import android.content.Intent;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends AppCompatActivity  {
    private Button submit;
    private EditText loginLabel;
    private EditText passwordLabel;
    private TextView errorLabel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        loginLabel = (EditText) findViewById(R.id.login);
        passwordLabel = (EditText) findViewById(R.id.password);
        errorLabel = (TextView) findViewById(R.id.errorLabel);
        submit = (Button) findViewById(R.id.button);

        submit.setOnClickListener(
                new View.OnClickListener()
                {
                    @Override
                    public void onClick(View v)  {
                        String login = loginLabel.getText().toString();
                        String password = passwordLabel.getText().toString();

                        // AsyncTask Execute
                        Auth user = new Auth();
                        user.execute(login, password);
                        String token = user.getToken();

                        // При попытке вывести token через out.println - NullPointerException и остановка программы.

                        if (token != null && !token.trim().isEmpty()) {
                            Intent intent = new Intent(v.getContext(), CourierActivity.class);
                            intent.putExtra("token", token);
                            startActivity(intent);
                        } else {
                            errorLabel.setText("Неправильный логин или пароль");
                        }
                    }

                }
            }
        );

        loginLabel.addTextChangedListener(
                new TextWatcher() {
                    @Override
                    public void beforeTextChanged(CharSequence s, int start, int count, int after) {}
                    public void onTextChanged(CharSequence s, int start, int before, int count) {}
                    public void afterTextChanged(Editable s) {
                        loginLabel.setError(null);
                    }
                }
        );
    }

    static class Auth extends AsyncTask<String, Void, String> {
        private String token;
        private String errorMessage;

        @Override
        protected String doInBackground(String... params) {
            String query = "Здесь URL";
            HttpURLConnection connection = null;
            try {
                connection = (HttpURLConnection) new URL(query).openConnection();
                connection.setRequestMethod("POST");
                connection.setDoOutput(true);
                String urlParameters = "username=" + params[0] + "&password=" + params[1];

                DataOutputStream dataOutputStream = new DataOutputStream(connection.getOutputStream());
                dataOutputStream.writeBytes(urlParameters);
                dataOutputStream.flush();
                dataOutputStream.close();

                StringBuilder sb = new StringBuilder();

                if (HttpURLConnection.HTTP_OK == connection.getResponseCode()) {
                    BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream(), "UTF-8"));
                    String line;
                    while ((line = in.readLine()) != null) {
                        sb.append(line);
                        sb.append("n");
                    }

                    return this.token = ServiceConnect.parse(sb.toString());
                } else {
                    this.errorMessage = "fail: " + connection.getResponseCode() + ", " + connection.getResponseMessage();
                }

            } catch (IOException e) {
                e.printStackTrace();
            }

            return this.token = null;
        }

        public String getToken() {
            return this.token;
        }

        public String getErrorMessage() {
            return this.errorMessage;
        }
    }
}

StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy);