Java provides a feature called method
reference in Java 8. Method reference 
is used to refer method of functional interface.
  It is compact and easy form
  of lambda expression. Each time when 
  you are using lambda expression to just
  referring a method, you can replace your
  lambda expression with method reference.

Types of Method References:
- Reference to a static method.  Syntax ==> ContainingClass::staticMethodName  
- Reference to an instance method.  Syntax ==> containingObject::instanceMethodName  
- Reference to a constructor.  Syntax ==> ClassName::new