COMPOSITION

 The composition is an association that represents a part of a whole relationship where a part cannot exist without a whole. If a whole is deleted then all parts are deleted. It has a stronger relationship.

Key Points

  • It represents a part-of relationship.
  • In composition, both the entities are dependent on each other.
  • When there is a composition between two entities, the composed object cannot exist without the other entity. For example, if order HAS-A line-items, then an order is a whole and line items are parts
  • If an order is deleted then all corresponding line items for that order should be deleted.
  •  Favor Composition over Inheritance.

Program Example of Java Composition

Let us consider the following program that demonstrates the concept of composition.

Step 1:

First we create a class Bike in which we declare and define data members and methods:

class Bike
{
// declaring data members and methods
private String color;
private int wheels;
public void bikeFeatures()
{
System.out.println("Bike Color= "+color + " wheels= " + wheels);
}
public void setColor(String color)
{
this.color = color;
}
public void setwheels(int wheels)
{
this.wheels = wheels;
}
}

Step 2:

Second, we create a class Honda which extends the above class Bike. Here Honda class uses HondaEngine class object start() method via composition. Now we can say that Honda class HAS-A HondaEngine:
class Honda extends Bike

{
//inherits all properties of bike class
public void setStart()
{
HondaEngine e = new HondaEngine();
e.start();
}
}


The next step in this Java Composition program is

Step 3:

Third, we create a class HondaEngine through which we use this class object in above class Honda:
class HondaEngine

{
public void start()
{
System.out.println("Engine has been started.");
}
public void stop()
{
System.out.println("Engine has been stopped.");
}
}

The final step of this Java Composition Program

Step 4:

Fourth we create a class CompositionDemo in which we make an object of Honda class and initialized it:
class CompositionDemo

{
public static void main(String[] args)
{
Honda h = new Honda();
h.setColor("Black");
h.setwheels(2);
h.bikeFeatures();
h.setStart();
}
}

Output- Java Composition - Edureka


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