What are functional interfaces? Which ones do you know? What do they accept and what do they return?

Functional interfaces in Java are interfaces with a single abstract method, which can be used as target types for lambda expressions and method references.

Main functional interfaces from the java.util.function package:

• Function<T, R> — takes an object of type T, returns an object of type R.
• Consumer<T> — takes an object of type T, returns nothing (void).
• Supplier<T> — takes no arguments, returns an object of type T.
• Predicate<T> — takes an object of type T, returns a boolean.
• UnaryOperator<T> — takes and returns an object of type T (a specialization of Function).
• BinaryOperator<T> — takes two objects of type T, returns an object of type T.

Example of using Function:

Function<String, Integer> lengthFunc = s -> s.length();
int len = lengthFunc.apply("hello"); // 5

Thus, functional interfaces allow writing more concise and expressive code using lambdas.