A simple project

Following Classes:

1. Student Class

2. Subject Class

3. ExamPaper Class

Aggregation example:

when you have field in one class that are also the object in another class, there comes a concept what is called aggregation.

We can access those object members from this class object using aggregation concept. See code in main method for aggregation:

System.out.println(paper.getSubject().getSubjectCode());
//similarily if you want to extract only the student number, see how you can
System.out.println(“Student number:”+paper.getStudent().getStudentNumber());

Let us see the complete project involving all the above classes in intellij

package com.example;

public class classAndObjects
{
public static void main(String[] args)
{
Subject subject1 = new Subject (“ETEE 310”, “MP&MC”);
Student std1=new Student(“Ravinder”, 1234);
Exampaper paper = new Exampaper(std1, subject1, 100);
//System.out.println(paper);
//we can extract more information from paper object, say we want to know the subject code.
//See the following code carefully
//this is use of aggregation in classes
System.out.println(paper.getSubject().getSubjectCode());
//similarily if you want to extract only the student number, see how you can
System.out.println(“Student number:”+paper.getStudent().getStudentNumber());
/*
Subject subject2= new Subject(“ETEE 310”, “MP&MC”);

Student std1=new Student(“Ravinder”, 1234);
Student std2=new Student(“Rajotiya”, 1235);
if(subject1.equal(subject2))
System.out.println(“They are same”);
System.out.println(subject1);
System.out.println(std1);
// this will print package , class and the object memory address and this we donot want.
//we have to override the toString default String method so that it can be printed in readable format.
//create toString method in class Subject that explains what is happening to
*/

}
}

//

package com.example;

public class Subject
{
private String subjectCode;
private String subjectName;
public Subject (String subjectCode, String subjectName)
{
this.subjectCode = subjectCode;
this.subjectName = subjectName;
}
//copy constructor
public Subject(Subject subject)
{
this.subjectCode = subject.subjectCode;
this.subjectName = subject.subjectName;
}
public boolean equal(Subject object)
{/*
if(this.subjectCode.equals(object.subjectCode) && (this.subjectName.equals(object.subjectName)))
{
return true;
}
else
{
return false;
}
*/
//we can replace everything in above comment by simple one line
return this.subjectCode.equals(object.subjectCode) && (this.subjectName.equals(object.subjectName));
}

public String getSubjectName() {
return subjectName;
}

public void setSubjectCode(String subjectCode) {
this.subjectCode = subjectCode;
}

public void setSubjectName(String subjectName) {
this.subjectName = subjectName;
}

public String getSubjectCode()
{
return subjectCode;
}
//is used to explain what is secific happening in object
// so we are overriding the class that inherits main class of all classes
//so we are trying our own toString method
@Override
public String toString()
{
return “Subject Information:\n” +
“Subject Code : “+ subjectCode +“\n“+
“Subject Name:” +subjectName;
//now run and see your output
}

}

// Student classification

package com.example;

public class Student
{
private String studentname;
private long studentNumber;

public Student(String studentname, long studentNumber) {
this.studentname = studentname;
this.studentNumber = studentNumber;
}
//copy constructor
public Student (Student obj1)
{
this.studentNumber= obj1.studentNumber;
this.studentname=obj1.studentname;
}
// equal method
public boolean equals(Student obj)
{
return this.studentNumber== obj.studentNumber && this.studentname.equals(obj.studentname);
}

public String getStudentname() {
return studentname;
}

public long getStudentNumber() {
return studentNumber;
}

public void setStudentname(String studentname) {
this.studentname = studentname;
}

public void setStudentNumber(long studentNumber) {
this.studentNumber = studentNumber;
}

@Override //override the inbuilt toString so that code is readable
public String toString()
{
return “student information”+“\n“+
“Student num:”+studentNumber +“\n“+
“student name”+studentname;
}
}

Aggrgation:

aggregation occurs in JAVA when fields in one class are objects of other class.

Public class Exampaper

{

private Student student; //field of this class but object of Student class

private Subject subject; // field of this class but object f Subject class

private int marks;

}

package com.example;

public class Exampaper
{
private Student student;
private Subject subject;
private int marks;

public Exampaper(Student student, Subject subject, int marks) {
this.student = student;
this.subject = subject;
this.marks = marks;
}

public Student getStudent() {
return new Student(student);// instead of passing back my student object pass a copy
}

public Subject getSubject() {
return new Subject(subject); // instead of passing back my subject object pass a copy of it
}

public int getMarks() {
return marks;
}

public void setStudent(Student student) {
this.student = new Student(student);
//also this.student, student that is passed in and student object on th of this.student exactly to the same address, so this is a security risk in handling object, so,
// for security purpose let us pass a copy of student object rather than setting the value directly
//similarily for subject object
this.subject = new Subject(subject);
this.marks=marks; // as marks are not of object type, so set is directly
}

public void setSubject(Subject subject) {
//this.subject = subject; //not this //instead of passing back(returning) the object itself pass a copy instead
this.subject = new Subject(subject); //this way is correct
}

public void setMarks(int marks) {
this.marks = marks;
}
@Override
public String toString()
{
return “Exam Paper Information ” + “\n” +
“student info “+student + “\n“+
“Subject info “+subject + “\n” +
“marks info” + marks;

}
}

main method use of aggregation…. see how subject code information is derived through paper objective

//similarly, how student number is extracted from paper object.getStudent().getStudentNumber()

public class classAndObjects
{
public static void main(String[] args)
{
Subject subject1 = new Subject (“ETEE 310”, “MP&MC”);
Student std1=new Student(“Ravinder”, 1234);
Exampaper paper = new Exampaper(std1, subject1, 100);
//System.out.println(paper);
//we can extract more information from paper object, say we want to know the subject code.
//See the following code carefully
//this is use of aggregation in classes
System.out.println(paper.getSubject().getSubjectCode());
//similarily if you want to extract only the student number, see how you can
System.out.println(“Student number:”+paper.getStudent().getStudentNumber());
}