Java Optional

Define

empty()

Optional<String> optStrA = Optional.empty();
System.out.println(optStrA);

Optional.empty

of()

Optional<Integer> optIntA = Optional.of(1);
Optional<Integer> optIntB = Optional.of(null);
System.out.println(optIntA);
System.out.println(optIntB);

Optional[1]
NullPointerException

ofNullable()

Optional<Integer> optIntA = Optional.ofNullable(null);
System.out.println(optIntA);

Optional.empty

Call

get()

Optional<Integer> optIntA = Optional.of(1);
Optional<Integer> optIntB = Optional.ofNullable(null);
System.out.println(optIntA.get());
System.out.println(optIntB.get()); 

1
NoSuchElementException

orElse()

• orElse(T other)
  • null일 경우, default 값을 설정

Optional<Integer> optIntA = Optional.of(1);
Optional<Integer> optIntB = Optional.ofNullable(null);
System.out.println(optIntA.orElse(0));
System.out.println(optIntB.orElse(0));

1
0

orElseGet()

• orElseGet(Supplier<? extends T> other)
  • null일 경우, 함수인자(Supplier)를 통해 default 값을 설정

Optional<Integer> optIntA = Optional.of(1);
Optional<Integer> optIntB = Optional.ofNullable(null);

Supplier<Integer> supplier = new Supplier<Integer>() {
    @Override
    public Integer get() {
        return 0;
    }
};
System.out.println(optIntA.orElseGet(supplier));
System.out.println(optIntB.orElseGet(supplier)); 

1
0

orElseThrow()

• orElseThrow(Supplier<? extends X> exceptionSupplier)
  • null일 경우, 함수인자(Supplier)를 통해 예외 반환

Optional<Integer> optIntA = Optional.of(1);
Optional<Integer> optIntB = Optional.ofNullable(null);

Supplier<Exception> supplier = new Supplier<Exception>() {
    @Override
    public Exception get() {
        return new Exception("orElseThrowException");
    }
};

try {
    System.out.println(optIntA.orElseThrow(supplier));
} catch (Exception e) {
    System.out.println(e.getMessage());
}

try {
    System.out.println(optIntB.orElseThrow(supplier));
} catch (Exception e) {
    System.out.println(e.getMessage());
}

1
orElseThrowException

isPresent()

• [1] ifPresent()
  • return -> boolean
• [2] ifPresent(Consumer<? super T> consumer)

Optional<Integer> optIntA = Optional.of(1);
Optional<Integer> optIntB = Optional.ofNullable(null);
System.out.println(optIntA.isPresent());
System.out.println(optIntB.isPresent());

true
false

Optional<String> optString = Optional.of("hello");
optString.ifPresent(str -> {
    System.out.println("length : " + str.length());
});

length : 5

Stream

Optional 객체는 원소가 1개인 Stream과 같이 사용 가능

map()

• Optional<U> map(Function<? super T, Optional<U» mapper)
  • return Optional

int length = Optional.ofNullable("hello")
    .map(String::trim)
    .map(String::length)
    .orElse(0);

5

flatMap()

• Optional<U> flatMap(Function<? super T, Optional<U» mapper)
  • return Optional.get()

Optional<String> optString = Optional.of("hello");

// map
System.out.println(optString.map(str->str));
System.out.println(optString.map(str->Optional.of(str)));

// flatMap
System.out.println(optString.flatMap(str->str)); // * Compile Exception
System.out.println(optString.flatMap(str->Optional.of(str)));

Optional[hello]
Optional[Optional[hello]]

Optional[hello]

filter()

int length = Optional.ofNullable("hello")
    .filter(o -> o.length() > 5) // * Optional.empty
    .map(String::length)
    .orElse(0);

0

Customizing

getAsOptional()

• 리스트중 특정 인덱스 항목을 요청할 때

public static <T> Optional<T> getAsOptional(List<T> list, int index) {
    try {
        return Optional.of(list.get(index));
    } catch (ArrayIndexOutOfBoundsException e) {
        return Optional.empty();
    }
}

참고

• https://www.daleseo.com/java8-optional-after/