Java8 - Functional interfaces

Functional interfaces
Java8 has added to it's kitty another concept know as functional interfaces.

So what exactly is an functional interfaces?

• Functional interfaces are interfaces with exactly a single abstract method (SAM). 
• It is represented by the annotation type @FunctionalInterface. 
• This annotation let's the compiler know that the interfaces should adhere to the contract of SAM.

NOTE:

• A couple of things here that this interface can also have other default functions but not another abstract method. 
• These interfaces can also be extended by another interfaces and can be called functional interface as long it does not have another abstract method.
• Also compiler will treat all SAM interfaces as functional regardless if the annotation is present or not.

So how do we use this interfaces?
These interfaces can be represented using

• Lamda expressions 
• Method reference and 
• Constructor reference.(Lamda expressions another important concept I have covered in another blog article)

Examples of functional interfaces
in Java8 are Runnable, Comparator, ActionListener, Callable etc.