Functional Interface:

A functional interface is an interface that has only one abstract method. Functional interfaces are the foundation of functional programming in Java and are used extensively with lambda expressions or method references.

To create a functional interface in Java, you can follow these steps:

1. Define an interface: Start by defining a regular interface using the interface keyword. This interface should contain a single abstract method that represents the functionality you want to encapsulate.

2. Add the @FunctionalInterface annotation (optional): Although not strictly required, it's a good practice to add the @FunctionalInterface annotation to indicate that your interface is intended to be a functional interface. This annotation serves as a documentation tool and provides a compile-time check to ensure that the interface satisfies the functional interface contract.

3. Define the abstract method: Inside your interface, declare the abstract method that represents the functionality of your functional interface. This method should not have an implementation (no method body) and should define the input parameters and return type based on your requirements.

Here's an example that demonstrates the creation of a functional interface:

java
@FunctionalInterface

interface MyFunctionalInterface {
void doSomething();
 }

In this example, MyFunctionalInterface is a functional interface that has a single abstract method called doSomething(). The @FunctionalInterface annotation indicates that this interface is intended to be used as a functional interface.

Functional interfaces in Java offer several advantages: Support for functional programming: Functional interfaces enable functional programming paradigms in Java. They allow you to treat functions as first-class citizens, enabling you to pass behavior as arguments, return behavior from methods, and store behavior in variables or data structures. Concise and expressive code: Functional interfaces, combined with lambda expressions or method references, allow you to write code in a more concise and expressive manner. This reduces boilerplate code and makes your code easier to read and understand. Improved code reusability: Functional interfaces promote code reusability by encapsulating behavior in a single abstract method. You can define multiple implementations of the functional interface and reuse them in different parts of your codebase. Flexibility and composability: Functional interfaces provide flexibility and composability by allowing you to combine multiple functions or behaviors using methods like andThen() or compose(). This enables you to create complex behavior by chaining together simpler functions. Leveraging multi-core processors: Functional interfaces, along with the Streams API introduced in Java 8, allow for efficient parallel execution of operations on collections. This enables you to take advantage of multi-core processors and improve performance in certain scenarios. Simplified testing: Functional interfaces make it easier to test your code. Since the behavior of a functional interface is encapsulated in a single method, you can easily write unit tests by providing different implementations of the interface and verifying the expected behavior. Integration with existing APIs: Many libraries and frameworks in Java, such as the Streams API, CompletableFuture, and JavaFX, are designed to work with functional interfaces. By using functional interfaces, you can seamlessly integrate your code with these existing APIs and leverage their functionality.

To determine which one is not a functional interface:

we need to identify an interface that does not satisfy the requirements of a functional interface, which is having only one abstract method. Let's consider the following examples:

  1. Example 1:
java
@FunctionalInterface
interface MyFunctionalInterface { 
void doSomething(); 
default void doSomethingElse() {
// Implementation 
 }
}

In this example, MyFunctionalInterface is still a functional interface even though it has a default method doSomethingElse(). Default methods are allowed in functional interfaces, as long as there is only one abstract method.

  1. Example 2:
java
@FunctionalInterface 
interface MyFunctionalInterface { 
void doSomething(); 
void doSomethingElse();
 }

In this example, MyFunctionalInterface is not a functional interface because it has two abstract methods (doSomething() and doSomethingElse()). To be a functional interface, there should be only one abstract method.

Therefore, the second example is not a functional interface since it violates the requirement of having only one abstract method.


Functional interfaces serve as a foundational concept in functional programming in Java and provide several important uses:

  1. Lambdas and Method References: Functional interfaces enable the use of lambda expressions and method references. These constructs allow you to express behavior concisely and directly in your code, making it more readable and expressive.

  2. Higher-Order Functions: Functional interfaces enable the creation of higher-order functions, which are functions that take other functions as arguments or return functions as results. This allows for powerful composition and abstraction of behavior.

  3. API Design: Functional interfaces are often used when designing APIs that require callback mechanisms or customizable behavior. By defining functional interfaces, API developers can provide flexibility to API users, allowing them to customize behavior by providing implementations of the functional interface.

  4. Stream Operations: Functional interfaces are widely used in stream operations introduced in Java 8. Stream operations allow for declarative and functional-style processing of collections or sequences of data. Functional interfaces such as Predicate, Function, and Consumer are used as parameters in various stream methods.