diff --git a/Day05/.gitignore b/Day05/.gitignore
new file mode 100644
index 0000000..f68d109
--- /dev/null
+++ b/Day05/.gitignore
@@ -0,0 +1,29 @@
+### IntelliJ IDEA ###
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store
\ No newline at end of file
diff --git a/Day05/Day05.iml b/Day05/Day05.iml
new file mode 100644
index 0000000..6db0c46
--- /dev/null
+++ b/Day05/Day05.iml
@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+    </content>
+    <orderEntry type="jdk" jdkName="17" jdkType="JavaSDK" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>
\ No newline at end of file
diff --git a/Day05/src/me/day05/practice/Practice01/Electronic.java b/Day05/src/me/day05/practice/Practice01/Electronic.java
new file mode 100644
index 0000000..e4c0a8e
--- /dev/null
+++ b/Day05/src/me/day05/practice/Practice01/Electronic.java
@@ -0,0 +1,122 @@
+package me.day05.practice.Practice01;
+
+
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.util.Arrays;
+import java.util.Objects;
+
+public class Electronic {
+
+    public enum companyName {SAMSUNG,LG,APPLE}
+    public enum authMethod {FINGERPRINT, PATTERN, PIN, FACE}
+    private authMethod[] authMethods;
+    private companyName company;
+    private String productNo;
+    private static int productionCount=1;
+
+    private String modelName;
+    private LocalDateTime dateOfMade;
+
+    public Electronic(companyName companyName, authMethod []authMehods, String modelName){
+
+        StringBuffer generateProductNo;
+        generateProductNo = new StringBuffer(LocalDate.now().getYear()%100);
+        generateProductNo.append(String.format("%02",LocalDate.now().getDayOfMonth()));
+        generateProductNo.append(String.format("%02",LocalDate.now().getDayOfMonth()));
+        generateProductNo.append(String.format("%04",productionCount++ ));
+        this.productNo = String.valueOf(getProductNo());
+        this.authMethods = authMehods;
+        this.company = companyName;
+        this.modelName = modelName;
+        this.dateOfMade = LocalDateTime.now(ZoneId.systemDefault());
+
+    }
+
+    public authMethod[] getAuthMethods() {
+        return authMethods;
+    }
+
+    public void setAuthMethods(authMethod[] authMethods) {
+        this.authMethods = authMethods;
+    }
+
+    public companyName getCompany() {
+        return company;
+    }
+
+    public void setCompany(companyName company) {
+        this.company = company;
+    }
+
+    public String getProductNo() {
+        return productNo;
+    }
+
+    public void setProductNo(String productNo) {
+        this.productNo = productNo;
+    }
+
+    public static int getProductionCount() {
+        return productionCount;
+    }
+
+    public static void setProductionCount(int productionCount) {
+        Electronic.productionCount = productionCount;
+    }
+
+    public LocalDateTime getDateOfMade() {
+        return dateOfMade;
+    }
+
+    public void setDateOfMade(LocalDateTime dateOfMade) {
+        this.dateOfMade = dateOfMade;
+    }
+
+    public String getModelName() {
+        return modelName;
+    }
+
+    public void setModelName(String modelName) {
+        this.modelName = modelName;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        Electronic that = (Electronic) o;
+
+        if (!Objects.equals(productNo, that.productNo)) return false;
+        if (!Objects.equals(dateOfMade, that.dateOfMade)) return false;
+        if (!Objects.equals(modelName, that.modelName)) return false;
+        if (company != that.company) return false;
+        // Probably incorrect - comparing Object[] arrays with Arrays.equals
+        return Arrays.equals(authMethods, that.authMethods);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = productNo != null ? productNo.hashCode() : 0;
+        result = 31 * result + (dateOfMade != null ? dateOfMade.hashCode() : 0);
+        result = 31 * result + (modelName != null ? modelName.hashCode() : 0);
+        result = 31 * result + (company != null ? company.hashCode() : 0);
+        result = 31 * result + Arrays.hashCode(authMethods);
+        return result;
+    }
+
+
+    @Override
+    public String toString() {
+        return "Electronic{" +
+                "productNo=" + productNo +
+                ", dateOfMade=" + dateOfMade +
+                ", modelName='" + modelName + '\'' +
+                ", company=" + company +
+                ", authMethods=" + Arrays.toString(authMethods) +
+                '}';
+    }
+
+}
diff --git a/Day05/src/me/day05/practice/Practice01/User.java b/Day05/src/me/day05/practice/Practice01/User.java
new file mode 100644
index 0000000..31e9be6
--- /dev/null
+++ b/Day05/src/me/day05/practice/Practice01/User.java
@@ -0,0 +1,105 @@
+package me.day05.practice.Practice01;
+
+import java.util.Arrays;
+import java.util.Objects;
+
+public class User {
+
+    private String userId;
+    private String userPassword;
+    private int userPhoneNumber;
+    private String userEmail;
+    private int userBirthDate;
+    private Electronic[] electronicDevices;
+
+
+    public User(){}// default 생성자
+
+    public User(String userId){
+        this.userId = userId;
+    }
+
+    public User(String userId, String userPassword, int userPhoneNumber, String userEmail, int userBirthDate, Electronic[] electronicDevices) {
+        this.userId = userId;
+        this.userPassword = userPassword;
+        this.userPhoneNumber = userPhoneNumber;
+        this.userEmail = userEmail;
+        this.userBirthDate = userBirthDate;
+        this.electronicDevices = electronicDevices;
+    }
+
+    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 int getUserPhoneNumber() {
+        return userPhoneNumber;
+    }
+
+    public void setUserPhoneNumber(int userPhoneNumber) {
+        this.userPhoneNumber = userPhoneNumber;
+    }
+
+    public String getUserEmail() {
+        return userEmail;
+    }
+
+    public void setUserEmail(String userEmail) {
+        this.userEmail = userEmail;
+    }
+
+    public int getUserBirthDate() {
+        return userBirthDate;
+    }
+
+    public void setUserBirthDate(int userBirthDate) {
+        this.userBirthDate = userBirthDate;
+    }
+
+    public Electronic[] getElectronicDevices() {
+        return electronicDevices;
+    }
+
+    public void setElectronicDevices(Electronic[] electronicDevices) {
+        this.electronicDevices = electronicDevices;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        User user = (User) o;
+
+        return Objects.equals(userId, user.userId);
+    }
+
+    @Override
+    public int hashCode() {
+        return userId.hashCode();
+    }
+
+    @Override
+    public String toString() {
+        return "User{" +
+                "userId='" + userId + '\'' +
+                ", userPassword='" + userPassword + '\'' +
+                ", userPhoneNumber=" + userPhoneNumber +
+                ", userEmail='" + userEmail + '\'' +
+                ", userBirthDate=" + userBirthDate +
+                ", electronicDevices=" + Arrays.toString(electronicDevices) +
+                '}';
+    }
+}
diff --git a/Day05/src/me/day05/practice/Practice02/Users.java b/Day05/src/me/day05/practice/Practice02/Users.java
new file mode 100644
index 0000000..5146e4a
--- /dev/null
+++ b/Day05/src/me/day05/practice/Practice02/Users.java
@@ -0,0 +1,212 @@
+package me.day05.practice.Practice02;
+
+import me.day05.practice.Practice01.User;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+
+public class Users {
+
+    private static Users allUsers;
+    // 싱글톤
+    public Users getInstance() {
+        if (allUsers == null) {
+            allUsers = new Users();
+        }
+        return allUsers;
+    }
+    private User[] userList;
+    private static final int DEFAULT = 10;
+    private int size;
+    private int capacity;
+
+    private Users() {
+        allUsers.userList = new User[DEFAULT];
+        this.capacity = DEFAULT;
+        this.size = 0;
+    }
+
+    private Users(int initial){
+        allUsers.userList = new User[initial];
+        this.capacity = initial;
+        this.size =0;
+    }
+
+    private Users(User[] userList){
+        this.userList = userList;
+        this.capacity = userList.length;
+        this.size = userList.length;
+    }
+
+    public User[] getUserList() {
+        return userList;
+    }
+
+    public void setUserList(User[] userList) {
+        this.userList = userList;
+    }
+
+    // add, set, get, pop, indexOf,size, capcity(for dynamic- size 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;
+    }
+
+    public User get(int index){
+        if(index<0||index>=this.size){
+            System.out.println("index의 범위가 올바르지 않습니다.");
+            return null;
+        }
+        return userList[index];
+    }
+
+    public void set(int index, User user){
+        if(index<0||index>=this.size){
+            System.out.println("index의 범위가 올바르지 않습니다.");
+            return;
+        }
+        if(user == null) {
+            System.out.println("저장이 불가합니다.");
+            return;
+        }
+        userList[index] = user;
+    }
+
+    public int indexOf(User user){
+        if(user == null) {
+            System.out.println("not found .");
+            return -1;
+        }
+
+        for(int i=0;i<this.size;i++){
+            if(userList[i] == null) continue;
+            if(userList[i].equals(user))return i;
+        }
+        return -1;
+    }
+
+    public void grow(){
+        capacity*=2;// 용량이 다 차면 2배씩 증가
+        // 방법 1.
+        userList = Arrays.copyOf(userList, capacity);
+        // 방법 2.
+
+    }
+    public void add(User user){
+        if(user == null) return ;// null에 add를 할 수 없음.
+        if(size<capacity){
+            userList[size]= user;
+            size++;
+        }else{
+            grow();// capcity배열의 크기를 늘림.
+            add(user);
+        }
+    }
+
+    public User pop(int idx){
+
+        if(size == 0) return null;
+        if(idx < 0 || idx >= size) return null;
+
+        User popElement = userList[idx];
+        userList[idx]= null;
+
+        for(int i = idx+1;i<size;i++){
+            userList[i-1] = userList[i];
+        }
+        userList[size-1]= null;
+        size--;
+        return popElement;
+
+
+    }
+
+
+
+    /// CRUD
+    // CREATE, READ, UPDATE, DELETE
+
+    public User select(String userId) {
+        if (userId == null) {
+            System.out.println("userId가 비어있습니다. 다시 입력해주세요!");
+            return null;
+        }
+
+        for(int i=0;i<this.size;i++){
+            if(allUsers.userList[i].getUserId().equals(userId)){
+                return allUsers.userList[i];
+            }
+        }
+        return null;
+    }
+
+    public boolean insert(User user){
+        if(user== null|| user.getUserId()==null) return false;
+
+        for(int i=0;i<allUsers.size;i++){
+            if(allUsers.userList[i].equals(user))return false;
+        }
+        allUsers.add(user);
+        return true;
+    }
+
+    public boolean update(User user){//
+        if(user== null|| user.getUserId()==null) return false;
+
+        for(int i=0;i<allUsers.size;i++){
+            if(allUsers.userList[i].equals(user))return false;
+        }
+        int idx = indexOf(user);
+        if(idx == -1) return false;
+
+        allUsers.set(idx, user);
+        return true;
+    }
+
+    public boolean delete (String userId){
+        if(userId == null) return false;
+
+        int idx = indexOf(new User(userId));
+        if(idx == -1 )return false;
+
+        allUsers.pop(idx);
+        return true;
+
+    }
+
+    public User findByUserId(String userId) {
+
+        for (User user : userList) {
+            if (user.equals(userId)) {
+                return user;
+            }
+        }
+        System.out.println("Not Found User");
+        return null;
+    }
+
+    public User copy(User user) {// Deep copy
+        User newUser = new User();
+        newUser.setUserId(user.getUserId());
+        newUser.setUserPassword(user.getUserPassword());
+        newUser.setUserPhoneNumber(user.getUserPhoneNumber());
+        newUser.setUserEmail(user.getUserEmail());
+        newUser.setUserBirthDate(user.getUserBirthDate());
+        newUser.setElectronicDevices(user.getElectronicDevices());
+
+        return newUser;
+    }
+
+
+}
diff --git a/Day05/src/me/day05/practice/Practice03/Electronics.java b/Day05/src/me/day05/practice/Practice03/Electronics.java
new file mode 100644
index 0000000..25cdc08
--- /dev/null
+++ b/Day05/src/me/day05/practice/Practice03/Electronics.java
@@ -0,0 +1,123 @@
+package me.day05.practice.Practice03;
+
+import me.day05.practice.Practice01.Electronic;
+
+import java.util.Arrays;
+
+public class Electronics {
+
+    private static Electronics allElectronics;
+    public Electronics getInstance(){
+        if(allElectronics==null){
+            allElectronics = new Electronics();
+        }
+        return allElectronics;
+    }
+    private Electronic[] electronicList;
+    private static final int DEFAULT =10;
+    private int size;
+    private int capacity;
+    private Electronics(){
+        electronicList = new Electronic[DEFAULT];
+        capacity= DEFAULT;
+        size = 0;
+    }
+
+    public static Electronics getAllElectronics() {
+        return allElectronics;
+    }
+
+    public static void setAllElectronics(Electronics allElectronics) {
+        Electronics.allElectronics = allElectronics;
+    }
+
+    public Electronic[] getElectronicList() {
+        return electronicList;
+    }
+
+    public void setElectronicList(Electronic[] electronicList) {
+        this.electronicList = electronicList;
+    }
+
+    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;
+    }
+
+    public Electronic findByProductionNo(String productNo){
+
+        for(int i=0;i<size;i++){
+            if(electronicList[i].getProductNo().equals(productNo)){
+                return electronicList[i];
+            }
+        }
+        return null;
+    }
+
+    public Electronic[] groupByCompanyName(Electronic.companyName company){
+
+        Electronic[] groupByCompanyNameArray= new Electronic[size];
+        int j=0;
+        for(int i=0;i<size;i++){
+            if(electronicList[i].getCompany().equals(company.name())){
+                groupByCompanyNameArray[j++]=electronicList[i];
+            }
+        }
+
+        return groupByCompanyNameArray;
+    }
+
+    public Electronic[] groupByAuthMethod(Electronic.authMethod authMethod){
+        Electronic[] groupByAuthMethodArray= new Electronic[size];
+        int j=0;
+        for(int i=0;i<size;i++){
+            if(electronicList[i].getCompany().equals(authMethod.name())){
+                groupByAuthMethodArray[j++]=electronicList[i];
+            }
+        }
+
+        return groupByAuthMethodArray;
+
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        Electronics that = (Electronics) o;
+
+        if (size != that.size) return false;
+        if (capacity != that.capacity) return false;
+        // Probably incorrect - comparing Object[] arrays with Arrays.equals
+        return Arrays.equals(electronicList, that.electronicList);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = Arrays.hashCode(electronicList);
+        result = 31 * result + size;
+        result = 31 * result + capacity;
+        return result;
+    }
+
+    @Override
+    public String toString() {
+        return "Electronics{" +
+                "electronicList=" + Arrays.toString(electronicList) +
+                ", size=" + size +
+                ", capacity=" + capacity +
+                '}';
+    }
+}