Step1 (#2323)

* [feat] 스트림, 람다, Optional

* comment

---------

Co-authored-by: sophia.song(송지현)/kakao <[email protected]>

Author: jihyun-s

src/main/java/nextstep/fp/Conditional.java

@@ -0,0 +1,6 @@
+package nextstep.fp;
+
+@FunctionalInterface
+public interface Conditional {
+  boolean check(Integer number);
+}

src/main/java/nextstep/fp/Lambda.java

@@ -27,30 +27,22 @@ public void run() {
     }
 
     public static int sumAll(List<Integer> numbers) {
-        int total = 0;
-        for (int number : numbers) {
-            total += number;
-        }
-        return total;
+        return sumAll(numbers, n -> true);
     }
 
     public static int sumAllEven(List<Integer> numbers) {
-        int total = 0;
-        for (int number : numbers) {
-            if (number % 2 == 0) {
-                total += number;
-            }
-        }
-        return total;
+        return sumAll(numbers, n -> n % 2 == 0);
     }
 
     public static int sumAllOverThree(List<Integer> numbers) {
-        int total = 0;
-        for (int number : numbers) {
-            if (number > 3) {
-                total += number;
-            }
-        }
-        return total;
+        return sumAll(numbers, n -> n > 3);
     }
+
+    public static int sumAll(List<Integer> numbers, Conditional condition) {
+        return numbers.stream()
+            .filter(condition::check)
+            .mapToInt(Integer::intValue)
+            .sum();
+    }
+
 }

src/main/java/nextstep/fp/StreamStudy.java

@@ -27,7 +27,13 @@ public static void printLongestWordTop100() throws IOException {
                 .get("src/main/resources/fp/war-and-peace.txt")), StandardCharsets.UTF_8);
         List<String> words = Arrays.asList(contents.split("[\\P{L}]+"));
 
-        // TODO 이 부분에 구현한다.
+        Arrays.stream(contents.split("[\\P{L}]+"))
+            .map(String::toLowerCase)
+            .filter(word -> word.length() > 12)
+            .distinct()
+            .sorted((w1, w2) -> Integer.compare(w2.length(), w1.length()))
+            .limit(100)
+            .forEach(System.out::println);
     }
 
     public static List<Integer> doubleNumbers(List<Integer> numbers) {
@@ -39,6 +45,9 @@ public static long sumAll(List<Integer> numbers) {
     }
 
     public static long sumOverThreeAndDouble(List<Integer> numbers) {
-        return 0;
+        return numbers.stream()
+            .filter(n -> n > 3)
+            .map(n -> n * 2)
+            .reduce(0, Integer::sum);
     }
-}
+}

src/main/java/nextstep/optional/Expression.java

@@ -1,5 +1,7 @@
 package nextstep.optional;
 
+import java.util.Arrays;
+
 enum Expression {
     PLUS("+"), MINUS("-"), TIMES("*"), DIVIDE("/");
 
@@ -14,12 +16,10 @@ private static boolean matchExpression(Expression e, String expression) {
     }
 
     static Expression of(String expression) {
-        for (Expression v : values()) {
-            if (matchExpression(v, expression)) {
-                return v;
-            }
-        }
-
-        throw new IllegalArgumentException(String.format("%s는 사칙연산에 해당하지 않는 표현식입니다.", expression));
+        return Arrays.stream(values())
+            .filter(e -> matchExpression(e, expression))
+            .findFirst()
+            .orElseThrow(() ->
+                new IllegalArgumentException(String.format("%s는 사칙연산에 해당하지 않는 표현식입니다.", expression)));
     }
 }

src/main/java/nextstep/optional/User.java

@@ -1,5 +1,7 @@
 package nextstep.optional;
 
+import java.util.Optional;
+
 public class User {
     private String name;
     private Integer age;
@@ -33,7 +35,10 @@ public static boolean ageIsInRange1(User user) {
     }
 
     public static boolean ageIsInRange2(User user) {
-        return false;
+        return Optional.ofNullable(user)
+            .map(User::getAge)
+            .filter(age -> age >= 30 && age <= 45)
+            .isPresent();
     }
 
     @Override

src/main/java/nextstep/optional/Users.java

@@ -13,11 +13,9 @@ public class Users {
             new User("honux", 45));
 
     User getUser(String name) {
-        for (User user : users) {
-            if (user.matchName(name)) {
-                return user;
-            }
-        }
-        return DEFAULT_USER;
+        return users.stream()
+            .filter(user -> user.matchName(name))
+            .findFirst()
+            .orElse(DEFAULT_USER);
     }
 }

src/test/java/nextstep/fp/CarTest.java

@@ -8,24 +8,14 @@ public class CarTest {
     @Test
     public void 이동() {
         Car car = new Car("pobi", 0);
-        Car actual = car.move(new MoveStrategy() {
-            @Override
-            public boolean isMovable() {
-                return true;
-            }
-        });
+        Car actual = car.move(() -> true);
         assertThat(actual).isEqualTo(new Car("pobi", 1));
     }
 
     @Test
     public void 정지() {
         Car car = new Car("pobi", 0);
-        Car actual = car.move(new MoveStrategy() {
-            @Override
-            public boolean isMovable() {
-                return false;
-            }
-        });
+        Car actual = car.move(() -> false);
         assertThat(actual).isEqualTo(new Car("pobi", 0));
     }
 }