Functional Programming in java

Java 8 introduced a new feature called lambda expressions. Before Java 8 it only followed object oriented programming and its concepts. Now it has allowed to write functional programming as well by passing code as an argument to a method. Functional programming is a hot topic for some other programming languages.  But Java doesn't had any feature like that before java 8.
In this blog I will try to cover following topics:

1. Lambda expression
2. Functional interface
3. Variable capture
4. Method References
5. Default method

Before starting anything, lets come to questions of WHY and then HOW.