Show nice error message if username already exists

Closes #35

src/main/java/catering/users/UserController.java

@@ -35,12 +35,18 @@ public class UserController {
 	}
 
 	@PostMapping("/register")
-	String register(@Valid RegistrationForm form, Errors result) {
+	String register(@Valid RegistrationForm form, Errors result, Model model) {
 		if (result.hasErrors()){
 			return "register";
 		}
-		userManagement.createCustomer(form.getUsername(),form.getAddress(),form.getPassword());
-		return "redirect:/login";
+		if (userManagement.userNameExists(form.getUsername())) {
+			model.addAttribute("userNameExists",true);
+			return "register";
+		} else {
+			model.addAttribute("userNameExists",false);
+			userManagement.createCustomer(form.getUsername(),form.getAddress(),form.getPassword());
+			return "redirect:/login";
+		}
 	}
 
 	@GetMapping("/login")

src/main/java/catering/users/UserManagement.java

@@ -51,4 +51,12 @@ public class UserManagement {
 	public Optional<User> getUserByAccount(UserAccount userAccount) {
 		return users.findAll().stream().filter(u -> u.getUserAccount().equals(userAccount)).findFirst();
 	}
+
+	public boolean userNameExists(String username){
+		return !users.findAll().stream()
+			.filter(u -> u.getUsername().equals(username))
+			.toList()
+			.isEmpty();
+	}
+
 }

src/main/resources/templates/register.html

@@ -9,6 +9,7 @@
 			<div class="mb-3">
 				<label class="form-label" for="username">Nutzername</label>
 				<input class="form-control" name="username" type="text" required>
+				<p th:if="${userNameExists}">Nutzername vergeben</p>
 			</div>
 			<div class="mb-3">
 				<label class="form-label" for="password">Passwort</label>