diff --git a/.idea/material_theme_project_new.xml b/.idea/material_theme_project_new.xml
new file mode 100644
index 0000000..21fcfac
--- /dev/null
+++ b/.idea/material_theme_project_new.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="MaterialThemeProjectNewConfig">
+    <option name="metadata">
+      <MTProjectMetadataState>
+        <option name="userId" value="7044f647:19873776121:-7ff2" />
+      </MTProjectMetadataState>
+    </option>
+  </component>
+</project>
\ No newline at end of file
diff --git a/src/employee_management/Department.java b/src/employee_management/Department.java
index 09c2577..81cf742 100644
--- a/src/employee_management/Department.java
+++ b/src/employee_management/Department.java
@@ -1,15 +1,67 @@
 package employee_management;
 
-/**
- * TODO : a department is a collection of employees. It has a name and a set of employees in it.
- */
+import java.util.ArrayList;
+import java.util.List;
+
 public class Department {
-    /**
-     * TODO : Employees should never ever be duplicated in a single department.
-     * Please check if it already exists in the department before inserting if need be. Duplicate
-     * here means : their employee id and name are the same.
-     *
-     * TODO : for our HR officer, give a function to show all details about employees, including
-     * their pay.
-     */
+    private final String name;
+    private List<Employee> employees = new ArrayList<>() ;
+
+    public Department(String name, List<Employee> employees) {
+        this.name = name;
+        this.employees = employees;
+    }
+
+    public Department(String name) {
+        this.name = name;
+    }
+
+    public void addEmployee(Employee employee){
+        boolean isExisting = false;
+        for (Employee e : employees) {
+            if (e.getEmployeeId() == employee.getEmployeeId() && e.getName().equals(employee.getName())) {
+                isExisting = true;
+                break;
+            }
+        }
+        if (!isExisting) {
+            employees.add(employee);
+        }
+    }
+
+    public String getDepartmentDetails(){
+        StringBuilder details = new StringBuilder();
+        details.append("Department Name : ").append(name).append("\n");
+        details.append("-----------------------------------\n");
+        for (Employee e : employees) {
+            details.append(e.toString()).append("\n");
+            details.append("-----------------------------------\n");
+        }
+        return details.toString();
+    }
+
+    public String getEmployeesFullDetails(){
+        StringBuilder details = new StringBuilder();
+        details.append("Department Name : ").append(name).append("\n");
+        details.append("-----------------------------------\n");
+        for (Employee e : employees) {
+            if (e instanceof FullTimeEmployee) {
+                details.append(e.toString())
+                        .append("\nPay : ")
+                        .append(((FullTimeEmployee) e).calculatePay())
+                        .append("\n-----------------------------------\n");
+            }
+            else if (e instanceof PartTimeEmployee) {
+                details.append(e.toString())
+                        .append("\nHourly Rate : ")
+                        .append(((PartTimeEmployee) e).getHourlyRate())
+                        .append("\nHours Worked : ")
+                        .append(((PartTimeEmployee) e).getHoursWorked())
+                        .append("\nPay : ")
+                        .append(((PartTimeEmployee) e).calculatePay())
+                        .append("\n-----------------------------------\n");
+            }
+        }
+        return details.toString();
+    }
 }
diff --git a/src/employee_management/Employee.java b/src/employee_management/Employee.java
new file mode 100644
index 0000000..188c2c3
--- /dev/null
+++ b/src/employee_management/Employee.java
@@ -0,0 +1,39 @@
+package employee_management;
+
+public abstract class Employee {
+    public static int lastEmployeeId = 0;
+    private final int employeeId;
+    private final String name;
+
+    public Employee(int employeeId, String name) {
+        if (employeeId <= 0 || employeeId < lastEmployeeId) {
+            throw new IllegalArgumentException("Invalid Employee Id");
+        }
+        this.employeeId = employeeId;
+        lastEmployeeId = employeeId;
+        this.name = name;
+    }
+
+    public Employee (String name){
+        this.employeeId = lastEmployeeId + 1;
+        lastEmployeeId++;
+        this.name = name;
+    }
+
+    public int getEmployeeId() {
+        return employeeId;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public abstract double calculatePay();
+    public abstract double calculateTax();
+    @Override
+    public String toString() {
+        return "EmployeeId : " + employeeId + "\nName : " + name;
+    }
+
+
+}
diff --git a/src/employee_management/FullTimeEmployee.java b/src/employee_management/FullTimeEmployee.java
index 1d5b60a..84d1ed3 100644
--- a/src/employee_management/FullTimeEmployee.java
+++ b/src/employee_management/FullTimeEmployee.java
@@ -1,18 +1,20 @@
 package employee_management;
 
-public class FullTimeEmployee {
-    public int employeeId;
-    public String name;
-    public double monthlySalary;
+public class FullTimeEmployee extends Employee{
+    private double monthlySalary;
 
     public FullTimeEmployee(int employeeId, String name, double monthlySalary) {
-        this.employeeId = employeeId;
-        this.name = name;
+        super(employeeId, name);
         this.monthlySalary = monthlySalary;
     }
-
+    @Override
     public double calculatePay() {
         return monthlySalary;
     }
+
+    @Override
+    public double calculateTax() {
+        return monthlySalary * 0.2;
+    }
 }
 
diff --git a/src/employee_management/PartTimeEmployee.java b/src/employee_management/PartTimeEmployee.java
index ab234c7..d25d60a 100644
--- a/src/employee_management/PartTimeEmployee.java
+++ b/src/employee_management/PartTimeEmployee.java
@@ -1,21 +1,34 @@
 package employee_management;
 
-public class PartTimeEmployee {
-    public int employeeId;
-    public String name;
-    public double hoursWorked;
-    public double hourlyRate;
-
+public class PartTimeEmployee extends Employee{
+    private double hoursWorked;
+    private double hourlyRate;
 
     public PartTimeEmployee(int employeeId, String name, double hoursWorked, double hourlyRate) {
-        this.employeeId = employeeId;
-        this.name = name;
+        super(employeeId, name);
         this.hoursWorked = hoursWorked;
         this.hourlyRate = hourlyRate;
     }
 
+    public double getHoursWorked() {
+        return hoursWorked;
+    }
+    public double getHourlyRate() {
+        return hourlyRate;
+    }
+    @Override
     public double calculatePay() {
         return hoursWorked * hourlyRate;
     }
+
+    @Override
+    public double calculateTax() {
+        return hoursWorked * hourlyRate * 0.15;
+    }
+
+    @Override
+    public String toString() {
+        return super.toString() +"\nHours Worked : " + hoursWorked;
+    }
 }
 
diff --git a/src/employee_management/TestCases.java b/src/employee_management/TestCases.java
index 4afd0b7..5f3da7d 100644
--- a/src/employee_management/TestCases.java
+++ b/src/employee_management/TestCases.java
@@ -13,15 +13,6 @@ public static void main(String[] args) {
         FullTimeEmployee koto = TestCases.fullTimeEmployeeKoto();
         PartTimeEmployee lita = TestCases.partTimeEmployeeLita();
 
-        // TODO : prevent the direct access here
-        koto.monthlySalary = 6_000_000;
-        lita.hoursWorked = 1_000;
-
-        // TODO : do not allow incoherent changes on salaries (applies to full time employees too)
-        lita.hourlyRate = -10_000_000;
-
-        // TODO : fix the string format of both full time and part time employees
-        // NOTE : do not show sensitive data such as hourlyRate or monthlySalary here !
         System.out.println("Details about Koto " + koto);
         System.out.println("Details about Lita " + lita);
     }