Introduction to OOP with Java - Abu Khleif · following UML class diagram: Introduction to OOP with...

Introduction to OOP with Java

Instructor: AbuKhleif, Mohammad Noor

Sep 2017

www.abukhleif.com

http://www.abukhleif.com/

OOP – Part 4


Sep 2017

www.abukhleif.com

Lecture 10:


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

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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


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


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

Immutable Objects and Classes


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• Normally, you create an object and (allow its contents to be changed later?).

• However, occasionally it is desirable to create an object whose contents cannot be changed once the object has been created.• Such an object is called immutable object and its class is called

immutable class.


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• For a class to be immutable, it must meet the following requirements:• All data fields must be private.

• There can’t be any mutator methods for data fields.

• No accessor methods can return a reference to a data field that is mutable.


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Example

Introduction to OOP with Java - AbuKhleiF www.abukhleif.com 8

public class Student {

private int id;

private String name;

private double[] grades;

public Student(int ssn, String newName) {

id = ssn;

name = newName;

grades = new double[3];

}

public int getId() {

return id;

}

public String getName() {

return name;

}

public double[] getGrades() {

return grades;

}

}

This method actually returns a reference to the array grades, which means it can be changed once returned.

Example,cont.


public class test {

public static void main(String[] args) {

Student student = new Student(112233, “John”);

double[] studentGrades = student.getGrades();

studentGrades[0] = 90.0;



}

}

Let’s Code

• Rewrite the previous class to be immutable.


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Composition and Aggregation


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• An object can contain another object. The relationship between the two is called composition.

• Composition is actually a special case of the aggregationrelationship.• Aggregation models has-a relationship and represents ownership

relationship between objects.

• The owner object is called an aggregating object.• And its class is called an aggregating class.

• The subject object is called an aggregated object.• And its class is called an aggregated class.


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• An object may be owned by several other aggregating objects.

• If an object is exclusively owned by an aggregating object, the relationship between them is referred to as composition.


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Name Address Student

Composition Aggregation

1 1..3 1 1


• An aggregation relationship is usually represented as a data field in the aggregating class.


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public class Name {

...

}

public class Student {

private Name name;

private Address address;

...

}

public class Address {

...

}

Aggregated class Aggregating class Aggregated class


• Aggregation may exist between objects of the same class.


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Person

Supervisor

1

1

public class Person { // The type for the data is the class itselfprivate Person supervisor; ...

}

Person

Supervisor

1

m

public class Person { // The type for the data is the class itselfprivate Person[] supervisors; ...

}

Let’s Code• Implement this class, using the Student class

example we studied before.

• Build a class ‘TestCourse’ to test your Course class.


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Course

-courseName: String

-students: String[]

-numberOfStudents: int

+Course(courseName: String)

+getCourseName(): String

+addStudent(student: String): void

+dropStudent(student: String): void

+getStudents(): String[]

+getNumberOfStudents(): int

The name of the course.

An array to store the students for the course.

The number of students (default: 0).

Creates a course with the specified name.

Returns the course name.

Adds a new student to the course.

Drops a student from the course.

Returns the students in the course.

Returns the number of students in the course.

Suggested Solution


public class Course {

private String courseName;

private String[] students = new String[100];

private int numberOfStudents;

public Course(String courseName) {

this.courseName = courseName;

}

public void addStudent(String student) {

students[numberOfStudents] = student;

numberOfStudents++;

}

public String[] getStudents() {

return students;

}

public int getNumberOfStudents() {

return numberOfStudents;

}

public String getCourseName() {

return courseName;

}

public void dropStudent(String student) {

// Left as an exercise

}

}

Suggested Solution


public class TestCourse {

public static void main(String[] args) {

Course course1 = new Course("Data Structures");

Course course2 = new Course("Database Systems");

course1.addStudent("Peter Jones");

course1.addStudent("Brian Smith");

course1.addStudent("Anne Kennedy");

course2.addStudent("Peter Jones");

course2.addStudent("Steve Smith");

System.out.println("Number of students in course1: "

+ course1.getNumberOfStudents());

String[] students = course1.getStudents();

for (int i = 0; i < course1.getNumberOfStudents(); i++)

System.out.print(students[i] + ", ");

System.out.println();

System.out.print("Number of students in course2: "

+ course2.getNumberOfStudents());

}

}

Tips in OOP Design(Self Study)


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Designing a Class

• (Coherence) A class should describe a single entity, and all the class operations should logically fit together to support a coherent purpose.

• You can use a class for students, for example, but you should not combine students and staff in the same class, because students and staff have different entities.


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Designing a Class

• (Separating responsibilities) A single entity with too many responsibilities can be broken into several classes to separate responsibilities.

• The classes String, StringBuilder, and StringBuffer all deal with strings, for example, but have different responsibilities.

• The String class deals with immutable strings, the StringBuilderclass is for creating mutable strings, and the StringBuffer class is similar to StringBuilder except that StringBuffer contains synchronized methods for updating strings.


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Designing a Class

• Classes are designed for reuse.

• Users can incorporate classes in many different combinations, orders, and environments.

• Therefore, you should design a class that imposes no restrictions on what or when the user can do with it, design the properties to ensure that the user can set properties in any order, with any combination of values, and design methods to function independently of their order of occurrence.


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Designing a Class

• Provide a public no-arg constructor and override the equalsmethod and the toString method defined in the Object class whenever possible.


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Designing a Class

• Follow standard Java programming style and naming conventions.

• Choose informative names for classes, data fields, and methods.

• Always place the data declaration before the constructor, and place constructors before methods.

• Always provide a constructor and initialize variables to avoid programming errors.


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Using Visibility Modifiers

• Each class can present two contracts• one for the users of the class, and• one for the extenders of the class.

• Make the fields private and accessor methods public if they are intended for the users of the class.

• Make the fields or method protected if they are intended for extenders of the class.


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• The contract for the extenders encompasses the contract for the users.

• The extended class may increase the visibility of an instance method from protected to public, or change its implementation, but you should never change the implementation in a way that violates that contract.


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• A class should use the private modifier to hide its data from direct access by clients.

• You can use get methods and set methods to provide users with access to the private data, but only to private data you want the user to see or to modify.

• A class should also hide methods not intended for client use.


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Using the static Modifier

• A property that is shared by all the instances of the class should be declared as a static property.


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Homework

Homework

Part 1- Write a Java program that implements the classes shown in the following UML class diagram:


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Homework

For the Item class, you are required to implement the following:

• A constructor that initializes the item name and price with the passed arguments.

• The mutator (set) method for the salesTax data field.

• The mutator (set) and accessor (get) methods for the price data field.

• The method getPriceWithTax which returns the price of the item including the tax.


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Homework

For the Market class you are required to implement the following:

• A constructor that initializes the name with the passed argument and create an array with 0 elements for the reference data field items.

• A constructor that initializes the name with the passed argument and create an array with the same number of elements of the passed array of items. Names and prices of items in the passed array should be copied to newly created objects in the new array.

• The method maxPrice which returns a reference to the item with the highest price.

• The method addItem which adds the passed item to the array items.


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Homework

Part 2- In your main class:

• Set the data field salesTax of the Item class to 0.14.

• Create an array with two items: • name: Chocolate, price: 0.5 • name: Rice, price: 4.0

• Create a market named “UJ” with these two items.

• Add the following item to the market: • name: Cheese, price: 1.3

• Print the name of the market, then the name and price of the item with the highest price, as follows:

“Market name: _______, item with highest price is ________ with price = _________.”

• For each item in the market, print the item name, price, and price with tax, as follows:

“Item name: _____, price: _________, price with tax:________”


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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.


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


Sep 2017

www.abukhleif.com

References:

End of Lecture =D


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