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Functional interface in Java 8 What are functional interfaces?

The most important thing to be remembered about functional interface is that it has only one abstract method and not more than this. @ FunctionalI nterface is an annotation that prevents the compiler by making them aware about the fact that there is only one abstract method in the given interface. With the help of functional interfaces Java implements the functional programming in Java. Functional interface enables the usage of ‘Lambda’ expression that eliminates the code.

What is the need of functional interface in Java 8?

Java 8 has launched a new feature called ‘Lambda’ expressions.

It is used for expressing the anonymous classes in Java 8 in short hand notations.  Functional interface is used for passing the functionality to Lambda expressions.  Functional interface help the Lambda expressions to in understanding which method needs to be pick. 

Points to be remembered about functional interface

 More than one abstract method is not at all allowed in a functional interface.  Interface with default or static methods can be a functional interface.

 Comparator interface has two main abstract methods; equals and compare where equal method has same method signature as an object class.

Different types of Lambda expressions 1. Lambda expression without parameters as well as without return type.

2. Lambda expression without parameters but with return type.

3. Lambda expression with parameters & without return type.

4. Lambda expression with parameters as well as with return type.

Why functional interface has only one abstract method?

Functional interface has only one abstract method because during the time of lambda expressions there is only one implementation possible at a time and hence it has only single function contract.

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