Optional class

Optionalはnullかnull以外かのどちらの値も格納できるclass
Optional.ofNullable(xxx)はnullならemptyに読み替えて格納する。
array refer時のArrayIndexOutOfBoundsExceptionやNullpointerExceptionを回避してくれるようなものではないので注意

public class Outer {
    public static void main(String[] args) {
        String strArray[] = {"1a","2a",null};
        System.out.println(get(strArray,3));
        System.out.println(get(strArray,2));
        System.out.println(get(strArray,1));
        System.out.println(get(null,1));        
    }
    static <T> Optional<T> get(T[] array, int index) {
        if(array == null) {
            return Optional.empty();
        } else {
            try {
                return Optional.ofNullable(array[index]);
            } catch (ArrayIndexOutOfBoundsException ex) {
                return Optional.empty();
            }
        }
    }
}

Optional.empty
Optional.empty
Optional[2a]
Optional.empty

emptyからget()すると、NoSuchElementException

        System.out.println(Optional.empty().get());

Exception in thread "main" java.util.NoSuchElementException: No value present
	at java.base/java.util.Optional.get(Optional.java:148)
	at com.mycompany.mavenproject1.Outer.main(Outer.java:12)
Command execution failed.

isPresent/isEmptyで空か空以外かわかる

    public static void main(String[] args) {
        Optional<String> op = Optional.of("xxx");
        System.out.println(op.isPresent());
        System.out.println(op.isEmpty());
        op = Optional.empty();
        System.out.println(op.isPresent());
        System.out.println(op.isEmpty());
        }

true
false
false
true