> For the complete documentation index, see [llms.txt](https://www.pranaypourkar.co.in/the-programmers-guide/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.pranaypourkar.co.in/the-programmers-guide/java/java-basics/basics/abstraction/interface/functional-interfaces.md).

# Functional Interfaces

## **About**

* A **functional interface** is an interface that has exactly one abstract method. Functional interfaces are ideal for use with **lambda expressions** and **method references** in Java 8 and beyond.
* The `@FunctionalInterface` annotation is used to indicate a functional interface, but it is optional. When annotation is provided, the compiler ensures that a functional interface contains only one abstract method.
* Examples: `Runnable`, `Callable`, `Comparator`, and custom single-method interfaces.
* **Lambda Expressions**: Since Java 8, lambdas are used to instantiate functional interfaces, which greatly simplifies code and improves readability.

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

public class Main {
    public static void main(String[] args) {
        Greeting greeting = name -> System.out.println("Hello, " + name);
        greeting.sayHello("Alice");
    }
}
```

```java
// Compilation error - Multiple non-overriding abstract methods found in interface test. SomeMain. Greeting
@FunctionalInterface
interface Greeting {
    void sayHello(String name);
    String sayBye();
}
```

For more details, refer to the below page -

{% content-ref url="/pages/kABXL8XaFAiPyyREJjE0" %}
[Functional Interfaces](/the-programmers-guide/java/java-concepts/functional-programming/functional-interfaces.md)
{% endcontent-ref %}


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