Explain functional interfaces.

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A functional interface in Java is an interface that contains only one abstract method. It can have multiple default or static methods, but only one method must be abstract. Functional interfaces are the foundation for lambda expressions and method references in Java.

Declaration:

You can use the @FunctionalInterface annotation to declare it. This is optional but helps prevent accidental addition of extra abstract methods.

Example:

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

You can use a lambda expression with this interface:

java
Greeting greet = () -> System.out.println("Hello!");
greet.sayHello();

Common Functional Interfaces in java.util.function package:

• Predicate<T> – returns a boolean (used for filtering).

• Function<T, R> – takes an input and returns a result.

• Consumer<T> – takes input but returns nothing.

• Supplier<T> – returns a value, takes no input.

Why Use Functional Interfaces?

• Enables functional programming in Java.

• Makes code cleaner and more concise.

• Works well with Streams and Lambda expressions.

Key Points:

• Only one abstract method allowed.

• Can have multiple default or static methods.

• Supports lambda expressions and method references.

In short, functional interfaces make it possible to write more expressive and concise code using lambdas in Java 8 and beyond.

Read more: 

What is the Stream API?

What are lambda expressions?

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