在Java Web應(yīng)用中，增刪改查是一個(gè)常見的需求。如果需要對(duì)帳號(hào)和密碼進(jìn)行操作，可以使用以下代碼示例：

public class User {
private String username;
private String password;
public User(String username, String password) {
this.username = username;
this.password = password;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}
public class UserDao {
private ListuserList;
public UserDao() {
userList = new ArrayList<>();
userList.add(new User("admin", "admin"));
userList.add(new User("test", "test"));
}
public ListgetUserList() {
return userList;
}
public void addUser(User user) {
userList.add(user);
}
public void updateUser(String username, User user) {
for (int i = 0; i< userList.size(); i++) {
if (userList.get(i).getUsername().equals(username)) {
userList.set(i, user);
break;
}
}
}
public void deleteUser(String username) {
for (int i = 0; i< userList.size(); i++) {
if (userList.get(i).getUsername().equals(username)) {
userList.remove(i);
break;
}
}
}
public User getUser(String username, String password) {
for (User user : userList) {
if (user.getUsername().equals(username) && user.getPassword().equals(password)) {
return user;
}
}
return null;
}
}

以上代碼定義了一個(gè)User類和一個(gè)UserDao類，User類用來封裝帳號(hào)和密碼，UserDao類用來對(duì)用戶進(jìn)行增刪改查操作。UserDao類中包括了初始化用戶列表、添加用戶、修改用戶、刪除用戶、獲取指定用戶的方法。

可以在Servlet中使用UserDao類提供的方法來實(shí)現(xiàn)對(duì)用戶的增刪改查操作，例如：

UserDao userDao = new UserDao(); // 創(chuàng)建UserDao對(duì)象
// 添加用戶
User user = new User("newuser", "newpassword");
userDao.addUser(user);
// 修改用戶
User updateUser = new User("newuser", "newpassword");
userDao.updateUser("olduser", updateUser);
// 刪除用戶
userDao.deleteUser("olduser");
// 獲取用戶
User user = userDao.getUser("username", "password");