and methods

than its superclass.

• Private data fields in a superclass are not accessible outside the class.• They cannot be used directly in a subclass.

• They can only be accessed/mutated through public accessors/mutators if defined in the superclass.


14

Important Notes

• Inheritance is used to model the is-a relationship.• Do not blindly extend a class just for the sake of reusing methods.

• Some programming languages allow you to derive a subclass from several classes. This capability is called multiple inheritance.• Java does not allow multiple inheritance.

• A Java class may inherit directly from only one class.

• (Multiple inheritance can be achieved through interfaces in Java?)


15

The super Keyword


16

The super Keyword

• The keyword super refers to the superclass and can be used to:• Call a superclass constructor.

• Call a superclass method.


17

The super Keyword

• Remember that a constructor is used to construct an instance of a class.

• Unlike properties and methods, the constructors of a superclass are not inherited by a subclass.• They can only be invoked from the constructors of the subclasses

using the keyword super.


18

The super Keyword

• The syntax to call a superclass’s constructor is:

super(); // to invoke the no-arg constructor

super(parameters); //to invoke a constructor with parameters

• The statement super() or super(parameters) must appear in the first line of the subclass’s constructor.

• If the keyword super is not explicitly used, the superclass's no-arg constructor is automatically invoked.


19

The super Keyword

• The statement super() or super(parameters) must appear in the first line of the subclass’s constructor.

• If the keyword super() is not explicitly used, the superclass's no-arg constructor is automatically invoked.


20

public A(double d) {

// some statements

}

is equivalent to

public A(double d) {

super();

// some statements

}

public A() {

}

is equivalent to

public A() {

super();

}

The super KeywordExample

• The following constructor can be added to the Circle class of the previous example:

public Circle (double radius){

super();

this.radius = radius;

}


21

Constructor Chaining


22


• A constructor may invoke an overloaded constructor (using this) or its superclass constructor (using super).

• If neither is invoked explicitly, the compiler automatically puts super() as the first statement in the constructor.


23


• In any case, constructing an instance of a class invokes the constructors of all the superclasses along the inheritance hierarchy.• When constructing an object of a subclass, the subclass constructor first

invokes its superclass constructor before performing its own tasks.

• If the superclass is derived from another class, the superclass constructor invokes its parent-class contsructor before performing its tasks.

• This process continues until the last constructor along the inheritance hierarchy is called.

• The process above is known as ‘Constructor Chaining’.


24

Example


public class Faculty extends Employee {


new Faculty();

}

public Faculty() {

System.out.println("(4) Faculty's no-arg constructor is invoked");

}

}

class Employee extends Person {

public Employee() {

this("(2) Invoke Employee’s overloaded constructor");

System.out.println("(3) Employee's no-arg constructor is invoked");

}

public Employee(String s) {

System.out.println(s);

}

}

class Person {

public Person() {

System.out.println("(1) Person's no-arg constructor is invoked");

}

}

Example


Output:(1) Person's no-arg constructor is invoked(2) Invoke Employee’s overloaded constructor(3) Employee's no-arg constructor is invoked(4) Faculty's no-arg constructor is invoked

Caution

• If a class is designed to be extended, it is better to provide a no-argconstructor to avoid programming errors.

• Example: this code cannot be compiled:public class Apple extends Fruit {

}

class Fruit {public Fruit(String name) {

System.out.println("Fruit's constructor is invoked");}

}


27

The default no-arg constructor of Apple will try to invoke a no-arg constructor of Fruit, which does not exist!

Using the super Keyword to Call a Superclass Method

• The keyword super can be used to reference a method other than the constructor in the superclass.

• The syntax is:super.method(parameters);

• The printCircleDetails method in the Circle class could be rewritten as follows:

public void printCircleDetails(){System.out.println("Circle color is "+ super.getColor() + " , circle filled? "+ super.isFilled() + " , circle radius is "+ radius);}


28

Using the super Keyword to Call a Superclass Method

• It is not necessary to put the super keyword before the methods getColor and isFilled in the previous example.• These methods are inherited by the Circle class.• Cases were the super keyword is needed to invoke the superclass

methods will be showed when methods overriding is introduced.


29

Overriding Methods


30

Overriding Methods

• A subclass inherits methods from a superclass.• Sometimes, it is necessary for the subclass to modify the

implementation of a method defined in the superclass.• In the previous example, the method printObjectDetails of the

GeometricObject class can be overridden in the Circle class as follows:

public void printObjectDetails(){super.printObjectDetails();System.out.println("Circle radius = "+radius);}


31

Overriding Methods

• An instance method can be overridden only if it is accessible.• Thus, a private method cannot be overridden, because it is not

accessible outside its own class.• If a method defined in a subclass is private in its superclass, the two

methods are completely unrelated.

• Like an instance method, a static method can be inherited. However a static method cannot be overridden.• If a static method defined in the superclass is redefined in a subclass,

the method defined in the superclass is hidden.• The hidden static methods can be invoked using the syntax

SuperClassName.staticMethodName(parameters);


32

Overriding vs. Overloading


33


• Overloading means to define multiple methods with the same name but different signatures.

• Overriding means to provide a new implementation for a method in the subclass.• The method should be defined in the subclass using the same

signature and the same return type.


34



35

public class Test {


A a = new A();

a.p(10);

a.p(10.0);

}

}

class B {

public void p(double i) {

System.out.println(i * 2);

}

}

class A extends B {

// This method overrides the method in B


System.out.println(i);

}

}

public class Test {


A a = new A();

a.p(10);

a.p(10.0);

}

}

class B {


System.out.println(i * 2);

}

}

class A extends B {

// This method overloads the method in B

public void p(int i) {

System.out.println(i);

}

}


• Overridden methods are in different classes related by inheritance.

• Overloaded methods can be either in the same class or different classes related by inheritance.

• Overridden methods have the same signature and return type.

• Overloaded methods have the same name but a different parameter list.


36

@override Annotation

• To avoid mistakes, you can use a special Java syntax, called override annotation:• Place @override before the method in the subclass.

• This annotation denotes that the annotated method is required to override a method in the superclass.• If a method with this annotation does not override its superclass’s method,

the compiler will report an error.


37

@override Annotation

• For example, in order to denote that the printObjectDetails is overridden in the Circle class:

@Override

public void printObjectDetails(){

super.printObjectDetails();

System.out.println("Circle radius = "+radius);

}


38

The Object Class and Its Methods


39

The Object Class and Its Methods

• Every class in Java is descended from the java.lang.Objectclass.

• If no inheritance is specified when a class is defined, the superclass of the class is Object.


40

public class Circle { ...

}

Equivalent public class Circle extends Object {

...

}

The toString() method in Object

• One of the most important methods provided by the Object class is the method toString().

• The signature of the toString() method is:

public String toString()

• Invoking toString() on an object returns a string that describes the object.


41


• By default, it returns a string consisting of a class name of which the object is an instance, and at sign (@), and the object’s memory address in hexadecimal.• For example, the output of the following codeCircle c = new Circle();

System.out.println(c.toString());

is something like:

Circle@780324ff


42


• Usually, we override the toString() method so that it returns a descriptive string representation of the object.

• For example, the toString() method in the Object class can be overridden for the Circle class as follows:

public String toString(){

return "Color is " + getColor() + ". Is filled? " + isFilled() + ". Radius is " + radius + ".";

}


43


• You can also pass an object to invoke the toString() method.

• Example:

System.out.println(object);

System.out.print(object);

• These are equivalent to invoking:

System.out.println(object.toString());

System.out.print(object.toString());


44

Let’s Code

Design a class named Triangle that extends GeometricObject. The class contains:

• Three double data fields named side1, side2, and side3 with default values 1.0 to denote three sides of the triangle.

• A no-arg constructor that creates a default triangle.

• A constructor that creates a triangle with the specified side1, side2, and side3.

• The accessor methods for all three data fields.

• A method named getArea() that returns the area of this triangle.

• A method named getPerimeter() that returns the perimeter of this triangle.

• A method named toString() that returns a string description for the triangle.


45

Let’s Code, cont.

• The formula for computing the area of a triangle is:

𝑠 = (𝑠𝑖𝑑𝑒1 + 𝑠𝑖𝑑𝑒2 + 𝑠𝑖𝑑𝑒3)/2;

𝑎𝑟𝑒𝑎 = 𝑠(𝑠 − 𝑠𝑖𝑑𝑒1)(𝑠 − 𝑠𝑖𝑑𝑒2)(𝑠 − 𝑠𝑖𝑑𝑒3)


46

Let’s Code, cont.

• Draw the UML diagrams for the classes Triangle and GeometricObject and implement the classes.

• Write a test program that prompts the user to enter three sides of the triangle, a color, and a Boolean value to indicate whether the triangle is filled.• The program should create a Triangle object with

these sides and set the color and filled properties using the input.

• The program should display the area, perimeter, color, and true or false to indicate whether it is filled or not.


47

Homework

Homework

Part 1- Implement the following classes using Java:


49

Homework

Part 2- In your main method:

• Define one object of type Employee, one object of type Faculty, and one object of type Staff.• Object-1 (Employee):

office: 404name: Ahmedsalary: 500

• Object-2 (Faculty):office: 310name: Samersalary: 750 rank: Professor


50

Homework

Part 2- In your main method:• Object-3 (Staff):

office: 205name: Salmasalary: 450rank: Secretary

• Print the details of each object by invoking the toString() method.


51

Homework Submission

• Submit only the .java files.

• Upload your file to the Facebook group.

• Submission due: Thursday, Oct 5 - 08:00 PM

• Late submission will not be reviewed by the instructor.

• Public solutions upload goal is to share knowledge, you can see other’s solutions, but, please, don’t cheat yourself!

• Don’t forget, your solution should be well-documented, well-designed, and well-styled.


52

- Eng. Asma Abdel Karim Computer Engineering Department, JU Slides and Sheets.- Liang, Introduction to Java Programming 10/e

Instructor: AbuKhleif, Mohammad Noor

Sep 2017

www.abukhleif.com

References:

End of Lecture =D


introduction to oop with java - abu khleif · 2017-09-30 · introduction to oop with java...

Documents