Functional Interface Introduction

• Java 8 introduced a new terminology called Functional Interface.
• An interface having only single abstract method is called a Functional Interface.
• Main use of Functional interface is that the implementation of the one abstract method
• can be passed as a Lamda expression.
• Functional interface provide behaviour parameterization feature.
• Java 8 has provided @FunctionalInterface, this annotation can be used on any interface
• which you want explicitly treated as functional interface. 
• Then compiler will give compile time error if the annotated interface does not
• satisfy the functional interface condition.
• Lamda expression can be applied to interface having only one abstract method.

Examples of existing Functional Interfaces

@FunctionalInterface
public interface Runnable{
public abstract void run();
}

@FunctionalInterface
public interface Callable<V>{
public V call() throws Exception;
}

@FunctionalInterface
public interface Predicate<T>{
boolean test(T t);
}

User defined examples

public interface Word{
public void printDetail();
}

@FunctionalInterface
public interface Forest{
public void printAnimals();
}