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.
e.g.,
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()
  {
    room.getNoMirrors();
    ....
   }
 }

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