LoginActivity.java

package neocom.dealerbook.controller;

//... Imports

public class LoginActivity extends AppCompatActivity {
  
  public void doLogin(View view){
    //.... Do Things
    
    loginShortCut(); //Method don't run in production || To Remove on Proguard
    String userName = this.user.getText().toString();
    String userPassword = this.password.getText().toString();
    api.generateToken(userName, userPassword, GRANT_TYPE, getLoginCallback(button));
    
  }
  
  private void loginShortCut() {
      if (user.getText().toString().equals("123")) {
          user.setText("some_user@some_provider.com");
          password.setText("some_password");
      }
  }
}

proguard-rules.pro

-assumenosideeffects class android.util.Log {
    public static boolean isLoggable(java.lang.String, int);
    public static int v(...);
    public static int i(...);
    public static int w(...);
    public static int d(...);
    public static int e(...);
}

-assumenosideeffects class neocom.dealerbook.controller.LoginActivity {
    private void loginShortCut();
}