diff --git a/src/Practice01/AuthMethod.java b/src/Practice01/AuthMethod.java
new file mode 100644
index 0000000..6578306
--- /dev/null
+++ b/src/Practice01/AuthMethod.java
@@ -0,0 +1,5 @@
+package Practice01;
+
+public enum AuthMethod {
+    FINGERPRINT, PATTERN, PIN, FACE
+}
diff --git a/src/Practice01/Company.java b/src/Practice01/Company.java
new file mode 100644
index 0000000..83e53a9
--- /dev/null
+++ b/src/Practice01/Company.java
@@ -0,0 +1,5 @@
+package Practice01;
+
+public enum Company {
+    SAMSUNG, LG, APPLE
+}
diff --git a/src/Practice01/Electronic.java b/src/Practice01/Electronic.java
new file mode 100644
index 0000000..0326905
--- /dev/null
+++ b/src/Practice01/Electronic.java
@@ -0,0 +1,105 @@
+package Practice01;
+
+import java.security.spec.EllipticCurve;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.time.format.DateTimeFormatter;
+import java.util.Arrays;
+import java.util.Objects;
+
+public class Electronic {
+    private String productNo;
+    private static int productNum = 1;
+    private String modelName;
+    private Company companyName;
+    private String dateOfMade;
+    private AuthMethod[] authMethod;
+
+    private static LocalDateTime localDateTime = LocalDateTime.now(ZoneId.systemDefault());;
+
+    private Electronic(){
+        this.productNo = localDateTime.format(DateTimeFormatter.ofPattern("yyMMdd")) + String.format("%04d",productNum);
+        productNum++;
+    }
+
+    public Electronic(String modelName, Company companyName){
+        this();
+        this.modelName = modelName;
+        this.companyName = companyName;
+    }
+
+    public Electronic(String modelName, Company companyName, String dateOfMade, AuthMethod[] authMethod){
+        this();
+        this.modelName = modelName;
+        this.companyName = companyName;
+        this.dateOfMade = dateOfMade;
+        this.authMethod = authMethod;
+    }
+
+    public String getProductNo() {
+        return productNo;
+    }
+
+    public void setProductNo(String productNo) {
+        this.productNo = productNo;
+    }
+
+    public String getModelName() {
+        return modelName;
+    }
+
+    public void setModelName(String modelName) {
+        this.modelName = modelName;
+    }
+
+    public Company getCompanyName() {
+        return companyName;
+    }
+
+    public void setCompanyName(Company companyName) {
+        this.companyName = companyName;
+    }
+
+    public String getDateOfMade() {
+        return dateOfMade;
+    }
+
+    public void setDateOfMade(String dateOfMade) {
+        this.dateOfMade = dateOfMade;
+    }
+
+    public AuthMethod[] getAuthMethod() {
+        return authMethod;
+    }
+
+    public void setAuthMethod(AuthMethod[] authMethod) {
+        this.authMethod = authMethod;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Electronic that = (Electronic) o;
+        return Objects.equals(productNo, that.productNo) && Objects.equals(modelName, that.modelName) && companyName == that.companyName && Objects.equals(dateOfMade, that.dateOfMade) && Arrays.equals(authMethod, that.authMethod);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = Objects.hash(productNo, modelName, companyName, dateOfMade);
+        result = 31 * result + Arrays.hashCode(authMethod);
+        return result;
+    }
+
+    @Override
+    public String toString() {
+        return "Electronic{" +
+                "productNo='" + productNo + '\'' +
+                ", modelName='" + modelName + '\'' +
+                ", companyName=" + companyName +
+                ", dateOfMade='" + dateOfMade + '\'' +
+                ", authMethod=" + Arrays.toString(authMethod) +
+                '}';
+    }
+}
diff --git a/src/Practice01/Electronics.java b/src/Practice01/Electronics.java
new file mode 100644
index 0000000..541a19c
--- /dev/null
+++ b/src/Practice01/Electronics.java
@@ -0,0 +1,148 @@
+package Practice01;
+
+import java.util.Arrays;
+import java.util.Objects;
+
+public class Electronics {
+    ////////
+    // singleton
+    private static Electronics allElectronics;
+
+    public static 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;
+    }
+
+    public Electronics(int initial){
+        electronicList = new Electronic[initial];
+        capacity = initial;
+    }
+
+    public Electronics(Electronic[] electronicList){
+        this.electronicList = electronicList;
+        capacity = electronicList.length;
+        size = electronicList.length;
+    }
+
+    public Electronic[] getElectronicList(){return electronicList;}
+
+    public int size() {
+        return size;
+    }
+    private int capacity() {
+        return capacity;
+    }
+
+    public Electronic get(int index){
+        if (index<0 || index >= size) return null;
+        return electronicList[index];
+    }
+
+    public void set(int index, Electronic electronic){
+        if(index<0 || index >=size) return;
+        if(electronic == null) return;
+
+        electronicList[index] = electronic;
+    }
+
+    public void add(Electronic electronic){
+        if(electronic == null) return;
+
+        if(size<capacity){
+            electronicList[size] = electronic;
+            size++;
+        }else{
+            grow();
+            add(electronic);
+        }
+    }
+
+    private void grow(){
+        capacity *= 2;
+        electronicList = Arrays.copyOf(electronicList, capacity);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Electronics that = (Electronics) o;
+        return size == that.size && capacity == that.capacity && Arrays.equals(electronicList, that.electronicList);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = Objects.hash(size, capacity);
+        result = 31 * result + Arrays.hashCode(electronicList);
+        return result;
+    }
+
+    @Override
+    public String toString() {
+        return "Electronics{" +
+                "electronicList=" + Arrays.toString(electronicList) +
+                ", size=" + size +
+                ", capacity=" + capacity +
+                '}';
+    }
+
+    public Electronic findByProductNo(String productNo){
+        if(productNo == null) return null;
+        if(allElectronics == null) return null;
+
+        for (int i = 0; i < allElectronics.size(); i++) {
+            if(allElectronics.get(i).getProductNo() == productNo){
+                return allElectronics.get(i);
+            }
+        }
+        return null;
+    }
+
+    public Electronic[] groupByCompanyName(Company company){
+        if(company == null) return null;
+        if(allElectronics == null) return null;
+
+        Electronic[] groupByCN = new Electronic[allElectronics.size()];
+        int count = 0;
+        for (int i = 0; i < allElectronics.size(); i++) {
+            if(allElectronics.get(i).getCompanyName() == company){
+                groupByCN[count] = allElectronics.get(i);
+                count++;
+            }
+        }
+        return groupByCN;
+    }
+
+
+
+    public Electronic[] groupByAuthMethod(AuthMethod authMethod){
+        if(authMethod == null) return null;
+        if(allElectronics == null) return null;
+
+        Electronic[] groupByAM = new Electronic[allElectronics.size()];
+        int count = 0;
+        for (int i = 0; i < allElectronics.size(); i++) {
+            for (int j = 0; j < allElectronics.get(i).getAuthMethod().length; j++) {
+                if(authMethod == allElectronics.get(i).getAuthMethod()[j]){
+                    groupByAM[count] = allElectronics.get(i);
+                    count++;
+                }
+            }
+        }
+        return groupByAM;
+    }
+}
diff --git a/src/Practice01/User.java b/src/Practice01/User.java
new file mode 100644
index 0000000..3c1bf19
--- /dev/null
+++ b/src/Practice01/User.java
@@ -0,0 +1,129 @@
+package Practice01;
+
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.util.Arrays;
+import java.util.Objects;
+
+public class User {
+    private String userId;
+    private String userPassword;
+    private int userPhoneNumber;
+    private String userEmail;
+    private String userBirthDate;
+    private Electronic[] electronicDevices;
+    private LocalDateTime registerTime;
+
+    private User(){
+        this.registerTime = LocalDateTime.now(ZoneId.systemDefault());
+    }
+
+    public User(String userId){
+        this();
+        this.userId = userId;
+    }
+
+    public User(String userId, String userPassword){
+        this();
+        this.userId = userId;
+        this.userPassword = userPassword;
+    }
+
+
+    public User(String userId, String userPassword, int userPhoneNumber, String userEmail, String userBirthDate, Electronic[] electronicDevices){
+        this();
+        this.userId = userId;
+        this.userPassword = userPassword;
+        this.userPhoneNumber = userPhoneNumber;
+        this.userEmail = userEmail;
+        this.userBirthDate = userBirthDate;
+        this.electronicDevices = electronicDevices;
+        this.registerTime = LocalDateTime.now(ZoneId.systemDefault());
+
+    }
+
+    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 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 o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        User user = (User) o;
+        return userPhoneNumber == user.userPhoneNumber && Objects.equals(userId, user.userId) && Objects.equals(userPassword, user.userPassword) && Objects.equals(userEmail, user.userEmail) && Objects.equals(userBirthDate, user.userBirthDate) && Arrays.equals(electronicDevices, user.electronicDevices) && Objects.equals(registerTime, user.registerTime);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = Objects.hash(userId, userPassword, userPhoneNumber, userEmail, userBirthDate, registerTime);
+        result = 31 * result + Arrays.hashCode(electronicDevices);
+        return result;
+    }
+
+    @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/src/Practice01/Users.java b/src/Practice01/Users.java
new file mode 100644
index 0000000..37a29b8
--- /dev/null
+++ b/src/Practice01/Users.java
@@ -0,0 +1,137 @@
+package Practice01;
+
+import java.lang.reflect.Member;
+import java.util.Arrays;
+import java.util.Objects;
+
+public class Users {
+    //////////////
+    //singleton
+    private static Users allUsers;
+
+    public static 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(){
+        userList = new User[DEFAULT];
+        capacity = DEFAULT;
+    }
+
+    public Users(int initial){
+        userList = new User[initial];
+        capacity = initial;
+    }
+
+    public Users(User[] userList){
+        this.userList = userList;
+        capacity = userList.length;
+        size = userList.length;
+    }
+
+    public User[] getUserList(){
+        return userList;
+    }
+
+    public void init() {
+        for (int i = 0; i < 10; i++) {
+            allUsers.add(new User(
+                    Character.toString('a'+i),
+                    Character.toString('a'+i) +"1234"
+            ));
+        }
+    }
+
+
+    public User findByUserId(String userId){
+        if(userId == null) return null;
+        if(allUsers == null) return null;
+
+
+        for (int i = 0; i < allUsers.size(); i++) {
+            if (allUsers.get(i).getUserId() == userId){
+                return allUsers.get(i);
+            }
+        }
+        return null;
+    }
+
+    public User copy(User user){
+        if(user == null) return null;
+        User copyUser = new User(user.getUserId(), user.getUserPassword(), user.getUserPhoneNumber(), user.getUserEmail(), user.getUserBirthDate(), user.getElectronicDevices());
+        return copyUser;
+    }
+
+
+
+    public int size() {
+        return size;
+    }
+
+    private int capacity() {
+        return capacity;
+    }
+
+    public User get(int index){
+        if (index<0 || index >= size) return null;
+        return userList[index];
+    }
+
+    public void set(int index, User user){
+        if(index<0 || index >=size) return;
+        if(user == null) return;
+
+        userList[index] = user;
+    }
+
+    public void add(User user){
+        if(user == null) return;
+
+        if(size<capacity){
+            userList[size] = user;
+            size++;
+        }else{
+            grow();
+            add(user);
+        }
+    }
+
+    private void grow(){
+        capacity *= 2;
+        userList = Arrays.copyOf(userList, capacity);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Users users = (Users) o;
+        return size == users.size && capacity == users.capacity && Arrays.equals(userList, users.userList);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = Objects.hash(size, capacity);
+        result = 31 * result + Arrays.hashCode(userList);
+        return result;
+    }
+
+    @Override
+    public String toString() {
+        return "Users{" +
+                "userList=" + Arrays.toString(userList) +
+                ", size=" + size +
+                ", capacity=" + capacity +
+                '}';
+    }
+}