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Can someone give me a practical example of Association, aggregation and composition

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Hi,

Can you give some practical example of association, aggregation and composition which will make this understandable.
A code sample wud be really helpfull.

regards,

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last answered one year ago

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There's an understandable explanation of these concepts in this article together with some simple C++ examples for all three.

Even if you don't know C++, you'll probably be able to understand these examples.

However, I've translated the first one to C# - I haven't time to translate the others right now *. Although it's unnecessary to manually delete objects in C# (the GC does it for us), I've added a Dispose() method to the classes so as to get the same output as the C++ code.

// association example

using System;


class Department : IDisposable
{
    string name_p;
  
    public Department(string dName)
    {
      Console.WriteLine("Department::Constructor\n");
      name_p = dName;
    }

    public string dName() 
    {
      return name_p;
    }

    public void Dispose()
    {
      Console.WriteLine("Department::Dispose\n");
    }

}

class Student :IDisposable
{
    string name_p;
 
    public Student(string sName)
    {
      Console.WriteLine("Student::Constructor\n");
      name_p = sName;
    }

    public string sName()
    {
      return name_p;
    }
    
    public void Dispose()
    {
      Console.WriteLine("Student::Dispose\n");      
    }
}


class Course : IDisposable
{
    Student std_p;
    Department dept_p;
    string courseName_p;

    static int index = 0;
    static Course[] courseList = new Course[4];

    public Course(string crseName, Student student, Department dept)      
    {
      courseName_p = null;     
      std_p = student;
      dept_p = dept;    
      Console.WriteLine("Course::Constructor\n");

      if (index < 4)
      {        
        courseName_p = crseName;
        
        //insert this Course in courseList
        courseList[index] = this;
        ++index;
      }
      else
      {
        Console.WriteLine("Cannot accomodate any more Courses\n");
      }
    }

    public void Dispose()
    { 
      Console.WriteLine("Course::Dispose\n");      
    }

    public static string findStudent(string crseName, string deptName)
    {
      for(int i=0; i<index; i++)
      {
        if ( (courseList[i].CourseName == crseName) && 
             (courseList[i].DeptName == deptName) )
        {
          return courseList[i].StdName;
        }
      }
      return ""; 
    }

    public string StdName{ get { return std_p.sName(); } }
    public string DeptName { get { return dept_p.dName(); } }
    public string CourseName { get{ return courseName_p; } }
}

static class Program
{ 

    static void Main()
    {
       int i;

       Console.WriteLine("\nExample of Association class...\n");
       Console.WriteLine("-----------------------------------\n\n");

       Console.WriteLine("We have got 4 students ...\n");
       Student[] studentNames = new Student[4]{new Student("Meera"), new Student("Moina"), new Student("Teena"), new Student("Mridula")} ;

       Console.WriteLine("\n");

       Console.WriteLine("We have got 2 Departments...\n");
       Department[] departNames = new Department[2]{new Department("Mathemetics"), new Department("ComputerScience")} ;
  
       Console.WriteLine("\n");

       Console.WriteLine("Here class Course Associates Student and Department, with a Course name ...\n");
       Course course1 = new Course("DataStructure",studentNames[0], departNames[1]);
       Course course2 = new Course("Maths",studentNames[3], departNames[0]);
       Course course3 = new Course("Geometry",studentNames[2], departNames[0]);
       Course course4 = new Course("CA",studentNames[1], departNames[1]);

       Console.WriteLine("\n");

       Console.WriteLine("Finding a Student using Course and Department...\n");
       Console.WriteLine("Student who has taken Maths Course in Mathemetics Department is : {0}", Course.findStudent("Maths", "Mathemetics"));
  
       Console.WriteLine("\n");

       Console.WriteLine("Disposal of objects...\n\n");

       for(i=0; i<4; ++i)
       {
         studentNames[i].Dispose();
       }

       Console.WriteLine("\n");

       for(i=0; i<2; ++i)
       {
         departNames[i].Dispose();
       }

       Console.WriteLine("\n");

       course1.Dispose();
       course2.Dispose();
       course3.Dispose();
       course4.Dispose();

       Console.WriteLine("\n");

       Console.ReadKey();
    }
}


EDIT

* I've found time now to translate the other two examples. Not quite the same because of the different object disposal semantics between C++ and C#:
// aggregation example

using System;

class Employee : IDisposable 
{
   public Employee(string name)
   {
      Console.WriteLine("Employee::Constructor\n");
      myName_p = name;
   }
    
   public string disp(){ return myName_p ;}

   public void Dispose()
   {
      Console.WriteLine("Employee::Dispose\n");
   }

   private string myName_p;
}

class Company : IDisposable
{

   public Company(string compName, Employee emp)
   {
      Console.WriteLine("Company::Constructor\n");
      name = compName;
      myEmp_p = emp;
   }

   public void Dispose()
   {     
      Console.WriteLine("Company::Dispose\n");
      myEmp_p.Dispose();
      myEmp_p = null;
   }
    
   private string name;
   private Employee myEmp_p;
}

static class Program
{
   static void Main()
   {
      Console.WriteLine("\nExample of Aggregation Relationship\n");
      Console.WriteLine("-----------------------------------------\n\n");
    
      Console.WriteLine("Here, an Employee-Keerti works for Company-MS \n");
      Employee emp = new Employee("Keerti");
  
   
      Company comp =  new Company("MS", emp);
      // here Company object and Employee objects will be disposed    
      Console.WriteLine("At this point Company gets disposed...\n");
      Console.WriteLine("and so does Employee-" + emp.disp() + "\n");
      comp.Dispose();
      Console.ReadKey();        
   }
}

// composition example

using System;

class Room : IDisposable
{
    public static void createRoom_v(out Room room, House hse, string name)
    {
      room = new Room(hse, name); 
    }

    public Room(){}
    
    public Room(House hse, string myName)
    {
      Console.WriteLine("Room::Constructor\n");
      myHse_p = hse;
      
      if(myHse_p != null)
      {       
        name_p = myName;
      }
      else
      {
        Console.WriteLine("Oops House itself is not Created Yet ...\n");
      }
    }

    public void Dispose()
    {
      Console.WriteLine("Room::Dispose\n");
      myHse_p = null;
    }

    public void disp()
    {
      Console.WriteLine(name_p);
      Console.WriteLine("\n");
    }
    
    public static void initList_v(out Room[] roomsList_p)
    {
      roomsList_p = new Room[3];
    }

    private House myHse_p;
    private string name_p;
}

class House : IDisposable
{
    public House(string myName)
    {
      Console.WriteLine("House::Constructor\n");
      name_p = myName;

      Room.initList_v(out roomsList_p);

      Room myRoom;
      Room.createRoom_v(out myRoom, this, "Kitchen");
      roomsList_p[0] = myRoom;
      
      Room.createRoom_v(out myRoom, this, "BedRoom");
      roomsList_p[1] = myRoom;

      Room.createRoom_v(out myRoom, this, "Drawing Room");
      roomsList_p[2] = myRoom;
    }
    
    public void Dispose()
    {
      Console.WriteLine("House::Dispose\n");
      int i;
      
      Console.WriteLine("Dispose all the Rooms ...\n");
      for(i=0; i<3; ++i)
      {
        if(roomsList_p[i] != null)
        {
          roomsList_p[i].Dispose();
          roomsList_p[i] = null;  
        }
          
      }
      roomsList_p = null;
    }

    public void disp()
    {
      Console.WriteLine("\n\nName of the House : " + name_p);
      
      if(roomsList_p != null)
      {
        int i;
        Console.WriteLine("\n\nRooms details...\n");
        for(i=0; i<3; ++i)
        {
          if(roomsList_p[i] != null)
          {
            roomsList_p[i].disp();
          }
        }
        Console.WriteLine("\n\n");
      }
    }

  private string name_p;
  private Room[] roomsList_p;
}


static class Program
{
  static void Main()
  {
     Console.WriteLine("\nExample of Composition Relationship\n");
     Console.WriteLine("-----------------------------------------\n\n");
     House hse = new House("Vishranti Nilaya");
     
     Console.WriteLine("\n\nHouse details...\n");
     hse.disp();
     
     Console.WriteLine("Here House itself creates the Rooms and disposes them as well, before it gets disposed...\n");
     hse.Dispose();
     Console.ReadKey();
  } 
}

  • answered one year ago by:

    vulpes
    17279
  • edited one year ago
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Thanks !!! the artical was good and thanks a lot for taking the effort to convert the code.....Really appreciate your time...hope i will be able to do the same sometime for others !!! :-)

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