Question

I’m sorry, I’m sure that this will be simple but I just cannot see where I’m going wrong here.

I’ve got a Fragment (a tool list) that can start a number of other fragments (tools) in a two pane set up. There is an activity that sits above & I am using it to manage the removal of tool fragments when their ‘close button’ is pressed. It then pops a tool from the backstack if one exists.

The strange thing is, that I have this working within one fragment but the ‘cut & pasted’ code into another fragment doesn’t work & throws a NPE. Any ideas why much appreciated.

The Activity snippet:

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;    

public class MyActivity extends FragmentActivity implements OnFragmentButtonClickedListener {

private final static int CLOSE_BUTTON = 0;
...


public void onFragmentButtonClicked(int button, Fragment fragId) {
    Log.i(TAG, "onFragmentButton");
    // process closing of fragment
    if (button == CLOSE_BUTTON) {
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        ft.remove(fragId);
        ft.commit();
    }
    // then restore most recent fragment from backstack
    if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
        getSupportFragmentManager().popBackStack();
    }
}
}

This fragment works:

import android.support.v4.app.Fragment;

public class FirstFragment extends Fragment implements OnClickListener{

OnFragmentButtonClickedListener fragmentButtonListener;
private final static int CLOSE_BUTTON = 0;


// empty constructor
public FirstFragment() {};

// Activity must implement this interface
public interface OnFragmentButtonClickedListener {
    public void onFragmentButtonClicked (int button, Fragment fragId);
}

@Override
public void onAttach(Activity activity) {
    Log.i(TAG, "onAttach");
    super.onAttach(activity);
    try {
        fragmentButtonListener = (OnFragmentButtonClickedListener) activity;
    } catch (ClassCastException e) {
        Log.w(TAG, (activity.toString() + " must implement OnFragmentButtonClickedListener"));
    }
}

...

public void onClick(View v) {
    Log.i(TAG, "onClick");
    switch (v.getId()) {
    case R.id.btn_st_close:
        //close activity
        eraseData(prefs);
        Log.i(TAG, "Frag Id = " + String.valueOf(getFragmentManager().findFragmentById(getId())));
        fragmentButtonListener.onFragmentButtonClicked(CLOSE_BUTTON, getFragmentManager().findFragmentById(getId()));
        break;
    }
}

But, this one throws a NPE in the onClick method when calling the onFragmentButtonClicked() method on the fragmentButtonListener.

import android.support.v4.app.Fragment;

public class SecondFragment extends Fragment implements OnClickListener {

OnFragmentButtonClickedListener fragmentButtonListener;
private final static int CLOSE_BUTTON = 0;

// empty constructor
public SecondFragment() {};

// Activity must implement this interface
public interface OnFragmentButtonClickedListener {
    public void onFragmentButtonClicked (int button, Fragment fragId);
}

@Override
public void onAttach(Activity activity) {
    Log.i(TAG, "onAttach");
    super.onAttach(activity);
    try {
        fragmentButtonListener = (OnFragmentButtonClickedListener) activity;
    } catch (ClassCastException e) {
        Log.w(TAG, (activity.toString() + " must implement OnFragmentButtonClickedListener"));
    }
}

...

 public void onClick(View v) {
     Log.i(TAG, "OnClick");
    switch (v.getId()) {
    case R.id.btn_wc_close:
        //close activity
        eraseData(prefs);
        Log.i(TAG, "Frag Id = " + String.valueOf(getFragmentManager().findFragmentById(getId())));
        try {
            fragmentButtonListener.onFragmentButtonClicked(CLOSE_BUTTON, getFragmentManager().findFragmentById(getId()));
        }
        catch (Exception e) {
            Log.i(TAG, "Exception: " + e);
        }
        break;
        }
    }

From logcat, I can see that the fragment Id is not null so I’m very confused.

08-20 09:13:49.096: I/Second Fragment(1691): onCreate
08-20 09:13:49.096: I/Second Fragment(1691): Frag Id = SecondFragment{41355d68 #1 id=0x7f090093}
08-20 09:13:49.106: I/Second Fragment(1691): onCreateView
08-20 09:13:49.406: I/Second Fragment(1691): onActivityCreated
08-20 09:13:49.560: I/Second Fragment(1691): onClick
0 09:13:51.766: I/Second Fragment(1691): Frag Id = SecondFragment{41355d68 #1 id=0x7f090093}
08-20 09:13:51.766: W/dalvikvm(1691): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
08-20 09:13:51.826: E/AndroidRuntime(1691): FATAL EXCEPTION: main
08-20 09:13:51.826: E/AndroidRuntime(1691): java.lang.NullPointerException
08-20 09:13:51.826: E/AndroidRuntime(1691):     at SecondFragment.onClick(IceTurbFragment.java:136)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.view.View.performClick(View.java:4084)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.view.View$PerformClick.run(View.java:16966)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.os.Handler.handleCallback(Handler.java:615)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.os.Looper.loop(Looper.java:137)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.app.ActivityThread.main(ActivityThread.java:4745)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at java.lang.reflect.Method.invokeNative(Native Method)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at java.lang.reflect.Method.invoke(Method.java:511)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at dalvik.system.NativeStart.main(Native Method)

Thanks for any help.

Answer 1

I can see you define 2 interface with the same name in each fragment.

public interface OnFragmentButtonClickedListener {
     public void onFragmentButtonClicked (int button, Fragment fragId);
}

But the activity can implement only one interface with this name (in this case, I believe your activity implement interface of first fragment) and maybe this cause error null pointer exception of second fragment.

//where is OnFragmentButtonClickedListener come from?
public class MyActivity extends FragmentActivity implements FirstFragment OnFragmentButtonClickedListener 

Try to write OnFragmentButtonClickedListener outside of 2 fragments ( maybe in another file)