diff --git a/src/main/kotlin/data/UserRepository.kt b/src/main/kotlin/data/UserRepository.kt
index 19ea027..6814e59 100644
--- a/src/main/kotlin/data/UserRepository.kt
+++ b/src/main/kotlin/data/UserRepository.kt
@@ -1,5 +1,11 @@
 package org.example.data
 
-interface UserRepository {
+import org.example.data.model.UserDTO
 
+interface UserRepository {
+    fun addUser()
+    fun removeUserById(userId: Int): Boolean
+    fun updateUserById(userId: Int, newUser: UserDTO): UserDTO
+    fun getAllUsers(): List<UserDTO>
+    fun findUserById(userId: Int): UserDTO
 }
\ No newline at end of file
diff --git a/src/main/kotlin/data/UserRepositoryImpl.kt b/src/main/kotlin/data/UserRepositoryImpl.kt
index 8cd1788..bd3122a 100644
--- a/src/main/kotlin/data/UserRepositoryImpl.kt
+++ b/src/main/kotlin/data/UserRepositoryImpl.kt
@@ -1,4 +1,33 @@
 package org.example.data
 
-class UserRepositoryImpl {
+import org.example.data.model.UserDTO
+
+class UserRepositoryImpl: UserRepository {
+    private val userSource: MutableList<UserDTO> = userList.toMutableList()
+
+    override fun addUser() {
+        TODO("Not yet implemented")
+    }
+
+    override fun removeUserById(userId: Int): Boolean {
+       return userSource.removeIf { it.userId == userId }
+    }
+
+    override fun updateUserById(userId: Int, newUser: UserDTO): UserDTO {
+        val oldUser = userSource.first { it.userId == userId }
+        oldUser.phone = newUser.phone
+        oldUser.address = newUser.address
+        oldUser.firstName = newUser.firstName
+        oldUser.lastName = newUser.lastName
+        oldUser.password = newUser.password
+        return oldUser
+    }
+
+    override fun getAllUsers(): List<UserDTO> = userList
+
+    override fun findUserById(userId: Int): UserDTO {
+        val findUser = userList.firstOrNull { it.userId == userId }
+        checkNotNull(findUser)
+        return findUser
+    }
 }
\ No newline at end of file
diff --git a/src/main/kotlin/data/model/UserDTO.kt b/src/main/kotlin/data/model/UserDTO.kt
index bc257f0..678a1ac 100644
--- a/src/main/kotlin/data/model/UserDTO.kt
+++ b/src/main/kotlin/data/model/UserDTO.kt
@@ -2,7 +2,7 @@ package org.example.data.model
 
 data class UserDTO(
     val userId: Int,
-    val firstName: String,
+    var firstName: String,
     var lastName: String? = null,
     var password: String,
     var phone: String? = null,