Java8 - Functional Interface

• An interface which only declare one abstract methods is a functional interface. Interfaces with single abstract also called SAM (Single Abstract Method) interface.
• @FunctionalInterface annotation can be used to declare an interface functional.
• If we put more than one abstract method and @FunctionalInterface in a same interface, it will give a compile time error.
• If we are not putting @FunctionalInterface annotation than also any interface with single abstract is a functional interface.
• Other than a single abstract method, other methods like Default and static methods can be declared in functional interface.
• If an functional interface override any Object class methods, than also it will be a valid functional interface. For example Comparator interface declared two methods compare()  and equals(), but its a functional interface because equals() method is overrided from Object class.
• Major benefit of functional interface is that functional interfaces can be instantiated by Lambda Expressions, which are simple and short than anonymous classes. I will create a new post for Lambda Expressions separately. 

Example of valid functional interfaces -  

Example 1 -

@FunctionalInterface

public interface ValidFunctionalInterface1 {

public void abstractMethod1();

}

Example 2 -

@FunctionalInterface

public interface ValidFunctionalInterface2 {

public void abstractMethod1();

public void defaultMethod1(){

System.out.println("This is a default method..")

}

}

Example of invalid functional interfaces -

@FunctionalInterface

public interface InvalidFunctionalInterface1 {

public void abstractMethod1();

public void abstractMethod2();

}

Above interface will thow a compile time error, InvalidFunctionalInterface1 is not a functional interface.

Example of valid but not functional interface -

public interface InvalidFunctionalInterface1 {

public void abstractMethod1();

public void abstractMethod2();

}

I hope Functional Interfaces concept is clear now. If you have any doubts please put in comments below. 

Check below links also-
Java8 - Default and static methods in Interface
How to create Immutable classes in JAVA
Method overloading versus method overriding in Java
Difference between Interface and Abstract class
Difference between Abstraction and Encapsulation

Difference between Comparable and Comparator in Java