What functional interfaces do you know? Can you give examples?

In Java, functional interfaces are interfaces with a single abstract method that can be used with lambda expressions and method references.

Main examples from the java.util.function package:

• Predicate<T> — takes an object of type T, returns boolean. Example: checking if a number is positive.

  Predicate<Integer> isPositive = x -> x > 0;
  System.out.println(isPositive.test(5)); // true
  

• Function<T, R> — takes T, returns R. Example: converting a string to its length.

  Function<String, Integer> length = s -> s.length();
  System.out.println(length.apply("Hello")); // 5
  

• Consumer<T> — takes T, returns nothing. Example: printing a string to the console.

  Consumer<String> printer = s -> System.out.println(s);
  printer.accept("Hello");
  

• Supplier<T> — takes no arguments, returns T. Example: generating a random number.

  Supplier<Double> random = () -> Math.random();
  System.out.println(random.get());
  

• UnaryOperator<T> — takes and returns the same type T. Example: doubling a number.

  UnaryOperator<Integer> doubler = x -> x * 2;
  System.out.println(doubler.apply(4)); // 8
  

• BinaryOperator<T> — takes two arguments of type T and returns T. Example: sum of two numbers.

  BinaryOperator<Integer> adder = (a, b) -> a + b;
  System.out.println(adder.apply(3, 7)); // 10
  

These interfaces are widely used for functional programming, collection processing, and streams.