Composition in Java Example

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(); 
         } 
   } 
 

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.