Android不允许转换多个片段：

Question

Following the tutorial here and some help from stackoverflow I was able to make a button switch the replace the current fragment with another. To elaborate I can replace the current fragment but if I wish to replace the one after that with a button the app crashes. My code:

MainActivity:

public class MainActivity extends Activity {

    public FragmentTransaction transaction = getFragmentManager().beginTransaction();
    public Fragment loginFragment = new LoginFragment();
    public Fragment mainFragment = new MainFragment();
    public Fragment settingsFragment = new SettingsFragment();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {

            getFragmentManager().beginTransaction().add(R.id.container, loginFragment).commit();

        } else {

            getFragmentManager().beginTransaction().add(R.id.container, mainFragment).commit();
        }
    }

    public void goToMain(View v) {

        transaction.replace(R.id.container, mainFragment);
        transaction.addToBackStack(null);
        transaction.commit();
    }

    public void goToSettings(View v) {

        transaction.replace(R.id.container, settingsFragment);
        transaction.addToBackStack(null);
        transaction.commit();
    }

    public static class LoginFragment extends Fragment {

        public LoginFragment() {

        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

            View rootView = inflater.inflate(R.layout.fragment_login, container, false);

            return rootView;
        }
    }

    public static class MainFragment extends Fragment {

        public MainFragment() {

        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

            View rootView = inflater.inflate(R.layout.fragment_main, container, false);

            return rootView;
        }
    }

    public static class SettingsFragment extends Fragment {

        public SettingsFragment() {

        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

            View rootView = inflater.inflate(R.layout.fragment_settings, container, false);

            return rootView;
        }
    }
}

Login fragment xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.pointlight.kingdomcraft.MainActivity$LoginFragment" >

    <Button
        android:id="@+id/button_submit"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:onClick="goToMain"
        android:text="@string/action_submit" />

</RelativeLayout>

Main fragment xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.pointlight.kingdomcraft.MainActivity$MainFragment" >

    <Button
        android:id="@+id/button_settings"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:onClick="goToSettings"
        android:text="@string/action_settings" />
</RelativeLayout>

As you can see the code for the two different transactions is exactly the same... Why will android not allow the switching of fragments more than once?

EDIT: Also for whatever reason my logcat is empty

Answer 1

You call commit() on the transaction but you don't call beginTransaction() before the new replace(...) command.

Edit: Maybe this was not clear. I meant:

public void goToMain(View v) {
    FragmentTransaction transaction = getFragmentManager().beginTransaction();
    transaction.replace(R.id.container, mainFragment);
    transaction.addToBackStack(null);
    transaction.commit();
}

and so on. You can not reuse your transaction, so don't save it. Create a new one.