Modern Java: Lambdas, Optional, Records and Sealed Classes

import java.util.*;
import java.util.function.*;

public class Lambdas {
    public static void main(String[] args) {
        Function<Integer, Integer> square = x -> x * x;
        Predicate<String> isLong = s -> s.length() > 4;
        Supplier<String> greeting = () -> "hello";
        Consumer<String> printer = s -> System.out.println("LOG: " + s);
        BiFunction<Integer, Integer, Integer> add = Integer::sum;

        System.out.println(square.apply(5));         // 25
        System.out.println(isLong.test("banana"));   // true
        System.out.println(greeting.get());          // hello
        printer.accept("started");                   // LOG: started
        System.out.println(add.apply(2, 3));         // 5

        List<String> names = new ArrayList<>(List.of("Ben", "asha", "Carla"));
        names.sort(String::compareToIgnoreCase);     // a method reference
        System.out.println(names);                   // [asha, Ben, Carla]

        int base = 10;                               // a captured local variable must be effectively final
        Function<Integer, Integer> addBase = n -> n + base;
        System.out.println(addBase.apply(5));        // 15
    }
}

public class Functional {
    @FunctionalInterface
    interface Validator {
        boolean validate(String input);                  // exactly one abstract method
    }

    public static void main(String[] args) {
        Validator notBlank = s -> !s.isBlank();
        System.out.println(notBlank.validate(" "));      // false

        Validator legacy = new Validator() {             // the anonymous class equivalent
            @Override
            public boolean validate(String input) { return !input.isBlank(); }
        };
        System.out.println(legacy.validate("x"));        // true
    }
}

import java.util.*;

public class OptionalDemo {
    public static void main(String[] args) {
        System.out.println(findUser("asha").map(String::toUpperCase).orElse("unknown"));    // ASHA
        System.out.println(findUser("zed").map(String::toUpperCase).orElse("unknown"));     // unknown

        findUser("zed").ifPresentOrElse(
            user -> System.out.println("found " + user),
            () -> System.out.println("nobody"));                                              // nobody

        try {
            findUser("zed").orElseThrow(() -> new NoSuchElementException("no user zed"));
        } catch (NoSuchElementException e) {
            System.out.println(e.getMessage());                                               // no user zed
        }
    }

    static Optional<String> findUser(String name) {
        return name.equals("asha") ? Optional.of("asha") : Optional.empty();
    }
}