If no constructors are explicitly defined for the class,
Java supplies a default no-arg constructor which does
nothing more than initialize the fields of the class to
their default value.

if there is no constructor was defined explicitly it will 
define the default constructor...the logic in this was we 
are creating a constructor with out our knowledge while 
creating an object as follows....

        <classname> <obj>=new <classname()>------this is 
the constructor we are defining .............

when ther is no other constructor defined

If there is no constructor in the class , then the compiler 
create a default constructor with access specifier of class.


 class A{ }  - compiler create default constructor for this 
class.