feat: adds add and remove employee , employee count , and assertion(s) test

Author: AmiyahJo

lesson_15/tdd/tdd_app/src/test/java/com/codedifferently/lesson15/EmployeeManagerTest.java

@@ -1,6 +1,7 @@
 package com.codedifferently.lesson15;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -44,18 +45,61 @@ void testUpdateEmployee() {
     @Test
     void testGetEmployee() {
         //Arrange
+         Employee employee = new Employee(1, "John Doe", null, 0);
+        
         //Act
+        employeeManager.addEmployee(employee);
+        Employee retrievedEmployee = employeeManager.getEmployee(1);
+
+        //Assert
+        assertEquals(employee.getId(), retrievedEmployee.getId());
+        assertEquals(employee.getName(), retrievedEmployee.getName());
+
     }
 
     @Test
     void testRemoveEmployee() {
-        //Arrange
-        //Act
+       
+        Employee employee = new Employee(1, "John Doe", null, 0);
+    
+        employeeManager.addEmployee(employee);
+        assertEquals(1, employeeManager.getEmployeeCount());
+
+        employeeManager.removeEmployee(1);       
+        assertEquals(0, employeeManager.getEmployeeCount());
+        
     }
 
     @Test
     void testGetEmployeeCount() {
-        //Arrange
-        //Act
+
+        assertEquals(0, employeeManager.getEmployeeCount());
+
+        employeeManager.addEmployee(new Employee(1, "John Doe", null, 0));
+        employeeManager.addEmployee(new Employee(2, "Jane Doe", null, 0));
+        
+        assertEquals(2, employeeManager.getEmployeeCount());
+    } 
+    
+    @Test
+    void testGetEmployeeThrowsExceptionForNonExistentId() {
+        assertThrows(IllegalArgumentException.class, () -> {
+            employeeManager.getEmployee(999);
+        });
+    }
+
+    @Test
+    void testUpdateEmployeeThrowsExceptionForNonExistentId() {
+        Employee employee = new Employee(1, "John Doe", null, 0);
+        assertThrows(IllegalArgumentException.class, () -> {
+            employeeManager.updateEmployee(employee);
+        });
+    }
+
+    @Test
+    void testRemoveEmployeeThrowsExceptionForNonExistentId() {
+        assertThrows(IllegalArgumentException.class, () -> {
+            employeeManager.removeEmployee(999);
+        });
     }
 }