2022.12.1 / Optional, Stream 연습

Optional

 

Optional<String> empty = Optional.empty();
System.out.println(empty.isPresent()); // false

Optional<String> empty2 = Optional.of("assertNull");
System.out.println(empty2.isPresent()); // true

Optional.of(null); // 에려남

Optional<String> empty3 = Optional.ofNullable(null);
System.out.println(empty3.isPresent()); // false

 

 

• ifPresent() : Optional 에서 꺼낸 객체가 존재한다면, 구문 수행

String name = null;
Optional<String> opt = Optional.ofNullable(name);

opt.ifPresent(n -> System.out.println(n.length)); // 에러남

 

• orElse() : Optional에서 꺼낸 객체가 존재한다면 꺼내고, 그렇지 않다면? orElse의 인자값을 반환

System.out.println(Optional.ofNullable(null).orElse("냐옹")); // 냐옹

System.out.println(Optional.ofNullable("Hey!").orElse("냐옹")); // Hey!

 

• orElseGet() : orElse()와 비슷하지만, 인자값으로 람다 표현식의 결과값을 출력

System.out.println(Optional.ofNullable(null).orElseGet(String::new)); // ""
System.out.println(Optional.ofNullable(null).orElseGet(() -> "냐옹")); // 냐옹
System.out.println(Optional.ofNullable("Hey!").orElseGet(() -> "냐옹")); // Hey!

 

• orElseThrow() : Optional에서 꺼낸 객체가 존재한다면 꺼내고, 그렇지 않다면? Exception 던지기

String nullName = null;
String name = Optional.ofNullable(nullName).orElseThrow(
        IllegalArgumentException::new);

 

 

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Stream

 

 

• filter + anyElement / firstElement

List<String> elements = Stream.of("a", "b", "c")
                .filter(element -> element.contains("b"))
                .collect(Collectors.toList()); // [b]

Optional<String> anyElement = elements.stream().findAny();
System.out.println(anyElement.orElse("암것두 없어")); // b

Optional<String> firstElement = elements.stream().findFirst();
System.out.println(firstElement.orElse("한개두 없어")); //b

 

• N개의 중개 연산자

Stream<String> onceModifiedStream = Stream.of("abcd", "bbcd", "cbcd");

        Stream<String> twiceModifiedStream = onceModifiedStream
                .skip(1)
                .map(element -> element.substring(0, 3));

        System.out.println(twiceModifiedStream); // java.util.stream.ReferencePipeline$3@7cef4e59
        System.out.println(twiceModifiedStream.collect(Collectors.toList())); // [bbc, cbc]

 

• map 활용

class Product {
    private int age;
    private String name;

    public Product(int age, String name) {
        this.age = age;
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public String getName() {
        return name;
    }
}

public class JavaPracApplication {
    public static void main(String[] args) {

        List<Product> productList = Arrays.asList(
                new Product(23, "potatoes"),
                new Product(14, "orange"), new Product(13, "lemon"),
                new Product(23, "bread"), new Product(13, "sugar")
        );

        List<String> collectorCollection = productList.stream()
                .map(Product::getName)
                .collect(Collectors.toList());

        System.out.println(collectorCollection); // [potatoes, orange, lemon, bread, sugar]

        double averageAge = productList.stream()
                .collect(Collectors.averagingInt(Product::getAge));
        System.out.println("나이 평균 : " + averageAge); // 나이 평균 : 17.2

        int summingAge = productList.stream().mapToInt(Product::getAge).sum();
        System.out.println("나이 총합 : " + summingAge); // 나이 총합 : 86

    }
}