What is the difference between a standard interface and a functional interface in programming?

The standard interface in programming is an interface that can contain multiple methods and describes a set of operations that a class or object must implement.

A functional interface is an interface with exactly one abstract method. In Java, starting from version 8, functional interfaces are widely used for lambda expressions and method references.

Main differences:

• Number of methods: A standard interface can have many methods, while a functional interface has only one abstract method.
• Usage: Functional interfaces allow the use of lambda expressions, which simplifies the code and makes it more expressive.

Example of a functional interface in Java:

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

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

Thus, a functional interface is a special case of a standard interface designed for functional programming.