Can you list and explain the main functional interfaces used in Java?

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

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

• Function<T, R> — takes an object of type T and returns an object of type R.

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

• Consumer<T> — takes an object of type T and returns nothing (performs an action).

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

• Supplier<T> — takes no arguments and returns an object of type T.

  Supplier<Double> randomSupplier = () -> Math.random();
  double val = randomSupplier.get();
  

• Predicate<T> — takes an object of type T and returns a boolean.

  Predicate<String> isEmpty = s -> s.isEmpty();
  boolean result = isEmpty.test(""); // true
  

• UnaryOperator<T> — extends Function, takes and returns an object of the same type T.

• BinaryOperator<T> — takes two objects of type T and returns an object of type T.

These interfaces allow writing concise and expressive code, especially when working with streams and collections.