by Dinesh Thakur

Inheritance is suitable only when classes are in a relationship in which subclass is a (kind of) superclass. For example: A Car is a Vehicle so the class Car has all the features of class Vehicle in addition to the features of its own class. However, we cannot always have. is a relationship between objects of different classes. For example: A car is not a kind of engine. To represent such a relationship, we have an alternative to inheritance known as composition. It is applied when classes are in a relationship in which subclass has a (part of) superclass.

Unlike inheritance in which a subclass extends the functionality of a superclass, in composition, a class reuses the functionality simply by creating a reference to the object of the class it wants to reuse. For example: A car has an engine, a window has a button, a zoo has a tiger.

Now consider. the following program that demonstrates the concept of composition

class Date

{
     private int day;
     private int month;
     private int year;
     Date(int dd, int mm,int yy)
     {
       System.out.println("Constructor of Data Class is Called");
       day=dd;
       month=mm;
       year=yy;
     }
     public String toString()
     {
       return (day+"/"+month+"/"+year);
     }
}
   class Employee
{
     private int id;
     private String name;
     private Date hireDate; //object of Data class
     Employee(int num,String n, Date hire)
     {
       System.out.println("Constructor of Employee Class is Called");
       id=num ;
       name=n ;
       hireDate=hire;
     }
     public void display()
     {
       System.out.println("id = "+id);
       System.out.println("Name = "+name);
       System.out.println("Hiredate = "+hireDate);
     }
}
   public class Composition
   {
       public static void main(String[] args)
        {
                Date d = new Date(01,01,2011);
                Employee emp = new Employee(1,"Dinesh Thakur",d);
                emp.display();
        }
  }

Composition in Java Example

In this example, we first define a class Date for maintaining the date information. Then we define a class Employee that not only contains members id, name and display () but also contains a reference variable hireDate to refer objects of class Date as its member, which maintains employee's hiredate.

The statement,

emp.display () ;

on execution will display employee's id, name and the hiredate. The hireDate is obtained with an implicit call to the Date class's toString () method.