diff --git a/me/smartstore/Main.java b/me/smartstore/Main.java
new file mode 100644
index 00000000..0f39e9fa
--- /dev/null
+++ b/me/smartstore/Main.java
@@ -0,0 +1,8 @@
+package me.smartstore;
+
+public class Main {
+    public static void main(String[] args) {
+        SmartStoreApp.getInstance().test().run(); // function chaining
+//         SmartStoreApp.getInstance().run();
+    }
+}
diff --git a/me/smartstore/SmartStoreApp.java b/me/smartstore/SmartStoreApp.java
new file mode 100644
index 00000000..f62dd0eb
--- /dev/null
+++ b/me/smartstore/SmartStoreApp.java
@@ -0,0 +1,66 @@
+package me.smartstore;
+
+import me.smartstore.customer.Customer;
+import me.smartstore.customer.Customers;
+import me.smartstore.group.Group;
+import me.smartstore.group.GroupType;
+import me.smartstore.group.Groups;
+import me.smartstore.group.Parameter;
+import me.smartstore.menu.MainMenu;
+
+public class SmartStoreApp {
+    private final Groups allGroups = Groups.getInstance();
+    private final Customers allCustomers = Customers.getInstance();
+
+    private final MainMenu mainMenu = MainMenu.getInstance();
+
+    private static SmartStoreApp smartStoreApp;
+
+    public static SmartStoreApp getInstance() {
+        if (smartStoreApp == null) {
+            smartStoreApp = new SmartStoreApp();
+        }
+        return smartStoreApp;
+    }
+
+    private SmartStoreApp() {}
+
+    public void details() {
+        System.out.println("\n" +
+                "███████╗███╗   ███╗ █████╗ ██████╗ ████████╗    ███████╗████████╗ ██████╗ ██████╗ ███████╗\n" +
+                "██╔════╝████╗ ████║██╔══██╗██╔══██╗╚══██╔══╝    ██╔════╝╚══██╔══╝██╔═══██╗██╔══██╗██╔════╝\n" +
+                "███████╗██╔████╔██║███████║██████╔╝   ██║       ███████╗   ██║   ██║   ██║██████╔╝█████╗  \n" +
+                "╚════██║██║╚██╔╝██║██╔══██║██╔══██╗   ██║       ╚════██║   ██║   ██║   ██║██╔══██╗██╔══╝  \n" +
+                "███████║██║ ╚═╝ ██║██║  ██║██║  ██║   ██║       ███████║   ██║   ╚██████╔╝██║  ██║███████╗\n" +
+                "╚══════╝╚═╝     ╚═╝╚═╝  ╚═╝╚═╝  ╚═╝   ╚═╝       ╚══════╝   ╚═╝    ╚═════╝ ╚═╝  ╚═╝╚══════╝\n");
+    }
+
+    public SmartStoreApp test() {
+        allGroups.add(new Group(new Parameter(10, 100000), GroupType.GENERAL));
+        allGroups.add(new Group(new Parameter(20, 200000), GroupType.VIP));
+        allGroups.add(new Group(new Parameter(30, 300000), GroupType.VVIP));
+
+        for (int i = 0; i < 26; i++) {
+            allCustomers.add(new Customer(
+                    Character.toString(
+                            (char) ('a' + i)),
+                    (char) ('a' + i) + "123",
+                    ((int) (Math.random() * 5) + 1) * 10,
+                    ((int) (Math.random() * 5) + 1) * 100000,
+                    null));
+        }
+
+        allCustomers.refresh(allGroups);
+
+        System.out.println("allCustomers = " + allCustomers);
+        System.out.println("allGroups = " + allGroups);
+
+        return this; // smartStoreApp
+    }
+
+    public void run() {
+        details();
+        mainMenu.manage();
+
+    }
+}
diff --git a/me/smartstore/arrays/Collections.java b/me/smartstore/arrays/Collections.java
new file mode 100644
index 00000000..847ada31
--- /dev/null
+++ b/me/smartstore/arrays/Collections.java
@@ -0,0 +1,16 @@
+package me.smartstore.arrays;
+
+public interface Collections<T> {
+    // 데이터를 가지고 있는 객체가 아님
+    // 구현 해야하는 메소드의 정보만 가지고 있음 (인터페이스)
+
+    int size();
+    T get(int index);
+    void set(int index, T object);
+    int indexOf(T object);
+    void add(T object);
+    void add(int index, T object);
+    T pop();
+    T pop(int index);
+    T pop(T object);
+}
diff --git a/me/smartstore/arrays/DArray.java b/me/smartstore/arrays/DArray.java
new file mode 100644
index 00000000..97f65ff9
--- /dev/null
+++ b/me/smartstore/arrays/DArray.java
@@ -0,0 +1,145 @@
+package me.smartstore.arrays;
+
+import me.smartstore.exception.ElementNotFoundException;
+import me.smartstore.exception.EmptyArrayException;
+import me.smartstore.exception.NullArgumentException;
+
+public class DArray<T> implements Collections<T> { // Dynamic Array
+
+    protected static final int DEFAULT = 10;
+
+    protected T[] arrays;
+    protected int size;
+    protected int capacity;
+
+    public DArray() throws ClassCastException {
+        arrays = (T[]) new Object[DEFAULT];
+        capacity = DEFAULT;
+    }
+
+    public DArray(int initial) throws ClassCastException {
+        arrays = (T[]) new Object[initial];
+        capacity = initial;
+    }
+
+    public DArray(T[] arrays) {
+        this.arrays = arrays;
+        capacity = arrays.length;
+        size = arrays.length;
+    }
+
+    /////////////////////////////////////////
+    // add, set, get, pop, indexOf, size, capacity (for dynamic-sized array)
+
+    @Override
+    public int size() {
+        return size;
+    }
+
+    // 배열에 얼마나 capacity 남아있는지 외부에 알려줄 필요가 없기 때문에 <protected>으로 정의
+    protected int capacity() {
+        return capacity;
+    }
+
+    @Override
+    public T get(int index) throws IndexOutOfBoundsException {
+        if (index < 0 || index >= size) throw new IndexOutOfBoundsException();
+        return arrays[index];
+    }
+
+    /**
+     * @param: ...
+     * @return: ...
+     * @throws: IndexOutOfBoundsException
+     * @throws: NullArgumentException
+     * */
+    @Override
+    public void set(int index, T object) throws IndexOutOfBoundsException, NullArgumentException {
+        if (index < 0 || index >= size) throw new IndexOutOfBoundsException();
+        if (object == null) throw new NullArgumentException();
+
+        arrays[index] = object;
+    }
+
+    @Override
+    public int indexOf(T object) throws NullArgumentException, ElementNotFoundException {
+        if (object == null) throw new NullArgumentException(); // not found (instead of throwing exception)
+
+        for (int i = 0; i < size; i++) {
+            if (arrays[i] == null) continue;
+            if (arrays[i].equals(object)) return i;
+        }
+        throw new ElementNotFoundException(); // not found
+    }
+
+    // 배열의 cap이 부족한 경우
+    @Override
+    public void add(T object) throws NullArgumentException {
+        if (object == null) throw new NullArgumentException(); // if argument is null, do not add null value in array
+
+        if (size < capacity) {
+            arrays[size] = object;
+            size++;
+        } else {
+            grow();
+            add(object);
+        }
+    }
+
+    @Override
+    public void add(int index, T object) throws IndexOutOfBoundsException, NullArgumentException {
+        if (index < 0 || index >= size) throw new IndexOutOfBoundsException();
+        if (object == null) throw new NullArgumentException();
+
+        if (size < capacity) {
+            for (int i = size-1; i >= index ; i--) {
+                arrays[i+1] = arrays[i];
+            }
+            arrays[index] = object;
+            size++;
+        } else {
+            grow();
+            add(index, object);
+        }
+    }
+
+    @Override
+    public T pop() {
+        return pop(size-1);
+    }
+
+    @Override
+    public T pop(int index) throws IndexOutOfBoundsException {
+        if (size == 0) throw new EmptyArrayException();
+        if (index < 0 || index >= size) throw new IndexOutOfBoundsException();
+
+        T popElement = arrays[index];
+        arrays[index] = null; // 삭제됨을 명시적으로 표현
+
+        for (int i = index+1; i < size; i++) {
+            arrays[i-1] = arrays[i];
+        }
+        arrays[size-1] = null;
+        size--;
+        return popElement;
+    }
+
+    @Override
+    public T pop(T object) {
+        return pop(indexOf(object));
+    }
+
+    protected void grow() {
+        capacity *= 2;
+        arrays = java.util.Arrays.copyOf(arrays, capacity);
+    }
+
+    @Override
+    public String toString() {
+        String toStr = "";
+        for (int i = 0; i < size; i++) {
+            toStr += (arrays[i] + "\n");
+        }
+        return toStr;
+    }
+}
diff --git a/me/smartstore/customer/Customer.java b/me/smartstore/customer/Customer.java
new file mode 100644
index 00000000..58a1cb51
--- /dev/null
+++ b/me/smartstore/customer/Customer.java
@@ -0,0 +1,121 @@
+package me.smartstore.customer;
+
+import me.smartstore.exception.InputEmptyException;
+import me.smartstore.group.Group;
+import me.smartstore.group.GroupType;
+import me.smartstore.group.Parameter;
+
+public class Customer {
+    private String cusName;
+    private String cusId;
+    private Integer cusTotalTime;
+    private Integer cusTotalPay;
+    private Group group;
+
+    public Customer() {
+        this.group = new Group(new Parameter(0, 0), GroupType.NONE);
+    }
+
+    public Customer(String cusId) {
+        this.cusId = cusId;
+    }
+
+    public Customer(String cusName, String cusId) {
+        this.cusName = cusName;
+        this.cusId = cusId;
+    }
+
+    public Customer(String cusName, String cusId, Integer cusTotalTime, Integer cusTotalPay, Group group) {
+        this.cusName = cusName;
+        this.cusId = cusId;
+        this.cusTotalTime = cusTotalTime;
+        this.cusTotalPay = cusTotalPay;
+        this.group = group;
+    }
+
+    public String getCusName() {
+        return cusName;
+    }
+
+    public void setCusName(String cusName) {
+        if(cusName.equals("")){
+            throw new InputEmptyException();
+        }else {
+            this.cusName = cusName;
+        }
+    }
+
+    public String getCusId() {
+        return cusId;
+    }
+
+    public void setCusId(String cusId) {
+        if(cusId.equals("")){
+            throw new InputEmptyException();
+        }else {
+            this.cusId = cusId;
+        }
+    }
+
+    public Integer getCusTotalTime() {
+        return cusTotalTime;
+    }
+
+    public void setCusTotalTime(Integer cusTotalTime) {
+        this.cusTotalTime = cusTotalTime;
+    }
+
+    public Integer getCusTotalPay() {
+        return cusTotalPay;
+    }
+
+    public void setCusTotalPay(Integer cusTotalPay) {
+        this.cusTotalPay = cusTotalPay;
+    }
+
+    public Group getGroup() {
+        return group;
+    }
+
+    public void setGroup(Group group) {
+        this.group = group;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        Customer customer = (Customer) o;
+
+        if (getCusName() != null ? !getCusName().equals(customer.getCusName()) : customer.getCusName() != null)
+            return false;
+        if (getCusId() != null ? !getCusId().equals(customer.getCusId()) : customer.getCusId() != null) return false;
+        if (getCusTotalTime() != null ? !getCusTotalTime().equals(customer.getCusTotalTime()) : customer.getCusTotalTime() != null)
+            return false;
+        if (getCusTotalPay() != null ? !getCusTotalPay().equals(customer.getCusTotalPay()) : customer.getCusTotalPay() != null)
+            return false;
+        return getGroup() != null ? getGroup().equals(customer.getGroup()) : customer.getGroup() == null;
+    }
+
+    @Override
+    public int hashCode() {
+        int result = getCusName() != null ? getCusName().hashCode() : 0;
+        result = 31 * result + (getCusId() != null ? getCusId().hashCode() : 0);
+        result = 31 * result + (getCusTotalTime() != null ? getCusTotalTime().hashCode() : 0);
+        result = 31 * result + (getCusTotalPay() != null ? getCusTotalPay().hashCode() : 0);
+        result = 31 * result + (getGroup() != null ? getGroup().hashCode() : 0);
+        return result;
+    }
+
+    @Override
+    public String toString() {
+        return "Customer{" +
+                "cusName='" + cusName + '\'' +
+                ", cusId='" + cusId + '\'' +
+                ", cusTotalTime=" + cusTotalTime +
+                ", cusTotalPay=" + cusTotalPay +
+                ", group=" + group.getGroupType() +
+                '}';
+    }
+}
diff --git a/me/smartstore/customer/Customers.java b/me/smartstore/customer/Customers.java
new file mode 100644
index 00000000..49370c4f
--- /dev/null
+++ b/me/smartstore/customer/Customers.java
@@ -0,0 +1,66 @@
+package me.smartstore.customer;
+
+
+import me.smartstore.arrays.DArray;
+import me.smartstore.group.Group;
+import me.smartstore.group.Groups;
+
+import java.util.Arrays;
+
+public class Customers extends DArray<Customer> {
+
+    private static Customers allCustomers;
+
+    public static Customers getInstance() {
+        if (allCustomers == null) {
+            allCustomers = new Customers();
+        }
+        return allCustomers;
+    }
+
+    private Customers() {}
+
+    // refresh 함수가 호출되는 경우
+    // 1. 분류기준 바뀔 때
+    // 2. 새로운 고객이 들어올 때
+    public void refresh(Groups groups) {
+
+        Group[] allGroup = groups.findAllGroup();
+
+        Arrays.sort(allGroup, (o1, o2) -> o1.getParameter().getMinPay() != o2.getParameter().getMinPay() ?
+                o1.getParameter().getMinPay() - o2.getParameter().getMinPay() :
+                o1.getParameter().getMinTime() - o2.getParameter().getMinTime());
+
+        for(int i = 0; i < this.size(); i++) {
+            for(int j = 0; j <= allGroup.length - 1; j++) {
+                if(this.get(i).getCusTotalTime() >= allGroup[j].getParameter().getMinTime() &&
+                        this.get(i).getCusTotalPay() >= allGroup[j].getParameter().getMinPay()) {
+                    this.get(i).setGroup(allGroup[j]);
+                }
+            }
+        }
+    }
+
+    public int groupCount(Group group) {
+        int count = 0;
+        for(int i = 0; i < this.size; i++) {
+            if(this.get(i).getGroup() == group) {
+                count++;
+            }
+        }
+        return count;
+    }
+
+    public Customer[] findCustomerByGroup(Group group) {
+        Customer[] customers = new Customer[groupCount(group)];
+
+        int index = 0;
+        for(int i = 0; i < this.size; i++) {
+            if(this.get(i).getGroup() == group) {
+                customers[index++] = this.get(i);
+            }
+        }
+        return customers;
+    }
+
+}
diff --git a/me/smartstore/exception/ArrayEmptyException.java b/me/smartstore/exception/ArrayEmptyException.java
new file mode 100644
index 00000000..737945ac
--- /dev/null
+++ b/me/smartstore/exception/ArrayEmptyException.java
@@ -0,0 +1,14 @@
+package me.smartstore.exception;
+
+import me.smartstore.util.Message;
+
+public class ArrayEmptyException extends RuntimeException {
+
+    public ArrayEmptyException() {
+        super(Message.ERR_MSG_INVALID_ARR_EMPTY);
+    }
+
+    public ArrayEmptyException(String message) {
+        super(message);
+    }
+}
diff --git a/me/smartstore/exception/ElementNotFoundException.java b/me/smartstore/exception/ElementNotFoundException.java
new file mode 100644
index 00000000..6e934998
--- /dev/null
+++ b/me/smartstore/exception/ElementNotFoundException.java
@@ -0,0 +1,22 @@
+package me.smartstore.exception;
+
+public class ElementNotFoundException extends RuntimeException {
+    public ElementNotFoundException() {
+    }
+
+    public ElementNotFoundException(String message) {
+        super(message);
+    }
+
+    public ElementNotFoundException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    public ElementNotFoundException(Throwable cause) {
+        super(cause);
+    }
+
+    public ElementNotFoundException(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
+        super(message, cause, enableSuppression, writableStackTrace);
+    }
+}
diff --git a/me/smartstore/exception/EmptyArrayException.java b/me/smartstore/exception/EmptyArrayException.java
new file mode 100644
index 00000000..b19aeac3
--- /dev/null
+++ b/me/smartstore/exception/EmptyArrayException.java
@@ -0,0 +1,22 @@
+package me.smartstore.exception;
+
+public class EmptyArrayException extends RuntimeException {
+    public EmptyArrayException() {
+    }
+
+    public EmptyArrayException(String message) {
+        super(message);
+    }
+
+    public EmptyArrayException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    public EmptyArrayException(Throwable cause) {
+        super(cause);
+    }
+
+    public EmptyArrayException(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
+        super(message, cause, enableSuppression, writableStackTrace);
+    }
+}
diff --git a/me/smartstore/exception/InputEmptyException.java b/me/smartstore/exception/InputEmptyException.java
new file mode 100644
index 00000000..2053e665
--- /dev/null
+++ b/me/smartstore/exception/InputEmptyException.java
@@ -0,0 +1,14 @@
+package me.smartstore.exception;
+
+import me.smartstore.util.Message;
+
+public class InputEmptyException extends RuntimeException {
+
+    public InputEmptyException() {
+        super(Message.ERR_MSG_INVALID_INPUT_EMPTY);
+    }
+
+    public InputEmptyException(String message) {
+        super(message);
+    }
+}
diff --git a/me/smartstore/exception/InputEndException.java b/me/smartstore/exception/InputEndException.java
new file mode 100644
index 00000000..ffba020a
--- /dev/null
+++ b/me/smartstore/exception/InputEndException.java
@@ -0,0 +1,13 @@
+package me.smartstore.exception;
+
+import me.smartstore.util.Message;
+
+public class InputEndException extends RuntimeException {
+    public InputEndException() {
+        super(Message.ERR_MSG_INPUT_END);
+    }
+
+    public InputEndException(String message) {
+        super(message);
+    }
+}
diff --git a/me/smartstore/exception/InputFormatException.java b/me/smartstore/exception/InputFormatException.java
new file mode 100644
index 00000000..edc44e44
--- /dev/null
+++ b/me/smartstore/exception/InputFormatException.java
@@ -0,0 +1,13 @@
+package me.smartstore.exception;
+
+import me.smartstore.util.Message;
+
+public class InputFormatException extends RuntimeException {
+    public InputFormatException() {
+        super(Message.ERR_MSG_INVALID_INPUT_FORMAT);
+    }
+
+    public InputFormatException(String message) {
+        super(message);
+    }
+}
diff --git a/me/smartstore/exception/InputRangeException.java b/me/smartstore/exception/InputRangeException.java
new file mode 100644
index 00000000..10d7b011
--- /dev/null
+++ b/me/smartstore/exception/InputRangeException.java
@@ -0,0 +1,13 @@
+package me.smartstore.exception;
+
+import me.smartstore.util.Message;
+
+public class InputRangeException extends RuntimeException {
+    public InputRangeException() {
+        super(Message.ERR_MSG_INVALID_INPUT_RANGE);
+    }
+
+    public InputRangeException(String message) {
+        super(message);
+    }
+}
diff --git a/me/smartstore/exception/InputTypeException.java b/me/smartstore/exception/InputTypeException.java
new file mode 100644
index 00000000..0226f9c3
--- /dev/null
+++ b/me/smartstore/exception/InputTypeException.java
@@ -0,0 +1,13 @@
+package me.smartstore.exception;
+
+import me.smartstore.util.Message;
+
+public class InputTypeException extends RuntimeException {
+    public InputTypeException() {
+        super(Message.ERR_MSG_INVALID_INPUT_TYPE);
+    }
+
+    public InputTypeException(String message) {
+        super(message);
+    }
+}
diff --git a/me/smartstore/exception/NullArgumentException.java b/me/smartstore/exception/NullArgumentException.java
new file mode 100644
index 00000000..e2a8531d
--- /dev/null
+++ b/me/smartstore/exception/NullArgumentException.java
@@ -0,0 +1,22 @@
+package me.smartstore.exception;
+
+public class NullArgumentException extends RuntimeException {
+    public NullArgumentException() {
+    }
+
+    public NullArgumentException(String message) {
+        super(message);
+    }
+
+    public NullArgumentException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    public NullArgumentException(Throwable cause) {
+        super(cause);
+    }
+
+    public NullArgumentException(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
+        super(message, cause, enableSuppression, writableStackTrace);
+    }
+}
diff --git a/me/smartstore/group/Group.java b/me/smartstore/group/Group.java
new file mode 100644
index 00000000..ee40263a
--- /dev/null
+++ b/me/smartstore/group/Group.java
@@ -0,0 +1,58 @@
+package me.smartstore.group;
+
+import me.smartstore.exception.NullArgumentException;
+
+import java.util.Objects;
+
+public class Group {
+    private Parameter parameter; // 분류기준
+    private GroupType groupType; // 그룹 타입
+
+    public Group() {
+    }
+
+    public Group(Parameter parameter, GroupType groupType) {
+        if(parameter.getMinTime() == null || parameter.getMinPay() == null || groupType == null){
+            throw new NullArgumentException();
+        }else {
+            this.parameter = parameter;
+            this.groupType = groupType;
+        }
+    }
+
+    public Parameter getParameter() {
+        return parameter;
+    }
+
+    public void setParameter(Parameter parameter) {
+        this.parameter = parameter;
+    }
+
+    public GroupType getGroupType() {
+        return groupType;
+    }
+
+    public void setGroupType(GroupType groupType) {
+        this.groupType = groupType;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Group group = (Group) o;
+        return Objects.equals(parameter, group.parameter) && groupType == group.groupType;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(parameter, groupType);
+    }
+
+    @Override
+    public String toString() {
+        return
+                parameter + "\n" +
+                "groupType=" + groupType + "\n";
+    }
+}
diff --git a/me/smartstore/group/GroupType.java b/me/smartstore/group/GroupType.java
new file mode 100644
index 00000000..d6ccca37
--- /dev/null
+++ b/me/smartstore/group/GroupType.java
@@ -0,0 +1,22 @@
+package me.smartstore.group;
+
+public enum GroupType {
+    NONE("해당없음"), GENERAL("일반고객"), VIP("우수고객"), VVIP("최우수고객"),
+    N("해당없음"), G("일반고객"), V("우수고객"), VV("최우수고객");
+
+
+    String groupType = "";
+
+
+    GroupType(String groupType) {
+        this.groupType = groupType;
+    }
+
+    public GroupType replaceFullName() {
+        if (this == N) return NONE;
+        else if (this == G) return GENERAL;
+        else if (this == V) return VIP;
+        else if (this == VV) return VVIP;
+        return this;
+    }
+}
diff --git a/me/smartstore/group/Groups.java b/me/smartstore/group/Groups.java
new file mode 100644
index 00000000..c67396b6
--- /dev/null
+++ b/me/smartstore/group/Groups.java
@@ -0,0 +1,33 @@
+package me.smartstore.group;
+
+import me.smartstore.arrays.DArray;
+
+public class Groups extends DArray<Group> {
+    private static Groups allGroups;
+
+    public static Groups getInstance() {
+        if (allGroups == null) {
+            allGroups = new Groups();
+        }
+        return allGroups;
+    }
+
+    private Groups() {}
+
+    public Group find(GroupType groupType) {
+        for(int i = 0; i < this.size; i++) {
+            if(this.get(i).getGroupType() == groupType) {
+                return this.get(i);
+            }
+        }
+        return null;
+    }
+
+    public Group[] findAllGroup() {
+        Group[] result = new Group[this.size];
+        for(int i = 0; i < this.size; i++) {
+            result[i] = this.get(i);
+        }
+        return result;
+    }
+}
diff --git a/me/smartstore/group/Parameter.java b/me/smartstore/group/Parameter.java
new file mode 100644
index 00000000..9b711f41
--- /dev/null
+++ b/me/smartstore/group/Parameter.java
@@ -0,0 +1,54 @@
+package me.smartstore.group;
+
+import java.util.Objects;
+
+public class Parameter {
+    private Integer minTime;
+    private Integer minPay;
+
+
+    public Parameter() {
+    }
+
+    public Parameter(Integer minTime, Integer minPay) {
+        this.minTime = minTime;
+        this.minPay = minPay;
+    }
+
+    public Integer getMinTime() {
+        return minTime;
+    }
+
+    public void setMinTime(Integer minTime) {
+        this.minTime = minTime;
+    }
+
+    public Integer getMinPay() {
+        return minPay;
+    }
+
+    public void setMinPay(Integer minPay) {
+        this.minPay = minPay;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Parameter parameter = (Parameter) o;
+        return Objects.equals(minTime, parameter.minTime) && Objects.equals(minPay, parameter.minPay);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(minTime, minPay);
+    }
+
+    @Override
+    public String toString() {
+        return "Parameter{" +
+                "minTime=" + minTime +
+                ", minPay=" + minPay +
+                '}';
+    }
+}
diff --git a/me/smartstore/menu/CustomerMenu.java b/me/smartstore/menu/CustomerMenu.java
new file mode 100644
index 00000000..2bc92ed2
--- /dev/null
+++ b/me/smartstore/menu/CustomerMenu.java
@@ -0,0 +1,180 @@
+package me.smartstore.menu;
+
+import me.smartstore.customer.Customer;
+import me.smartstore.customer.Customers;
+import me.smartstore.exception.InputEmptyException;
+import me.smartstore.exception.InputEndException;
+import me.smartstore.group.Groups;
+import me.smartstore.util.Message;
+
+public class CustomerMenu implements Menu {
+    // singleton
+
+    private final Groups allGroups = Groups.getInstance();
+    private final Customers allCustomers = Customers.getInstance();
+    private static CustomerMenu customerMenu;
+
+    public static CustomerMenu getInstance() {
+        if (customerMenu == null) {
+            customerMenu = new CustomerMenu();
+        }
+        return customerMenu;
+    }
+
+    private CustomerMenu() {}
+
+    @Override
+    public void manage() {
+        while ( true ) { // 서브 메뉴 페이지를 유지하기 위한 while
+            int choice = chooseMenu(new String[]{
+                    "Add Customer",
+                    "View Customer",
+                    "Update Customer",
+                    "Delete Customer",
+                    "Back"
+            });
+            if(choice == 1) addCustomer();
+            else if(choice == 2) viewCustomer();
+            else if(choice == 3) updateCustomer();
+            else if(choice == 4) deleteCustomer();
+            else break;
+        }
+    }
+
+    private void addCustomer() {
+        while(true) {
+            try {
+                System.out.println("How many customers to input : ");
+                int inputCount = Integer.parseInt(nextLine(Message.END_MSG));
+                for(int i = 0; i < inputCount; i++) {
+                    System.out.printf("=============== Customer %d Info. ===============\n", i + 1);
+                    Customer addCustomer = customerInfoInput(new Customer());
+                    allCustomers.add(addCustomer);
+                    allCustomers.refresh(allGroups);
+                    System.out.printf(
+                            "＞　Success !!! %s  ＜\n" +
+                                    "           .A__A    ✨\uD83C\uDF82✨    A__A\n" +
+                                    "           ( •⩊•)  _______ (•⩊• )\n" +
+                                    "           (>\uD83C\uDF70>)   |   |   (<\uD83D\uDD2A<)\n", addCustomer);
+                }
+                break;
+            }catch (InputEndException e) {
+                System.out.println(Message.ERR_MSG_INPUT_END);
+                System.out.println();
+                break;
+            }
+        }
+    }
+
+    private Customer customerInfoInput(Customer customer) {
+        while(true) {
+            try {
+                int choice = chooseMenu(new String[]{
+                        "Customer Name",
+                        "Customer ID",
+                        "Customer Spent Time",
+                        "Customer Total Pay",
+                        "Back"
+                });
+                if(choice == 1) {
+                    System.out.print("Input Customer Name : ");
+                    customer.setCusName(nextLine(Message.END_MSG));
+                }else if (choice == 2) {
+                    System.out.print("Input Customer ID : ");
+                    customer.setCusId(nextLine(Message.END_MSG));
+                }else if(choice == 3) {
+                    System.out.print("Input Customer Spent Time : ");
+                    customer.setCusTotalTime(Integer.parseInt(nextLine(Message.END_MSG)));
+                }else if(choice == 4) {
+                    System.out.print("Input Customer Total Pay : ");
+                    customer.setCusTotalPay(Integer.parseInt(nextLine(Message.END_MSG)));
+                }else {
+                    if(customer.getCusId() == null || customer.getCusName() == null ||
+                            customer.getCusTotalTime() == null || customer.getCusTotalPay() == null) {
+                        System.out.println(Message.ERR_MSG_INVALID_INPUT_NULL);
+                    }else {
+                        return customer;
+                    }
+                }
+            }catch (InputEmptyException e){
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_EMPTY);
+            }catch (NumberFormatException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_FORMAT);
+            } catch (IndexOutOfBoundsException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_RANGE);
+            }
+        }
+    }
+
+    private void viewCustomer() {
+        if(allCustomers.size() == 0) {
+            System.out.println(Message.ERR_MSG_INVALID_ARR_EMPTY);
+            System.out.println();
+        }else {
+            System.out.println("======================================== Customer Info. ========================================");
+            int size = allCustomers.size();
+            for(int i = 0; i < size; i++) {
+                System.out.printf("No. %3d => ", i + 1 );
+                System.out.println(allCustomers.get(i));
+                System.out.println();
+            }
+            System.out.println("==================================================================================================");
+        }
+    }
+
+    private void updateCustomer() {
+        if(allCustomers.size() == 0) {
+            System.out.println(Message.ERR_MSG_INVALID_ARR_EMPTY);
+            System.out.println();
+        }else {
+            while(true) {
+                try {
+                    viewCustomer();
+                    System.out.printf("Which customer ( 1 ~ %d ) ? : ", allCustomers.size());
+                    int updateIndex = Integer.parseInt(nextLine(Message.END_MSG));
+                    Customer updateCustomer = allCustomers.get(updateIndex - 1);
+                    customerInfoInput(updateCustomer);
+                    allCustomers.refresh(allGroups);
+                }catch (InputEndException e) {
+                    System.out.println(Message.ERR_MSG_INPUT_END);
+                    System.out.println();
+                    break;
+                }
+            }
+        }
+    }
+
+    private void deleteCustomer() {
+        if(allCustomers.size() == 0) {
+            System.out.println(Message.ERR_MSG_INVALID_ARR_EMPTY);
+            System.out.println();
+        }else {
+            while(true) {
+                try {
+                    viewCustomer();
+                    if(allCustomers.size() == 0) {
+                        System.out.println(Message.ERR_MSG_INVALID_ARR_EMPTY);
+                        break;
+                    }
+                    System.out.printf("Which customer ( 1 ~ %d ) ? : ", allCustomers.size());
+                    int updateIndex = Integer.parseInt(nextLine(Message.END_MSG));
+                    allCustomers.pop(updateIndex - 1);
+                    System.out.println(
+                            "SUCCESS DELETE!!\n" +
+                                    "두다다다다다다다\n" +
+                                    "두다다다다다다다\n" +
+                                    "　(∩`・ω・)\n" +
+                                    "＿/_ミつ/￣￣￣/\n" +
+                                    "　　＼/＿＿＿/\n");
+                }catch (InputEndException e) {
+                    System.out.println(Message.ERR_MSG_INPUT_END);
+                    break;
+                }catch (NumberFormatException e) {
+                    System.out.println(Message.ERR_MSG_INVALID_INPUT_FORMAT);
+                }catch (IndexOutOfBoundsException e) {
+                    System.out.println(Message.ERR_MSG_INVALID_INPUT_RANGE);
+                }
+            }
+        }
+    }
+}
diff --git a/me/smartstore/menu/GroupMenu.java b/me/smartstore/menu/GroupMenu.java
new file mode 100644
index 00000000..69c067a8
--- /dev/null
+++ b/me/smartstore/menu/GroupMenu.java
@@ -0,0 +1,174 @@
+package me.smartstore.menu;
+
+import me.smartstore.customer.Customers;
+import me.smartstore.exception.InputEndException;
+import me.smartstore.exception.NullArgumentException;
+import me.smartstore.group.Group;
+import me.smartstore.group.GroupType;
+import me.smartstore.group.Groups;
+import me.smartstore.group.Parameter;
+import me.smartstore.util.Message;
+
+public class GroupMenu implements Menu {
+    private final Groups allGroups = Groups.getInstance();
+    private final Customers allCustomers = Customers.getInstance();
+    // singleton
+    private static GroupMenu groupMenu;
+
+    public static GroupMenu getInstance() {
+        if (groupMenu == null) {
+            groupMenu = new GroupMenu();
+        }
+        return groupMenu;
+    }
+
+    private GroupMenu() {}
+
+    @Override
+    public void manage() {
+        while ( true ) { // 서브 메뉴 페이지를 유지하기 위한 while
+            int choice = chooseMenu(new String[]{
+                    "Set Parameter",
+                    "View Parameter",
+                    "Update Parameter",
+                    "Back"});
+
+            if (choice == 1) setParameter();
+            else if (choice == 2) viewParameter();
+            else if (choice == 3) updateParameter();
+            else break; // choice == 4
+        }
+
+    }
+
+    public void setParameter() { // 초기화할 때만 호출 가능
+        while ( true ) {
+            try {
+                GroupType groupType = chooseGroup();
+                if (groupType == null) {
+                    break;
+                }
+                // GroupType에 해당하는 group 객체를 찾아야 함
+                Group group = allGroups.find(groupType);
+                if (group != null && group.getParameter() != null) { // group.getParameter()이 null이 아니면 이미 초기화됨
+                    System.out.println("\n" + group.getGroupType() + " group already exists.");
+                    System.out.println("\n" + group);
+                } else {
+                    Parameter parameter = new Parameter();
+                    // time, pay 사용자 입력받은 후, 설정 필요
+                    inputParameter(parameter);
+
+                    Group addGroup = new Group(parameter, groupType);
+                    allGroups.add(addGroup);
+                    allCustomers.refresh(allGroups); // 파라미터가 변경되었거나 추가되는 경우, 고객 분류를 다시 해야함
+                    break;
+
+                }
+            }catch (NullArgumentException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_NULL);
+            }
+        }
+    }
+
+    private void inputParameter(Parameter parameter) {
+        while(true) {
+            try {
+                int choice = chooseMenu(new String[]{
+                        "Minimum Spent Time",
+                        "Minimum Total Pay",
+                        "Back"
+                });
+                if (choice == 1) {
+                    System.out.println("Input Minimum Spent Time");
+                    parameter.setMinTime(Integer.parseInt(nextLine(Message.END_MSG)));
+                    System.out.println(parameter);
+                } else if (choice == 2) {
+                    System.out.println("Input Minimum Total Pay");
+                    parameter.setMinPay(Integer.parseInt(nextLine(Message.END_MSG)));
+                    System.out.println(parameter);
+                } else {
+                    if(parameter.getMinPay() == null || parameter.getMinTime() == null) {
+                        break;
+                    }else {
+                        System.out.printf(
+                                "＿人人人人人人人人人人人人人人人人人人人人人人人人人人人人人人＿\n" +
+                                "＞　Success !!! %s  ＜\n" +
+                                "￣^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y￣\n" +
+                                "               .A__A    ✨\uD83C\uDF82✨    A__A\n" +
+                                "               ( •⩊•)  _______ (•⩊• )\n" +
+                                "               (>\uD83C\uDF70>)   |   |   (<\uD83D\uDD2A<)\n", parameter);
+                        break;
+                    }
+                }
+            }catch (InputEndException e) {
+                System.out.println(Message.ERR_MSG_INPUT_END);
+                break;
+            }catch (NumberFormatException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_FORMAT);
+            }
+        }
+    }
+
+    private void viewParameter() {
+        while(true) {
+            GroupType groupType = chooseGroup();
+            if(groupType == null) {
+                break;
+            }
+            Group group = allGroups.find(groupType);
+            if (group != null && group.getParameter() != null) {
+                System.out.println("\n ─────────[" + group.getGroupType() + "]─────────");
+                System.out.println(group);
+                System.out.println("──────────────────────────────────────────");
+            } else {
+                System.out.printf("Please Set Parameter first %s\n", groupType);
+                System.out.println();
+            }
+        }
+
+    }
+
+    private void updateParameter() {
+        while ( true ) {
+            GroupType groupType = chooseGroup();
+            if(groupType == null) {
+                break;
+            }
+            Group group = allGroups.find(groupType);
+            if (group != null && group.getParameter() != null) {
+                inputParameter(group.getParameter());
+                allCustomers.refresh(allGroups);
+            } else {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_NULL);
+                System.out.println();
+            }
+        }
+    }
+
+    public GroupType chooseGroup() {
+        while ( true ) {
+            try {
+                System.out.println(
+                        "┬ ┬┬ ┬┬┌─┐┬ ┬  ┌─┐┬─┐┌─┐┬ ┬┌─┐\n" +
+                        "│││├─┤││  ├─┤  │ ┬├┬┘│ ││ │├─┘\n" +
+                        "└┴┘┴ ┴┴└─┘┴ ┴  └─┘┴└─└─┘└─┘┴   \n" +
+                        "─────────────────────────────────");
+                GroupType[] groupTypes = GroupType.values();
+                for(int i = 0; i < groupTypes.length / 2; i++) {
+                    System.out.printf("         → %s (%s) ? \n", groupTypes[i], groupTypes[i + groupTypes.length / 2]);
+                }
+                System.out.println("─────────────────────────────────");
+                String choice = nextLine(Message.END_MSG);
+                // group (str) -> GroupType (enum)
+                // "VIP" -> GroupType.VIP
+                GroupType groupType = GroupType.valueOf(choice).replaceFullName();
+                return groupType;
+            } catch (InputEndException e) {
+                System.out.println(Message.ERR_MSG_INPUT_END);
+                return null;
+            } catch (IllegalArgumentException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_RANGE);
+            }
+        }
+    }
+}
diff --git a/me/smartstore/menu/MainMenu.java b/me/smartstore/menu/MainMenu.java
new file mode 100644
index 00000000..a7768657
--- /dev/null
+++ b/me/smartstore/menu/MainMenu.java
@@ -0,0 +1,39 @@
+package me.smartstore.menu;
+
+public class MainMenu implements Menu {
+
+    private final CustomerMenu customerMenu = CustomerMenu.getInstance();
+    private final GroupMenu groupMenu = GroupMenu.getInstance();
+    private final SummaryMenu summaryMenu = SummaryMenu.getInstance();
+
+    // singleton
+    private static MainMenu mainMenu;
+
+    public static MainMenu getInstance() {
+        if (mainMenu == null) {
+            mainMenu = new MainMenu();
+        }
+        return mainMenu;
+    }
+
+    private MainMenu() {}
+
+    @Override
+    public void manage() {
+        while ( true ) { // 프로그램 실행 while
+            int choice = mainMenu.chooseMenu(new String[] {
+                    "Parameter",
+                    "Customer",
+                    "Classification Summary",
+                    "Quit"});
+
+            if (choice == 1) groupMenu.manage();
+            else if (choice == 2) customerMenu.manage();
+            else if (choice == 3) summaryMenu.manage();
+            else { // choice == 4
+                System.out.println("Program Finished");
+                break;
+            }
+        }
+    }
+}
diff --git a/me/smartstore/menu/Menu.java b/me/smartstore/menu/Menu.java
new file mode 100644
index 00000000..15051939
--- /dev/null
+++ b/me/smartstore/menu/Menu.java
@@ -0,0 +1,55 @@
+package me.smartstore.menu;
+
+import me.smartstore.exception.InputEndException;
+import me.smartstore.exception.InputRangeException;
+import me.smartstore.util.Message;
+
+import java.util.InputMismatchException;
+import java.util.Scanner;
+
+public interface Menu {
+    Scanner scanner = new Scanner(System.in);
+
+    default String nextLine() { // 하나의 프로그램 상에서 nextLine() 함수를 통해서 사용자 입력을 받음
+        return scanner.nextLine().toUpperCase();
+    }
+
+    default String nextLine(String end) {
+        System.out.println("** Press 'end', if you want to exit! **");
+        System.out.print(">>>> input : ");
+        String str = nextLine();
+        if (str.equals(end)) throw new InputEndException();
+        return str;
+    }
+
+    default int chooseMenu(String[] menus) {
+        while ( true ) { // 예외 복구 while
+            try {
+                System.out.println(
+                        "                 ╔╦╗╔═╗╔╗╔╦ ╦                 \n" +
+                        "───────────────  ║║║║╣ ║║║║ ║  ───────────────\n" +
+                        "                 ╩ ╩╚═╝╝╚╝╚═╝                 "
+                );
+
+                for (int i = 0; i < menus.length; i++) {
+                    System.out.printf("         %d. %s\n", i + 1, menus[i]);
+                }
+                System.out.println(
+                        "                              \n" +
+                        "──────────────────────────────────────────────\n" +
+                        "                              "
+                );
+                System.out.print("Choose One: ");
+                int choice = Integer.parseInt(nextLine());
+                if (choice >= 1 && choice <= menus.length) return choice;
+                throw new InputRangeException(); // choice 가 범위에 벗어남
+            } catch (InputMismatchException | NumberFormatException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_FORMAT);
+            } catch (InputRangeException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_RANGE);
+            }
+        }
+    }
+
+    void manage(); // 각 서브메뉴들을 관리하는 함수 (각 서브메뉴의 최상위 메뉴)
+}
diff --git a/me/smartstore/menu/OrderType.java b/me/smartstore/menu/OrderType.java
new file mode 100644
index 00000000..494c59a0
--- /dev/null
+++ b/me/smartstore/menu/OrderType.java
@@ -0,0 +1,20 @@
+package me.smartstore.menu;
+
+public enum OrderType {
+
+    A("오름차순"), D("내림차순"),
+    ASCENDING("오름차순"), DESCENDING("내림차순");
+
+
+    String orderType = "";
+
+    OrderType(String orderType) {
+        this.orderType = orderType;
+    }
+
+    public OrderType replaceFullName() {
+        if (this == A) return ASCENDING;
+        else if (this == D) return DESCENDING;
+        return this;
+    }
+}
diff --git a/me/smartstore/menu/SummaryMenu.java b/me/smartstore/menu/SummaryMenu.java
new file mode 100644
index 00000000..7ccd1ba5
--- /dev/null
+++ b/me/smartstore/menu/SummaryMenu.java
@@ -0,0 +1,196 @@
+package me.smartstore.menu;
+
+import me.smartstore.customer.Customer;
+import me.smartstore.customer.Customers;
+import me.smartstore.exception.InputEndException;
+import me.smartstore.group.Group;
+import me.smartstore.group.Groups;
+import me.smartstore.util.Message;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+public class SummaryMenu implements Menu {
+    // singleton
+    private static SummaryMenu summaryMenu;
+
+    private final Groups allGroups = Groups.getInstance();
+    private final Customers allCustomers = Customers.getInstance();
+
+    public static SummaryMenu getInstance() {
+        if (summaryMenu == null) {
+            summaryMenu = new SummaryMenu();
+        }
+        return summaryMenu;
+    }
+
+    private SummaryMenu() {}
+
+    @Override
+    public void manage() {
+        if(allCustomers.size() == 0) {
+            System.out.println(Message.ERR_MSG_INVALID_ARR_EMPTY);
+        }else {
+            while ( true ) { // 서브 메뉴 페이지를 유지하기 위한 while
+                int choice = chooseMenu(new String[]{
+                        "Summary",
+                        "Summary (Sorted By Name)",
+                        "Summary (Sorted By Time)",
+                        "Summary (Sorted By Pay)",
+                        "Back"});
+                if (choice == 1) summary();
+                else if (choice == 2) summaryName();
+                else if (choice == 3) summaryTime();
+                else if (choice == 4) summaryPay();
+                else break;
+            }
+        }
+    }
+
+    private void summary() {
+        int groupSize = allGroups.size();
+        int customerSize = allCustomers.size();
+        for (int i = 0; i < groupSize; i++) {
+            Group findGroup = showGroupInfo(i);
+            int count = 1;
+            for (int j = 0; j < customerSize; j++) {
+                if (allCustomers.get(j).getGroup() == findGroup) {
+                    System.out.printf("No. %3d => %s \n", count++, allCustomers.get(j).toString());
+                }
+            }
+            System.out.println();
+        }
+    }
+
+    private void summaryName() {
+        int groupSize = allGroups.size();
+        while(true) {
+            try {
+                System.out.println("Which order (ASCENDING (A), DESCENDING (D))?");
+                String order = nextLine(Message.END_MSG);
+
+                OrderType orderType = findOrder(order);
+                if(orderType == null) {
+                    System.out.println(Message.ERR_MSG_INVALID_INPUT_RANGE);
+                }else {
+                    for(int i = 0; i < groupSize; i++) {
+                        Group findGroup = showGroupInfo(i);
+
+                        Customer[] customers = allCustomers.findCustomerByGroup(findGroup);
+
+                        if (String.valueOf(orderType).equals("ASCENDING")) {
+                            Arrays.sort(customers, Comparator.comparing(Customer::getCusName));
+                            for (int j = 0; j < customers.length; j++) {
+                                System.out.printf("No. %3d => %s \n", j + 1, customers[j].toString());
+                            }
+                        } else {
+                            Arrays.sort(customers, (o1, o2) -> o2.getCusName().compareTo(o1.getCusName()));
+                            for (int j = 0; j < customers.length; j++) {
+                                System.out.printf("No. %3d => %s \n", j + 1, customers[j].toString());
+                            }
+                        }
+                    }
+                }
+            }catch (InputEndException e) {
+                System.out.println(Message.ERR_MSG_INPUT_END);
+                break;
+            }catch (IndexOutOfBoundsException e) {
+                System.out.println(Message.ERR_MSG_INVALID_INPUT_EMPTY);
+                break;
+            }
+        }
+    }
+
+    private OrderType findOrder(String order) {
+        OrderType[] values = OrderType.values();
+        for(int i = 0; i < values.length; i++) {
+            if(values[i].toString().equals(order)) {
+               return values[i].replaceFullName();
+            }
+        }
+        return null;
+    }
+
+    private void summaryTime() {
+        int groupSize = allGroups.size();
+        while(true) {
+            try {
+                System.out.println("Which order (ASCENDING (A), DESCENDING (D))?");
+                String order = nextLine(Message.END_MSG);
+
+                OrderType orderType = findOrder(order);
+                if(orderType == null) {
+                    System.out.println(Message.ERR_MSG_INVALID_INPUT_RANGE);
+                }else {
+                    for(int i = 0; i < groupSize; i++) {
+                        Group findGroup = showGroupInfo(i);
+
+                        Customer[] customers = allCustomers.findCustomerByGroup(findGroup);
+
+                        if (String.valueOf(orderType).equals("ASCENDING")) {
+                            Arrays.sort(customers, Comparator.comparing(Customer::getCusTotalTime));
+                            for (int j = 0; j < customers.length; j++) {
+                                System.out.printf("No. %3d => %s \n", j + 1, customers[j].toString());
+                            }
+                        } else {
+                            Arrays.sort(customers, (o1, o2) -> o2.getCusTotalTime().compareTo(o1.getCusTotalTime()));
+                            for (int j = 0; j < customers.length; j++) {
+                                System.out.printf("No. %3d => %s \n", j + 1, customers[j].toString());
+                            }
+                        }
+                    }
+                }
+            }catch (InputEndException e) {
+                System.out.println(Message.ERR_MSG_INPUT_END);
+                break;
+            }
+        }
+    }
+
+    private void summaryPay() {
+        int groupSize = allGroups.size();
+        while(true) {
+            try {
+                System.out.println("Which order (ASCENDING (A), DESCENDING (D))?");
+                String order = nextLine(Message.END_MSG);
+
+                OrderType orderType = findOrder(order);
+
+                if(orderType == null) {
+                    System.out.println(Message.ERR_MSG_INVALID_INPUT_RANGE);
+                }else {
+                    for(int i = 0; i < groupSize; i++) {
+                        Group findGroup = showGroupInfo(i);
+
+                        Customer[] customers = allCustomers.findCustomerByGroup(findGroup);
+
+                        if(String.valueOf(orderType).equals("ASCENDING")) {
+                            Arrays.sort(customers, Comparator.comparing(Customer::getCusTotalPay));
+                            for(int j = 0; j < customers.length; j++) {
+                                System.out.printf("No. %3d => %s \n", j + 1, customers[j].toString());
+                            }
+                        } else {
+                            Arrays.sort(customers, (o1, o2) -> o2.getCusTotalPay().compareTo(o1.getCusTotalPay()));
+                            for(int j = 0; j < customers.length; j++) {
+                                System.out.printf("No. %3d => %s \n", j + 1, customers[j].toString());
+                            }
+                        }
+                    }
+                }
+            }catch (InputEndException e) {
+                System.out.println(Message.ERR_MSG_INPUT_END);
+                break;
+            }
+        }
+    }
+
+    private Group showGroupInfo(int i) {
+        System.out.print("==============================");
+        Group findGroup = allGroups.get(i);
+        System.out.printf("Group : %s ( Time : %d, Pay : %d )", findGroup.getGroupType(),
+                findGroup.getParameter().getMinTime(),
+                findGroup.getParameter().getMinPay());
+        System.out.println("==============================");
+        return findGroup;
+    }
+}
diff --git a/me/smartstore/util/Message.java b/me/smartstore/util/Message.java
new file mode 100644
index 00000000..1c67d21e
--- /dev/null
+++ b/me/smartstore/util/Message.java
@@ -0,0 +1,13 @@
+package me.smartstore.util;
+
+public interface Message {
+    String ERR_MSG_INVALID_ARR_EMPTY = "No Customers. Please input one first.";
+    String ERR_MSG_NULL_ARR_ELEMENT = "Elements in Array has null. Array can't be sorted.";
+    String ERR_MSG_INVALID_INPUT_NULL = "Null Input. Please input something.";
+    String ERR_MSG_INVALID_INPUT_EMPTY = "Empty Input. Please input something.";
+    String ERR_MSG_INVALID_INPUT_RANGE = "Invalid Input. Please try again.";
+    String ERR_MSG_INVALID_INPUT_TYPE = "Invalid Type for Input. Please try again.";
+    String ERR_MSG_INVALID_INPUT_FORMAT = "Invalid Format for Input. Please try again.";
+    String ERR_MSG_INPUT_END = "END is pressed. Exit this menu.";
+    String END_MSG = "END";
+}