1️⃣ 자바8 스트림, 람다, Optional

src/main/java/nextstep/fp/Condition.java

@@ -0,0 +1,5 @@
+package nextstep.fp;
+
+public interface Condition {
+    boolean condition(Integer number);
+}

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

@@ -4,53 +4,51 @@
 
 public class Lambda {
     public static void printAllOld(List<Integer> numbers) {
-        System.out.println("printAllOld");
-
-        for (int number : numbers) {
-            System.out.println(number);
-        }
+        print("printAllOld");
+        numbers.forEach(System.out::println);
     }
 
     public static void printAllLambda(List<Integer> numbers) {
-        System.out.println("printAllLambda");
-
+        print("printAllLambda");
         numbers.forEach(System.out::println);
     }
 
     public static void runThread() {
-        new Thread(new Runnable() {
-            @Override
-            public void run() {
-                System.out.println("Hello from thread");
-            }
-        }).start();
+        new Thread(() -> print("Hello from thread")).start();
     }
 
+
     public static int sumAll(List<Integer> numbers) {
-        int total = 0;
-        for (int number : numbers) {
-            total += number;
-        }
-        return total;
+        return sumAll(numbers, Lambda::getAll);
     }
 
     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, Lambda::getEven);
     }
 
     public static int sumAllOverThree(List<Integer> numbers) {
-        int total = 0;
-        for (int number : numbers) {
-            if (number > 3) {
-                total += number;
-            }
-        }
-        return total;
+        return sumAll(numbers, Lambda::getOverThree);
+    }
+
+    private static int sumAll(List<Integer> numbers, Condition condition) {
+        return numbers.stream()
+            .filter(condition::condition)
+            .reduce(Integer::sum).orElse(0);
+    }
+
+    private static void print(String Hello_from_thread) {
+        System.out.println(Hello_from_thread);
+    }
+
+    private static boolean getAll(Integer integer) {
+        return true;
+    }
+
+    private static boolean getEven(Integer integer) {
+        return integer % 2 == 0;
+    }
+
+    private static boolean getOverThree(Integer integer) {
+        return integer > 3;
     }
 }

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

@@ -4,15 +4,20 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Paths;
+import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Comparator;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 
 public class StreamStudy {
 
     public static long countWords() throws IOException {
         String contents = new String(Files.readAllBytes(Paths
-                .get("src/main/resources/fp/war-and-peace.txt")), StandardCharsets.UTF_8);
+            .get("src/main/resources/fp/war-and-peace.txt")), StandardCharsets.UTF_8);
         List<String> words = Arrays.asList(contents.split("[\\P{L}]+"));
 
         long count = 0;
@@ -24,21 +29,40 @@ public static long countWords() throws IOException {
 
     public static void printLongestWordTop100() throws IOException {
         String contents = new String(Files.readAllBytes(Paths
-                .get("src/main/resources/fp/war-and-peace.txt")), StandardCharsets.UTF_8);
+            .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.
+
+        Map<String, Integer> wordMap = new HashMap<>();
+
+        for (String word : words) {
+            if (!wordMap.containsKey(word)) {
+                wordMap.put(word, 0);
+            }
+            wordMap.put(word, wordMap.get(word) + 1);
+        }
+
+        List<String> collect = wordMap.entrySet()
+            .stream().filter(entry -> entry.getValue() > 12)
+            .sorted(Comparator.comparing(entry -> - entry.getValue()))
+            .map(entry -> entry.getKey().toLowerCase())
+            .distinct()
+            .collect(Collectors.toList());
+
+        List<String> subList = new ArrayList<>(collect.subList(0, 100));
+        System.out.println(subList);
+        System.out.println(subList.size());
     }
 
     public static List<Integer> doubleNumbers(List<Integer> numbers) {
         return numbers.stream().map(x -> 2 * x).collect(Collectors.toList());
     }
 
     public static long sumAll(List<Integer> numbers) {
-        return numbers.stream().reduce(0, (x, y) -> x + y);
+        return numbers.stream().reduce(0, Integer::sum);
     }
 
     public static long sumOverThreeAndDouble(List<Integer> numbers) {
-        return 0;
+        return Lambda.sumAllOverThree(numbers) * 2L;
     }
-}
+}

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 +16,11 @@ 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(expression1 -> matchExpression(expression1, 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;
@@ -25,15 +27,15 @@ public static boolean ageIsInRange1(User user) {
         boolean isInRange = false;
 
         if (user != null && user.getAge() != null
-                && (user.getAge() >= 30
-                && user.getAge() <= 45)) {
+            && (user.getAge() >= 30
+            && user.getAge() <= 45)) {
             isInRange = true;
         }
         return isInRange;
     }
 
     public static boolean ageIsInRange2(User user) {
-        return false;
+        return user != null && user.getAge() != null && (user.getAge() >= 30 && user.getAge() <= 45);
     }
 
     @Override

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

@@ -2,6 +2,7 @@
 
 import java.util.Arrays;
 import java.util.List;
+import java.util.Optional;
 
 public class Users {
     static final User DEFAULT_USER = new User("codesquad", 100);
@@ -12,12 +13,7 @@ public class Users {
             new User("jk", 40),
             new User("honux", 45));
 
-    User getUser(String name) {
-        for (User user : users) {
-            if (user.matchName(name)) {
-                return user;
-            }
-        }
-        return DEFAULT_USER;
+    Optional<User> getUser(String name) {
+        return users.stream().filter(user -> user.matchName(name)).findFirst();
     }
 }

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

@@ -8,24 +8,15 @@ 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));
     }
 }

src/test/java/nextstep/optional/ExpressionTest.java

@@ -15,8 +15,8 @@ public void of() {
     @Test
     public void notValidExpression() {
         assertThatIllegalArgumentException()
-                .isThrownBy(() -> {
-                    Expression.of("&");
-                });
+            .isThrownBy(() -> {
+                Expression.of("&");
+            });
     }
 }

src/test/java/nextstep/optional/UsersTest.java

@@ -9,13 +9,17 @@ public class UsersTest {
     @Test
     public void getUser() {
         Users users = new Users();
-        assertThat(users.getUser("crong")).isEqualTo(new User("crong", 35));
+        assertThat(getUser(users, "crong")).isEqualTo(new User("crong", 35));
     }
 
 
     @Test
     public void getDefaultUser() {
         Users users = new Users();
-        assertThat(users.getUser("codesquard")).isEqualTo(Users.DEFAULT_USER);
+        assertThat(getUser(users, "codesquard")).isEqualTo(Users.DEFAULT_USER);
+    }
+
+    private User getUser(Users users, String codesquard) {
+        return users.getUser(codesquard).orElse(Users.DEFAULT_USER);
     }
 }