Can you describe the main features of functional interfaces in programming?

Functional interfaces in programming are interfaces with a single abstract method. They are widely used to support functional programming, especially in Java 8 and above.

Main features:

• One abstract method — allows using lambda expressions or method references for implementation.
• @FunctionalInterface annotation — not mandatory, but recommended for explicitly indicating a functional interface and for compiler checks.
• Can contain default and static methods — but only one abstract method.

Example of a functional interface and its usage:

@FunctionalInterface
interface Converter<F, T> {
    T convert(F from);
}

Converter<String, Integer> stringToInteger = Integer::valueOf;
int number = stringToInteger.convert("123");

This simplifies the code, makes it more readable, and supports functional programming style.