An interface that contains no methods. Eg: Serializable,
Cloneable, SingleThreadModel etc. It is used to just mark
java classes that support certain capability.

this is one type on null interface, it doesn't contain any 
methods

Marker interface is an interface which gives a mark to the
runtime environment. This gives a mark that "I am going to
happen". It contains no methods.

markble interface does not contains any methods,but 
implementing of these interfaces will be recognise  by Jvm 
then it will decide that "this class is going do 
something".for example for cloneable interface implements 
by some class x which consists of a,b variable ,But I need 
to interchange instance variables a to b and b to a (pass 
by value)of class x and i want again the same values for a 
and b.At this situations we can implement cloneable 
interface.Like this markble interface are very useful as my 
knowledge........