Which annotation is used to create functional interface in Java 8?

Which annotation is used to create functional interface in Java 8?

I'm happy to respond in English for you!

In Java 8, the annotation used to create a functional interface is @FunctionalInterface. This annotation is used on an interface that can be implemented with a single abstract method (SAM). A SAM is a method that takes no arguments and returns a value.

The @FunctionalInterface annotation was introduced in Java 8 to support lambda expressions. A functional interface is an interface that has a single abstract method, which makes it easier to create lambda expressions that can be used as function objects.

Here's an example of how you might use the @FunctionalInterface annotation:

@FunctionalInterface
interface Greeting {
String sayHello(String name);
}

In this example, the Greeting interface has a single abstract method sayHello, which takes a String argument and returns a String. You can implement this interface with a lambda expression like this:

Greeting greeting = (name) -> "Hello, " + name;
System.out.println(greeting.sayHello("John")); // prints "Hello, John"

The @FunctionalInterface annotation is not required to use functional interfaces in Java. You can still create and use functional interfaces without it. However, using the annotation helps you catch errors at compile-time rather than runtime.

When you use the @FunctionalInterface annotation, the compiler will check that your interface only has a single abstract method. If your interface has more than one abstract method, or if it doesn't have any abstract methods at all, the compiler will generate an error.

In summary, the @FunctionalInterface annotation is used to mark an interface as functional in Java 8, which means it can be implemented with a single abstract method (SAM) using lambda expressions. It's not required to use functional interfaces, but it helps you catch errors at compile-time and makes your code more readable.

Why functional interfaces in Java 8 have one abstract method?

In Java 8, functional interfaces are a special type of interface that can have exactly one abstract method. This design choice has several implications and benefits.

Firstly, the reason for having only one abstract method is to ensure that instances of these interfaces can be used as lambda expressions or method references. In other words, functional interfaces provide a way to convert a piece of code into a first-class citizen that can be passed around as an argument to methods, returned from methods, and stored in data structures.

When you define an interface with multiple abstract methods, it's difficult to determine which method should be used when the interface is implemented using a lambda expression or method reference. This ambiguity arises because lambda expressions and method references are essentially single-method abstractions that can represent any one of the abstract methods defined in the interface.

By requiring functional interfaces to have only one abstract method, Java ensures that these interfaces can be safely used as lambda expressions or method references without causing ambiguity or confusion. When you define a functional interface with only one abstract method, it's clear which method should be invoked when the interface is implemented using a lambda expression or method reference.

Another benefit of having only one abstract method in functional interfaces is that it allows for more efficient compilation and execution. When a functional interface is implemented using a lambda expression, the compiler can statically determine which abstract method to invoke based on the context in which the interface is used. This reduces the overhead associated with dynamic method dispatch, making your code run faster and more efficiently.

In addition, the one-abstract-method restriction helps to avoid common pitfalls and edge cases that might arise when working with multiple abstract methods. For instance, if a functional interface has multiple abstract methods, it can be tricky to determine which method should be invoked first, last, or at all. By limiting interfaces to a single abstract method, Java eliminates these complexities and makes it easier for developers to work with functional programming concepts.

Finally, having only one abstract method in functional interfaces aligns well with the spirit of functional programming, which emphasizes working with individual functions that perform specific tasks rather than complex objects with multiple methods. This design choice reflects the idea that a function should do one thing and do it well, rather than trying to be a jack-of-all-trades.

In summary, Java 8's requirement for functional interfaces to have only one abstract method is a deliberate design choice aimed at ensuring safe and efficient use of lambda expressions and method references. It eliminates ambiguity and complexity, makes code run faster and more efficiently, and aligns with the principles of functional programming.