The following code demonstrates the implementation of user authentication within an Android chat application named YQ.

The login activity handles user input and initiates the authentication process:

public class LoginActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        
        Button loginButton = (Button) findViewById(R.id.btn_login);
        loginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                EditText accountField = (EditText) findViewById(R.id.et_account);
                EditText passwordField = (EditText) findViewById(R.id.et_password);
                
                int accountId = Integer.parseInt(accountField.getText().toString());
                String userPassword = passwordField.getText().toString();
                
                performAuthentication(accountId, userPassword);
            }
        });
    }
    
    private void performAuthentication(int userId, String userPass) {
        User userInfo = new User();
        userInfo.setAccount(userId);
        userInfo.setPassword(userPass);
        userInfo.setOperation("login");
        
        boolean success = new YQConServer().sendLoginInfo(userInfo);
        
        if (success) {
            try {
                // Request list of online contacts
                // ---
            } catch (IOException e) {
                e.printStackTrace();
            }
            
            Intent intent = new Intent(this, MainActivity.class);
            startActivity(intent);
        } else {
            Toast.makeText(this, "Authentication failed!", Toast.LENGTH_SHORT).show();
        }
    }
}

The client-side implementation manages network communication with the server:

public class YQClient {
    private Socket connection;
    
    public boolean sendLoginInfo(Object data) {
        boolean result = false;
        try {
            connection = new Socket();
            
            try {
                connection.connect(new InetSocketAddress("10.0.2.2", 5469), 2000);
            } catch (SocketTimeoutException e) {
                return false; // Connection timeout
            }
            
            ObjectOutputStream outputStream = new ObjectOutputStream(connection.getOutputStream());
            outputStream.writeObject(data);
            
            ObjectInputStream inputStream = new ObjectInputStream(connection.getInputStream());
            YQMessage message = (YQMessage) inputStream.readObject();
            
            if (message.getType().equals(YQMessageType.SUCCESS)) {
                // Establish persistent connection thread
                // ---
                result = true;
            } else if (message.getType().equals(YQMessageType.FAIL)) {
                result = false;
            }
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
        return result;
    }
}

The server-side component processes incoming authentication requests:

public class YQServer {
    public YQServer() {
        ServerSocket serverSocket = null;
        try {
            serverSocket = new ServerSocket(5469);
            System.out.println("Server started, listening on port 5469...");
            
            while (true) {
                Socket clientConnection = serverSocket.accept();
                
                ObjectInputStream inputStream = new ObjectInputStream(clientConnection.getInputStream());
                User userData = (User) inputStream.readObject();
                
                YQMessage responseMessage = new YQMessage();
                ObjectOutputStream outputStream = new ObjectOutputStream(clientConnection.getOutputStream());
                
                if (userData.getOperation().equals("login")) {
                    int accountId = userData.getAccount();
                    boolean authResult = new UserDao().login(accountId, userData.getPassword());
                    
                    if (authResult) {
                        System.out.println("[" + accountId + "] logged in");
                        responseMessage.setType(YQMessageType.SUCCESS);
                        outputStream.writeObject(responseMessage);
                        
                        // Spawn dedicated thread for continuous communication
                        // ---
                    } else {
                        responseMessage.setType(YQMessageType.FAIL);
                        outputStream.writeObject(responseMessage);
                    }
                } else if (userData.getOperation().equals("register")) {
                    // Handle registration
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

The application requires handling socket timeouts to prevent UI blocking during network failures.