Functional Interfaces In Java

What to do with it?

A functional interface is an interface that contains only one abstract method. They can have only one functionality to exhibit. From Java 8 onwards, lambda expressions can be used to represent the instance of a functional interface. A functional interface can have any number of default methods. Runnable , ActionListener , Comparable are some of the examples of functional interfaces.

Before Java 8, we had to create anonymous inner class objects or implement these interfaces.

// Java program to demonstrate functional interface
class Test
{
public static void main(String args[])
{
// create anonymous inner class object
new Thread(new Runnable()
{
@Override
public void run()
{
System.out.println("New thread created");
}
}).start();
}
}
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Java 8 onwards, we can assign lambda expression to its functional interface object like this:

// Java program to demonstrate Implementation of
// functional interface using lambda expressions
class Test
{
public static void main(String args[])
{
// lambda expression to create the object
new Thread(()->
{System.out.println("New thread created");}).start();
}
}

@FunctionalInterface Annotation

@FunctionalInterface annotation is used to ensure that the functional interface can’t have more than one abstract method. In case more than one abstract methods are present, the compiler flags an ‘Unexpected @FunctionalInterface annotation’ message. However, it is not mandatory to use this annotation.

// Java program to demonstrate lamda expressions to implement
// a user defined functional interface.
@FunctionalInterface
interface Square
{
int calculate(int x);
}
class Test
{
public static void main(String args[])
{
int a = 5;
// lambda expression to define the calculate method
Square s = (int x)->x*x;
// parameter passed and return type must be
// same as defined in the prototype
int ans = s.calculate(a);
System.out.println(ans);
}
}

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@Override

public void run()

{

System.out.println("New thread created");

}

}).start();

}

}

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