'is a' relationship and a 'has a' relationship in Java

 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.