diff --git a/me/day05/practice/Electronic.java b/me/day05/practice/Electronic.java
new file mode 100644
index 0000000..54aa545
--- /dev/null
+++ b/me/day05/practice/Electronic.java
@@ -0,0 +1,105 @@
+package me.day05.practice;
+
+import java.time.LocalDate;
+import java.time.format.DateTimeFormatter;
+import java.util.Arrays;
+import java.util.Objects;
+
+public class Electronic {
+
+    enum CompanyName { SAMSUNG, LG, APPLE }
+    enum AuthMethod { FINGERPRINT, PIN, PATTERN, FACE }
+
+    private static final int MAX_REGISTRATION_NUMBER = 9999;
+
+    private static int registrationNo;
+
+    private String productNo;
+    private String modelName;
+    private CompanyName companyName;
+    private String dateOfMade;
+    private AuthMethod[] authMethod;
+
+    Electronic () {
+        registrationNo++;
+        setDateOfMade();
+        setProductNo();
+    }
+
+    Electronic (String modelName, CompanyName companyName, AuthMethod[] authMethod) {
+        this();
+        this.modelName = modelName;
+        this.companyName = companyName;
+        this.authMethod = authMethod;
+    }
+
+    private void setDateOfMade(){
+        DateTimeFormatter timeFormatter = DateTimeFormatter.ofPattern("yyMMdd");
+        dateOfMade = timeFormatter.format(LocalDate.now());
+    }
+
+    private void setProductNo(){
+        if (registrationNo > MAX_REGISTRATION_NUMBER) registrationNo = 1;
+        productNo = dateOfMade + String.format("%4d", registrationNo).replace(" ", "0");
+    }
+
+    public boolean isContainsAuthMethod(AuthMethod authMethod){
+        for (AuthMethod auth : this.authMethod)
+            if (authMethod.equals(auth)) return true;
+        return false;
+    }
+
+    public String getProductNo() {
+        return productNo;
+    }
+
+    public String getModelName() {
+        return modelName;
+    }
+
+    public void setModelName(String modelName) {
+        this.modelName = modelName;
+    }
+
+    public CompanyName getCompanyName() {
+        return companyName;
+    }
+
+    public void setCompanyName(CompanyName companyName) {
+        this.companyName = companyName;
+    }
+
+    public String getDateOfMade() {
+        return dateOfMade;
+    }
+
+    public AuthMethod[] getAuthMethod() {
+        return authMethod;
+    }
+
+    public void setAuthMethod(AuthMethod[] authMethod) {
+        this.authMethod = authMethod;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(productNo, modelName, companyName, dateOfMade, Arrays.hashCode(authMethod));
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) return true;
+        if (obj == null || getClass() != obj.getClass()) return false;
+        return Objects.equals(productNo, ((Electronic)obj).productNo);
+    }
+
+    @Override
+    public String toString() {
+        return "Electronic { " +
+                "productNo=" + productNo +
+                ", modelName=" + modelName +
+                ", companyName= " + companyName +
+                ", dateOfMade=" + dateOfMade +
+                ", authMethod=" + Arrays.toString(authMethod) + " }";
+    }
+}
diff --git a/me/day05/practice/Electronics.java b/me/day05/practice/Electronics.java
new file mode 100644
index 0000000..22986a5
--- /dev/null
+++ b/me/day05/practice/Electronics.java
@@ -0,0 +1,119 @@
+package me.day05.practice;
+
+import me.day05.practice.Electronic.AuthMethod;
+import me.day05.practice.Electronic.CompanyName;
+
+import java.util.*;
+
+public class Electronics {
+
+    private static final Electronic[] EMPTY_ELECTRONIC_LIST = {};
+
+    private static Electronic[] electronicList;
+    private static Electronics electronicsInstance;
+
+    private int size;
+    private int capacity;
+
+    private Electronics(){
+        electronicList = EMPTY_ELECTRONIC_LIST;
+    }
+
+    // TODO: 1. Electronics 클래스의 객체를 싱글톤으로 생성하는 함수를 작성하시오.
+    public static Electronics getInstance() {
+        if (electronicsInstance == null) electronicsInstance = new Electronics();
+        return electronicsInstance;
+    }
+
+    // TODO: 2. 전자제품 일련번호 productNo를 통해 인자로 주어진 일련번호에 해당하는 전자제품을 반환하는 함수를 작성하시오.
+    public Optional<Electronic> findByProductNo(String productNo){
+        for (Electronic electronic : electronicList)
+            if (productNo.equals(electronic.getProductNo()))
+                return Optional.of(electronic);
+
+        return Optional.empty();
+    }
+
+    // TODO: 3. 전자제품들 중 인자로 주어진 제조 회사를 찾아서 하나의 배열에 반환하는 함수를 작성하시오.
+    public Optional<Electronic[]> groupByCompanyName(CompanyName company){
+
+        List<Electronic> temp = new ArrayList<>();
+
+        for (Electronic electronic : electronicList)
+            if (electronic.getCompanyName().equals(company))
+                temp.add(electronic);
+
+        Electronic[] companyNameGroup =
+                temp.isEmpty() ? null : listToArray(temp);
+
+        return Optional.ofNullable(companyNameGroup);
+    }
+
+    public Optional<Electronic> findByCompanyName(CompanyName company){
+        for (Electronic electronic : electronicList)
+            if (company.equals(electronic.getCompanyName()))
+                return Optional.of(electronic);
+
+        return Optional.empty();
+    }
+
+    // TODO: 4. 전자제품들 중 인자로 주어진 인증 방법을 찾아서 하나의 배열에 반환하는 함수를 작성하시오.
+    public Optional<Electronic[]> groupByAuthMethod(AuthMethod authMethod){
+
+        List<Electronic> temp = new ArrayList<>();
+
+        for (Electronic electronic : electronicList)
+            if (electronic.isContainsAuthMethod(authMethod))
+                temp.add(electronic);
+
+        Electronic[] authMethodNameGroup =
+                temp.isEmpty() ? null : listToArray(temp);
+
+        return Optional.ofNullable(authMethodNameGroup);
+    }
+
+    private Electronic[] listToArray(List<Electronic> list){
+        Electronic[] array = new Electronic[list.size()];
+
+        for (int i = 0; i < array.length; i++)
+            array[i] = list.get(i);
+
+        return array;
+    }
+
+    public int getSize() {
+        return size;
+    }
+
+    public void setSize(int size) {
+        this.size = size;
+    }
+
+    public int getCapacity() {
+        return capacity;
+    }
+
+    public void setCapacity(int capacity) {
+        this.capacity = capacity;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(Arrays.hashCode(electronicList), size, capacity);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) return true;
+        if (obj == null || getClass() != obj.getClass()) return false;
+        return Objects.equals(hashCode(), ((Electronics)obj).hashCode());
+    }
+
+    @Override
+    public String toString() {
+        return "Users { " +
+                "size=" + size +
+                ", capacity=" + capacity +
+                ", electronicList= " + Arrays.toString(electronicList) + " }";
+    }
+}
diff --git a/me/day05/practice/User.java b/me/day05/practice/User.java
new file mode 100644
index 0000000..527500e
--- /dev/null
+++ b/me/day05/practice/User.java
@@ -0,0 +1,126 @@
+package me.day05.practice;
+
+import java.time.LocalDate;
+import java.util.Arrays;
+import java.util.Objects;
+
+public class User {
+
+    private String userId;
+    private String userPassword;
+    private String userPhoneNumber;
+    private String userEmail;
+    private String userBirthDate;
+    private Electronic[] electronicDevice;
+    private LocalDate registerTime;
+
+    User() {
+        setRegisterTime();
+    }
+
+    User(String userId, String userPassword, String userPhoneNumber, String userEmail, String userBirthDate, Electronic[] electronicDevice) {
+        this();
+        this.userId = userId;
+        this.userPassword = userPassword;
+        this.userPhoneNumber = userPhoneNumber;
+        this.userEmail = userEmail;
+        this.userBirthDate = userBirthDate;
+        this.electronicDevice = electronicDevice;
+    }
+
+    private User(String userId, String userPassword, String userPhoneNumber, String userEmail, String userBirthDate, Electronic[] electronicDevice, LocalDate registerTime){
+        this.userId = userId;
+        this.userPassword = userPassword;
+        this.userPhoneNumber = userPhoneNumber;
+        this.userEmail = userEmail;
+        this.userBirthDate = userBirthDate;
+        this.electronicDevice = electronicDevice;
+        this.registerTime = registerTime;
+    }
+
+    private void setRegisterTime() {
+         registerTime = LocalDate.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[] getElectronicDevice() {
+        return electronicDevice;
+    }
+
+    public void setElectronicDevice(Electronic[] electronicDevice) {
+        this.electronicDevice = electronicDevice;
+    }
+
+    public LocalDate getRegisterTime() {
+        return registerTime;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(userId, userPassword, userEmail, userBirthDate, Arrays.hashCode(electronicDevice), registerTime);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) return true;
+        if (obj == null || getClass() != obj.getClass()) return false;
+        return Objects.equals(hashCode(), ((User)obj).hashCode());
+    }
+
+    @Override
+    public String toString() {
+        return "User { " +
+                "userId=" + userId +
+                ", userPassword=" + userPassword +
+                ", userPhoneNumber= " + userPhoneNumber +
+                ", userEmail=" + userEmail +
+                ", userBirthDate=" + userBirthDate +
+                ", electronicDevice=" + Arrays.toString(electronicDevice) +
+                ", registerTime=" + registerTime + " }";
+    }
+
+    @Override
+    protected User clone() {
+        return new User(userId, userPassword, userPhoneNumber, userEmail, userBirthDate, electronicDevice, registerTime);
+    }
+}
+
diff --git a/me/day05/practice/Users.java b/me/day05/practice/Users.java
new file mode 100644
index 0000000..92fc998
--- /dev/null
+++ b/me/day05/practice/Users.java
@@ -0,0 +1,78 @@
+package me.day05.practice;
+
+import java.util.Arrays;
+import java.util.Objects;
+import java.util.Optional;
+
+public class Users {
+
+    private static final int DEFAULT_CAPACITY = 10; // Default initial capacity
+    private static final User[] EMPTY_USER_LIST = {};
+
+    private static User[] userList;
+    private static Users usersInstance;
+
+    private int size;
+    private int capacity;
+
+    private Users(){
+        userList = EMPTY_USER_LIST;
+    }
+
+    // TODO: 1. Users 클래스의 객체를 싱글톤으로 생성하는 함수를 작성하시오.
+    public static Users getInstance() {
+        if (usersInstance == null) usersInstance = new Users();
+        return usersInstance;
+    }
+
+    // TODO: 2. 회원 아이디 userId를 통해 인자로 주어진 회원번호에 해당하는 회원을 반환하는 함수를 작성하시오.
+    public Optional<User> findById(String userId){
+        for (User user : userList)
+            if (userId.equals(user.getUserId()))
+                return Optional.of(user);
+
+        return Optional.empty();
+    }
+
+    // TODO: 3. 인자로 주어진 회원 정보를 깊은 복사 (deepCopy) 하는 함수를 작성하시오.
+    public User copy(User user) {
+        User copyUser = findById(user.getUserId()).orElseThrow(NullPointerException::new); // 예외 처리 추가 구현 필요
+        return copyUser.clone();
+    }
+
+    public int getSize() {
+        return size;
+    }
+
+    public void setSize(int size) {
+        this.size = size;
+    }
+
+    public int getCapacity() {
+        return capacity;
+    }
+
+    public void setCapacity(int capacity) {
+        this.capacity = capacity;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(Arrays.hashCode(userList), size, capacity);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) return true;
+        if (obj == null || getClass() != obj.getClass()) return false;
+        return Objects.equals(hashCode(), ((Users)obj).hashCode());
+    }
+
+    @Override
+    public String toString() {
+        return "Users { " +
+                "size=" + size +
+                ", capacity=" + capacity +
+                ", userList= " + Arrays.toString(userList) + " }";
+    }
+}