Thursday, January 14, 2010

what is 'is a' and 'has a' relationship:

The is a relationship is expressed with inheritance and has a relationship is expressed with composition.

Both inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for code reuse are class inheritance and object composition.
is a --- House is a Building
class Building {

class House extends Building {

has a -- House has a bathroom
class House
  Bathroom room = new Bathroom() ;
  public void getTotMirrors()
Inheritance Vs Composition
Inheritance is uni-directional. For example House is a Building. But Building is not a House. Inheritance uses
extends key word. Composition: is used when House has a Bathroom. It is incorrect to say House is a Bathroom.
Composition simply means using instance variables that refer to other objects. The class House will have an instance variable, 
which refers to a Bathroom object.

Article taken from Oracle Magazine march 2008 issue

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