java_main.md

Java

Optional

use Optional as a return type but never als a method - argument;

ifPresent()

   Optional<String> opt = Optional.of("baeldung");
    opt.ifPresent(name -> System.out.println(name.length()));

orElseGet()

 String nullName = null;
    String name = Optional.ofNullable(nullName).orElseGet(() -> "john");
    assertEquals("john", name);

String result = Optional.ofNullable(text).orElseGet(this::getMyDefault);

throw Exception

 String nullName = null;
 // throws a NoSuchElementException
    String name = Optional.ofNullable(nullName).orElseThrow();

filter and map

Optional.ofNullable(modem2)
       .map(Modem::getPrice)
       .filter(p -> p >= 10)
       .filter(p -> p <= 15)
       .isPresent(price->doSomeThing(price));

    List<String> companyNames = Arrays.asList(
      "paypal", "oracle", "", "microsoft", "", "apple");
    Optional<List<String>> listOptional = Optional.of(companyNames);

    int size = listOptional
      .map(List::size)
      .orElse(0);

ifPresentOrElse(,)

// returns empty Optional
Optional<Guitarist> lookupResult = guitaristService.findByLastName("Page");
lookupResult.ifPresentOrElse(
        guitarist -> System.out.println(guitarist.getSignatureSong()),
        () -> System.out.println("Guitarist not found!")
);