item 55 JungHyunLyoo - JAVA-JIKIMI/EFFECTIVE-JAVA3 GitHub Wiki

String str = "hello";

Optional<String> o = Optional.ofNullable(str); // str이 null이면 빈 Optional 객체 반환

public static <E extends Comparable<E>> E max(Collection<E> c) {

    if (c.isEmpty())
        throw new IllegalArgumentException("빈 컬렉션");

    E result = null;

    for (E e : c)
        if (result == null || e.compareTo(result) > 0)
            result = Objects.requireNonNull(e);

    return result;

}

public static <E extends Comparable<E>> Optional<E> max(Collection<E> c) {

    if (c.isEmpty())
        return Optional.empty();//적절한 정정 팩터리 메서드 사용

    E result = null;

    for (E e : c)
        if (result == null || e.compareTo(result) > 0)
            result = Objects.requireNonNull(e);

    return Optional.of(result);//적절한 정정 팩터리 메서드 사용

}

public static <E extends Comparable<E>> Optional<E> max(Collection<E> c) {
    return c.stream().max(Comparator.naturalOrder());
}

String str = max(words).orElse("no words");

Toy myToy = max(toys).orElseThrow(SimpleException::new); 
//예외 팩터리 전달. 실제로 예외가 일어나지 않으면 예외 생성 비용은 들지 않음

Element lastNobleGas = max(Elements.NOBLE_GASES).get();

private static String wontRunThis() { 
    System.out.println("Won't run this"); 
    return "foo"; 
} 
public void optional1() { 
    String o = Optional.of("Hello World!").orElse(wontRunThis()); 
    System.out.println("Result : " + o); 
}

private static String wontRunThis() { 
    System.out.println("Won't run this"); 
    return "foo"; 
} 
public void optional2() { 
    String o = Optional.of("Hello World!").orElseGet(() -> wontRunThis()); 
    System.out.println("Result : " + o); 
}

Optional<ProcessHandle> parentProcess = ph.parent();
System.out.println(
    "부모 PID: " + (parentProcess.isPresent() ?
    String.valueOf(parentProcess.get().pid()) : "N/A")
);

System.out.println("부모 PID: " +
    ph.parent().map(h -> String.valueOf(h.pid())).orElse("N/A"));

streamOfOptionals.filter(Optional::isPresent).map(Optional::get)

streamOfOptionals.flatMap(Optional::stream)