diff --git a/src/main/java/nextstep/fp/Lambda.java b/src/main/java/nextstep/fp/Lambda.java
index bd68fe1c..dce72e76 100644
--- a/src/main/java/nextstep/fp/Lambda.java
+++ b/src/main/java/nextstep/fp/Lambda.java
@@ -27,30 +27,23 @@ public void run() {
     }
 
     public static int sumAll(List<Integer> numbers) {
-        int total = 0;
-        for (int number : numbers) {
-            total += number;
-        }
-        return total;
+        return numbers.stream().reduce(Integer::sum).get();
+    }
+
+    public interface Conditional {
+        boolean test(Integer number);
     }
 
+    public static int sumAll(List<Integer> numbers,
+                             Conditional c) {
+        // c.test(number)를 활용해 구현할 수 있다.
+        return numbers.stream().filter(c::test).reduce(Integer::sum).get();
+    }
     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, (integer)->{return integer%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, (integer)->{return integer>3;});
     }
 }
diff --git a/src/main/java/nextstep/fp/StreamStudy.java b/src/main/java/nextstep/fp/StreamStudy.java
index b446983a..86b66d53 100644
--- a/src/main/java/nextstep/fp/StreamStudy.java
+++ b/src/main/java/nextstep/fp/StreamStudy.java
@@ -5,6 +5,7 @@
 import java.nio.file.Files;
 import java.nio.file.Paths;
 import java.util.Arrays;
+import java.util.Comparator;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -28,6 +29,7 @@ public static void printLongestWordTop100() throws IOException {
         List<String> words = Arrays.asList(contents.split("[\\P{L}]+"));
 
         // TODO 이 부분에 구현한다.
+        words.stream().filter(w -> w.length() > 12).sorted(Comparator.comparing(String::length).reversed()).distinct().limit(100).map(String::toLowerCase).forEach(System.out::println);
     }
 
     public static List<Integer> doubleNumbers(List<Integer> numbers) {
@@ -39,6 +41,7 @@ public static long sumAll(List<Integer> numbers) {
     }
 
     public static long sumOverThreeAndDouble(List<Integer> numbers) {
-        return 0;
+        return doubleNumbers(numbers.stream().filter(x -> x>3).collect(Collectors.toList()))
+                .stream().reduce(Integer::sum).get();
     }
-}
\ No newline at end of file
+}
diff --git a/src/main/java/nextstep/optional/Expression.java b/src/main/java/nextstep/optional/Expression.java
index 1c98cd6a..2ca322a0 100644
--- a/src/main/java/nextstep/optional/Expression.java
+++ b/src/main/java/nextstep/optional/Expression.java
@@ -1,5 +1,8 @@
 package nextstep.optional;
 
+import java.util.Arrays;
+import java.util.Optional;
+
 enum Expression {
     PLUS("+"), MINUS("-"), TIMES("*"), DIVIDE("/");
 
@@ -14,12 +17,6 @@ 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(v -> matchExpression(v, expression)).findFirst().orElseThrow(() -> new IllegalArgumentException(String.format("%s는 사칙연산에 해당하지 않는 표현식입니다.", expression)));
     }
 }
diff --git a/src/main/java/nextstep/optional/User.java b/src/main/java/nextstep/optional/User.java
index 9614c2f4..e7fcc660 100644
--- a/src/main/java/nextstep/optional/User.java
+++ b/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 +35,7 @@ public static boolean ageIsInRange1(User user) {
     }
 
     public static boolean ageIsInRange2(User user) {
-        return false;
+        return Optional.ofNullable(user).map(x -> x.getAge()).filter(x -> x >=30 && x <= 45).isPresent();
     }
 
     @Override
diff --git a/src/main/java/nextstep/optional/Users.java b/src/main/java/nextstep/optional/Users.java
index 6293040d..42183b7c 100644
--- a/src/main/java/nextstep/optional/Users.java
+++ b/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);
@@ -13,11 +14,6 @@ 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)).findAny().orElse(DEFAULT_USER);
     }
 }
diff --git a/src/test/java/nextstep/fp/CarTest.java b/src/test/java/nextstep/fp/CarTest.java
index 1ab1106f..9ab4e3bd 100644
--- a/src/test/java/nextstep/fp/CarTest.java
+++ b/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(() -> {return 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(() ->{return false;});
         assertThat(actual).isEqualTo(new Car("pobi", 0));
     }
 }