diff --git a/Practice_03/Electronic.java b/Practice_03/Electronic.java
new file mode 100644
index 0000000..c31d852
--- /dev/null
+++ b/Practice_03/Electronic.java
@@ -0,0 +1,88 @@
+package Practice_03;
+
+import java.util.*;
+import java.time.*;
+import java.time.format.DateTimeFormatter;
+
+public class Electronic {
+
+    public enum Company { SAMSUNG, LG, APPLE }
+    public enum AuthMethod { FINGERPRINT, PIN, PATTERN, FACE }
+
+    private static int registrationNo;
+    private String productNo;
+    private String modelName;
+    private Company company;
+    private LocalDate dateOfMade;
+    private AuthMethod[] authMethods;
+
+    public Electronic(String modelName, Company company, AuthMethod[] authMethods) {
+        this.modelName = modelName;
+        this.company = company;
+        this.authMethods = authMethods;
+        this.productNo = generateProductNo();
+    }
+
+    private String generateProductNo() {
+        String date = LocalDate.now().format(DateTimeFormatter.ofPattern("yyMMdd"));
+        registrationNo++;
+        registrationNo %= 100000;
+        String registrationNoStr = String.format("%04d", registrationNo);
+        return date + registrationNoStr;
+    }
+
+    public boolean containsAuthMethod(AuthMethod authMethod) {
+        return Arrays.asList(authMethods).contains(authMethod);
+    }
+
+    public String getProductNo() {
+        return productNo;
+    }
+
+    public String getModelName() {
+        return modelName;
+    }
+
+    public void setModelName(String modelName) {
+        this.modelName = modelName;
+    }
+
+    public Company getCompany() {
+        return company;
+    }
+
+    public void setCompany(Company company) {
+        this.company = company;
+    }
+
+    public LocalDate getDateOfMade() {
+        return dateOfMade;
+    }
+
+    public AuthMethod[] getAuthMethods() {
+        return authMethods;
+    }
+
+    public void setAuthMethods(AuthMethod[] authMethods) {
+        this.authMethods = authMethods;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(productNo, modelName, company, authMethods);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) return true;
+        if (obj == null || getClass() != obj.getClass()) return false;
+        Electronic that = (Electronic) obj;
+        return Objects.equals(this.productNo, that.productNo);
+    }
+
+    @Override
+    public String toString() {
+        return String.format("Electronic { productNo=%s, modelName=%s, company=%s, authMethods=%s }",
+                productNo, modelName, company, Arrays.toString(authMethods));
+    }
+}
diff --git a/Practice_03/Electronics.java b/Practice_03/Electronics.java
new file mode 100644
index 0000000..23c28f7
--- /dev/null
+++ b/Practice_03/Electronics.java
@@ -0,0 +1,71 @@
+package Practice_03;
+
+import java.util.*;
+import Practice_03.Electronic.*;
+
+public class Electronics {
+    private static Electronics instance;
+    private static Electronic[] electronicList;
+
+    private Electronics() {
+    }
+
+    public static Electronics getInstance() {
+        if (instance == null) {
+            instance = new Electronics();
+        }
+        return instance;
+    }
+
+    public Electronic findByProductNo(String productNo) {
+        for (Electronic electronic : electronicList) {
+            if (electronic.getProductNo().equals(productNo)) {
+                return electronic;
+            }
+        }
+        return null;
+    }
+
+    public Electronic[] groupByCompanyName(Company company) {
+        List<Electronic> electronics = new ArrayList<>();
+        for (Electronic electronic : electronicList) {
+            if (electronic.getCompany() == company) {
+                electronics.add(electronic);
+            }
+        }
+        return electronics.toArray(new Electronic[electronics.size()]);
+    }
+
+    public Electronic[] groupByAuthMethod(AuthMethod authMethod) {
+        List<Electronic> groupAuthList = new ArrayList<>();
+        for (Electronic electronic : electronicList) {
+            if (Arrays.equals(electronic.getAuthMethods(), new AuthMethod[] {authMethod})) {
+                groupAuthList.add(electronic);
+            }
+        }
+        return groupAuthList.toArray(new Electronic[groupAuthList.size()]);
+    }
+
+    public static Electronic[] getElectronicList() {
+        return electronicList;
+    }
+
+    public static void setElectronicList(Electronic[] electronicList) {
+        Electronics.electronicList = electronicList;
+    }
+
+    @Override
+    public int hashCode() {
+        return super.hashCode();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        return super.equals(obj);
+    }
+
+    @Override
+    public String toString() {
+        return "Electronics{}";
+    }
+}
diff --git a/Practice_03/User.java b/Practice_03/User.java
new file mode 100644
index 0000000..3333bd0
--- /dev/null
+++ b/Practice_03/User.java
@@ -0,0 +1,118 @@
+package Practice_03;
+
+import java.time.*;
+import java.util.*;
+
+public class User {
+    private String userId;
+    private String userPassword;
+    private String userPhoneNumber;
+    private String userEmail;
+    private String userBirthDate;
+    private Electronic[] electronicDevices;
+    private LocalDateTime registerTime;
+
+    public User() {
+        this.electronicDevices = new Electronic[10];
+        this.registerTime = LocalDateTime.now();
+    }
+
+    public User(String userId, String userPassword, String userPhoneNumber, String userEmail, String userBirthDate) {
+        this.userId = userId;
+        this.userPassword = userPassword;
+        this.userPhoneNumber = userPhoneNumber;
+        this.userEmail = userEmail;
+        this.userBirthDate = userBirthDate;
+        this.electronicDevices = new Electronic[10];
+        this.registerTime = LocalDateTime.now();
+    }
+
+    public String getUserId() {
+        return userId;
+    }
+
+    public void setUserId(String userId) {
+        this.userId = userId;
+    }
+
+    public String getUserPassword() {
+        return userPassword;
+    }
+
+    public void setUserPassword(String userPassword) {
+        this.userPassword = userPassword;
+    }
+
+    public String getUserPhoneNumber() {
+        return userPhoneNumber;
+    }
+
+    public void setUserPhoneNumber(String userPhoneNumber) {
+        this.userPhoneNumber = userPhoneNumber;
+    }
+
+    public String getUserEmail() {
+        return userEmail;
+    }
+
+    public void setUserEmail(String userEmail) {
+        this.userEmail = userEmail;
+    }
+
+    public String getUserBirthDate() {
+        return userBirthDate;
+    }
+
+    public void setUserBirthDate(String userBirthDate) {
+        this.userBirthDate = userBirthDate;
+    }
+
+    public Electronic[] getElectronicDevices() {
+        return electronicDevices;
+    }
+
+    public void setElectronicDevices(Electronic[] electronicDevices) {
+        this.electronicDevices = electronicDevices;
+    }
+
+    public LocalDateTime getRegisterTime() {
+        return registerTime;
+    }
+
+    public void setRegisterTime(LocalDateTime registerTime) {
+        this.registerTime = registerTime;
+    }
+
+    @Override
+    public boolean equals(Object object) {
+        if (this == object) return true;
+        if (object == null || this.getClass() != object.getClass()) return false;
+
+        User user = (User) object;
+
+        if (!Objects.equals(userId, user.userId)) return false;
+        if (!Objects.equals(userPassword, user.userPassword)) return false;
+        if (!Objects.equals(userPhoneNumber, user.userPhoneNumber)) return false;
+        if (!Objects.equals(userEmail, user.userEmail)) return false;
+        if (!Objects.equals(userBirthDate, user.userBirthDate)) return false;
+        return Objects.equals(registerTime, user.registerTime);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(userId, userPassword, userPhoneNumber, userEmail, userBirthDate, registerTime);
+    }
+
+    @Override
+    public String toString() {
+        return "User{" +
+                "userId='" + userId + '\'' +
+                ", userPassword='" + userPassword + '\'' +
+                ", userPhoneNumber='" + userPhoneNumber + '\'' +
+                ", userEmail='" + userEmail + '\'' +
+                ", userBirthDate='" + userBirthDate + '\'' +
+                ", electronicDevices=" + Arrays.toString(electronicDevices) +
+                ", registerTime=" + registerTime +
+                '}';
+    }
+}
diff --git a/Practice_03/Users.java b/Practice_03/Users.java
new file mode 100644
index 0000000..1a1a344
--- /dev/null
+++ b/Practice_03/Users.java
@@ -0,0 +1,69 @@
+package Practice_03;
+
+import java.util.*;
+
+public class Users {
+    private static Users instance;
+    private User[] userList;
+
+    private Users() {
+    }
+
+    public static Users getInstance() {
+        if (instance == null) {
+            instance = new Users();
+        }
+        return instance;
+    }
+
+    public User[] getUserList() {
+        return userList;
+    }
+
+    public void setUserList(User[] userList) {
+        this.userList = userList;
+    }
+
+    public User findByUserId(String userId) {
+        if (userList == null) {
+            return null;
+        }
+        for (User userInfo : userList) {
+            if (userInfo.getUserId().equals(userId)) {
+                return userInfo;
+            }
+        }
+        return null;
+    }
+
+    public User copy(User user) {
+        User copiedUser = new User();
+        copiedUser.setUserId(user.getUserId());
+        copiedUser.setUserEmail(user.getUserEmail());
+        copiedUser.setUserPassword(user.getUserPassword());
+        copiedUser.setUserBirthDate(user.getUserBirthDate());
+        copiedUser.setUserPhoneNumber(user.getUserPhoneNumber());
+        copiedUser.setElectronicDevices(Arrays.copyOf(user.getElectronicDevices(), user.getElectronicDevices().length));
+        return copiedUser;
+    }
+
+    @Override
+    public boolean equals(Object object) {
+        if (this == object) return true;
+        if (object == null || getClass() != object.getClass()) return false;
+        Users users = (Users) object;
+        return Arrays.equals(userList, users.userList);
+    }
+
+    @Override
+    public int hashCode() {
+        return Arrays.hashCode(userList);
+    }
+
+    @Override
+    public String toString() {
+        return "Users{" +
+                "userList=" + Arrays.toString(userList) +
+                '}';
+    }
+}