u know that java not supporting multiple inheritance.if u
extend thread class,u can't extend any class which you
required other than thread class.if u are implementing
runnable interface u can extend any class which you required
other than thread class.this is main advantage of
implementing runnable interface.

Many Java programmers feel that classes should be extended 
only when they are enhanced or modified in some way.So,if 
you will not be overriding any of Thread's other methods,it 
is probably best simply to implement Runnable.

imolimen Runnabule inerface and extends Thread functionally 
both are same.But implimenting RunnebleInterface more 
advantage because of there is possibility extends more then 
one class and also we can override run() only once.

if we extend thread then we can't extend other class.
but if we implement runnable interface we can implement as much interface as we can, there is no restriction.

thread call the run method automatically(if called with no-arg).but for runnable we need to call run method.
thread cause problem in multiple inheritance.
but runnable interface don't.

you know that java  does not supporting multiple inheritance
through classes .if u extend Thread class,u can't extend any
class which you required other than thread class.if u are
implementing runnable interface u can extend any class which
you required other than thread class.this is main advantage of
implementing java.lang.Runnable interface.