Theory

Aggregation relationship:

In Object-Oriented programming, an Object communicates to another object to use functionality and services provided by that object.

• If a class have an entity reference, it is known as Aggregation.

• Aggregation represents HAS-A relationship.

• It is a unidirectional association i.e. a one-way relationship.

• In Aggregation, both the entries can survive individually which means ending one entity will not affect the other entity.

Consider the class diagram

Student class
          public class Student
          {
          public String name; // Student's name.
          public double test1, test2, test3; // Grades on three tests.
          public Student()
          {
          System.out.printin(“ Creating Student 1! *);
          }
          public double getAverage()
          { // compute average test grade
          return (test1 + test2 + test3) / 3;
          }
          
          }// end of class Student
          

Course Class
          public class Course{
          private String courseName;
          private int courselevel;
          private final int courseCredits;
          public Course (String courseName, int courselevel, int courseCredits )
          {
          this.courseNeme= courseName;
          this.courselevel=courselevel;
          this. courseCredits=courseCredits;
          
          }
          }
          

Course Registration
          public class CourseRegistartion
          
          {
          private Course courseObj:
          private Student studentObj;
          
          public Course ( Course courseObj, Student studentObj)
          {
          
           
          this.courseObj
          this. studentObj
          }
          }
          

Composition relationship:

Composition is a restricted form of Aggregation in which two entities are highly dependent on each other.

• It represents part-of relationship.

• In composition, both entities are dependent on each other.

• When there is a composition between two entities, the composed object cannot exist without the other entity.

Composition is a strong Association whereas Aggregation is a weak Association.

Consider the following class diagram:

The class diagram shows that the Engine is a part-of Vehicle and the Vehicle cannot exist without the Engine object. The Engine object cannot be changed once assigned to a Vehicle.

Use the final Keyword on the variable as a modifier. Using final means the value once assigned to a variable cannot be changed.