feat: upgrade user management demo

src/main/java/com/example/demo/service/UserService.java

@@ -1,65 +1,112 @@
 package com.example.demo.service;
 
+import com.example.demo.dto.UserStatsResponse;
+import com.example.demo.exception.DuplicateEmailException;
 import com.example.demo.exception.ResourceNotFoundException;
 import com.example.demo.model.User;
 import org.springframework.stereotype.Service;
 
-import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.stream.Collectors;
 
 @Service
 public class UserService {
 
-    private final List<User> users = new ArrayList<>();
+    private final List<User> users = new CopyOnWriteArrayList<>();
     private final AtomicLong idGenerator = new AtomicLong(1);
 
     public UserService() {
-        users.add(new User(idGenerator.getAndIncrement(), "张三", "zhangsan@example.com", 25));
-        users.add(new User(idGenerator.getAndIncrement(), "李四", "lisi@example.com", 30));
-        users.add(new User(idGenerator.getAndIncrement(), "王五", "wangwu@example.com", 28));
+        users.add(new User(idGenerator.getAndIncrement(), "Alice Chen", "alice@example.com", 25));
+        users.add(new User(idGenerator.getAndIncrement(), "Brandon Li", "brandon@example.com", 30));
+        users.add(new User(idGenerator.getAndIncrement(), "Carol Wang", "carol@example.com", 28));
     }
 
     public List<User> findAll() {
-        return new ArrayList<>(users);
+        return users.stream()
+                .sorted((left, right) -> Long.compare(left.getId(), right.getId()))
+                .collect(Collectors.toList());
     }
 
     public User findById(Long id) {
         return users.stream()
-                .filter(u -> u.getId().equals(id))
+                .filter(user -> user.getId().equals(id))
                 .findFirst()
-                .orElseThrow(() -> new ResourceNotFoundException("用户不存在: id=" + id));
+                .orElseThrow(() -> new ResourceNotFoundException("User not found: id=" + id));
     }
 
-    public List<User> findByName(String name) {
+    public List<User> search(String keyword) {
+        if (keyword == null || keyword.isBlank()) {
+            return findAll();
+        }
+
+        String normalizedKeyword = keyword.trim().toLowerCase();
         return users.stream()
-                .filter(u -> u.getName().contains(name))
+                .filter(user -> user.getName().toLowerCase().contains(normalizedKeyword)
+                        || user.getEmail().toLowerCase().contains(normalizedKeyword))
+                .sorted((left, right) -> Long.compare(left.getId(), right.getId()))
                 .collect(Collectors.toList());
     }
 
     public User create(User user) {
-        user.setId(idGenerator.getAndIncrement());
-        users.add(user);
-        return user;
+        ensureEmailAvailable(user.getEmail(), null);
+        User normalized = normalize(user, idGenerator.getAndIncrement());
+        users.add(normalized);
+        return normalized;
     }
 
     public User update(Long id, User user) {
         findById(id);
-        user.setId(id);
-        for (int i = 0; i < users.size(); i++) {
-            if (users.get(i).getId().equals(id)) {
-                users.set(i, user);
-                return user;
+        ensureEmailAvailable(user.getEmail(), id);
+
+        User normalized = normalize(user, id);
+        for (int index = 0; index < users.size(); index++) {
+            if (users.get(index).getId().equals(id)) {
+                users.set(index, normalized);
+                return normalized;
             }
         }
-        throw new ResourceNotFoundException("用户不存在: id=" + id);
+
+        throw new ResourceNotFoundException("User not found: id=" + id);
     }
 
     public void delete(Long id) {
-        boolean removed = users.removeIf(u -> u.getId().equals(id));
+        boolean removed = users.removeIf(user -> user.getId().equals(id));
         if (!removed) {
-            throw new ResourceNotFoundException("用户不存在: id=" + id);
+            throw new ResourceNotFoundException("User not found: id=" + id);
         }
     }
+
+    public UserStatsResponse getStats() {
+        long totalUsers = users.size();
+        long adults = users.stream().filter(user -> user.getAge() >= 18).count();
+        long underThirty = users.stream().filter(user -> user.getAge() < 30).count();
+        double averageAge = users.stream()
+                .mapToInt(User::getAge)
+                .average()
+                .orElse(0.0);
+
+        return new UserStatsResponse(totalUsers, adults, underThirty, averageAge);
+    }
+
+    private void ensureEmailAvailable(String email, Long currentUserId) {
+        String normalizedEmail = email == null ? "" : email.trim().toLowerCase();
+        boolean exists = users.stream()
+                .anyMatch(user -> user.getEmail().equalsIgnoreCase(normalizedEmail)
+                        && (currentUserId == null || !user.getId().equals(currentUserId)));
+
+        if (exists) {
+            throw new DuplicateEmailException("A user with email " + normalizedEmail + " already exists.");
+        }
+    }
+
+    private User normalize(User user, Long id) {
+        return new User(
+                id,
+                user.getName().trim(),
+                user.getEmail().trim().toLowerCase(),
+                user.getAge()
+        );
+    }
 }