java8 : optional

java8to11/src/optional/App.java

@@ -0,0 +1,69 @@
+package optional;
+
+import java.time.Duration;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+
+public class App {
+
+    private static OnlineClass createNewClass() {
+        System.out.println("create new class");
+        return new OnlineClass(10, "New class", false);
+    }
+    public static void main(String[] args) {
+        List<OnlineClass> springClasses = new ArrayList<>();
+        springClasses.add(new OnlineClass(1, "spring boot", true));
+        springClasses.add(new OnlineClass(5, "rest api development", false));
+
+        Optional<OnlineClass> optional = springClasses.stream()
+                .filter(oc -> oc.getTitle().startsWith("spring"))
+                .findFirst();
+
+        // flatMap
+        Optional<Progress> progress = optional.flatMap(OnlineClass::getProgress);
+        Optional<Optional<Progress>> progress1 = optional.map(OnlineClass::getProgress);
+        Optional<Progress> progress2 = progress1.orElse(Optional.empty());
+
+//        // map
+//        Optional<Optional<Progress>> progress = optional.map(OnlineClass::getProgress);
+//        Optional<Progress> progress1 = progress.orElseThrow();
+//        Progress progress2 = progress1.orElseThrow();
+//        System.out.println(progress2);
+
+//        Optional<Integer> integer = optional.map(OnlineClass::getId);
+//        System.out.println(integer.isPresent());
+
+//        //filter
+//        Optional<OnlineClass> onlineClass = optional.filter(OnlineClass::isClosed);
+//        System.out.println(onlineClass.isPresent());
+
+//        // orElseThrow
+//        OnlineClass onlineClass = optional.orElseThrow(IllegalArgumentException::new); // 없으면 error
+//        System.out.println(onlineClass.getTitle());
+
+//        // orElseGet
+//        OnlineClass onlineClass = optional.orElseGet(App::createNewClass); // 있으면 실행 안함
+//        System.out.println(onlineClass.getTitle());
+
+//        // orElse
+//        OnlineClass onlineClass = optional.orElse(createNewClass()); // 있으면 꺼내고 없으면 ~~ 리턴 -> 있어도 ~~ 를 실행한다.
+//        System.out.println(onlineClass.getTitle());
+
+//        // ifPresent, get
+//        optional.ifPresent(oc -> System.out.println(oc.getTitle())); // 있으면 ~~ 실행
+
+//        boolean present = optional.isPresent();
+//        System.out.println(present);
+//        OnlineClass onlineClass = optional.get(); // optional 의 내용 꺼내기
+//        System.out.println(onlineClass.getTitle());
+
+//        // null 체크는 실수 가능성이 높다
+//        // 또한 null 을 리턴하는 자체가 문제
+//        OnlineClass spring_boot = new OnlineClass(1, "spring boot", true);
+//        Progress progress = spring_boot.getProgress();
+//        if (progress != null) {
+//            System.out.println(progress.getStudyDuration());
+//        }
+    }
+}

java8to11/src/optional/OnlineClass.java

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+package optional;
+
+import java.util.Optional;
+
+public class OnlineClass {
+    private Integer id;
+    private String title;
+    private boolean closed;
+    private Progress progress;
+
+    public OnlineClass(Integer id, String title, boolean closed) {
+        this.id = id;
+        this.title = title;
+        this.closed = closed;
+    }
+
+    public Optional<Progress> getProgress() {
+        return Optional.ofNullable(progress);
+    }
+
+    public void setProgress(Progress progress) {
+        this.progress = progress;
+    }
+
+    public Integer getId() {
+        return id;
+    }
+
+    public void setId(Integer id) {
+        this.id = id;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public void setTitle(String title) {
+        this.title = title;
+    }
+
+    public boolean isClosed() {
+        return closed;
+    }
+
+    public void setClosed(boolean closed) {
+        this.closed = closed;
+    }
+}

java8to11/src/optional/Progress.java

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+package optional;
+
+import java.time.Duration;
+
+public class Progress {
+
+    private Duration studyDuration;
+
+    private boolean finished;
+
+    public Duration getStudyDuration() {
+        return studyDuration;
+    }
+
+    public void setStudyDuration(Duration studyDuration) {
+        this.studyDuration = studyDuration;
+    }
+}