Challenge :: Fatal Exception after swipe to right to delete a crime

Android Programming (3rd Edition) 17. Two-Pane Master-Detail Interfaces
1

Hi @cstewart,
I am getting the following fatal exception when I swipe right to delete a Crime listed in the CrimeListFragment:

09-15 17:31:35.801 4526-4526/com.bignerdranch.android.criminalintent E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.bignerdranch.android.criminalintent, PID: 4526
java.lang.NullPointerException: Attempt to invoke virtual method ‘java.lang.Class java.lang.Object.getClass()’ on a null object reference
at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:380)
at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:430)
at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:421)
at com.bignerdranch.android.criminalintent.CrimeListActivity.onCrimeDeleted(CrimeListActivity.java:39)
at com.bignerdranch.android.criminalintent.CrimeListFragment$1.onSwiped(CrimeListFragment.java:82)
at android.support.v7.widget.helper.ItemTouchHelper$4.run(ItemTouchHelper.java:686)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Here is the code for the following:

CrimeListActivity.java 
package com.bignerdranch.android.criminalintent;

import android.content.Intent;
import android.support.v4.app.Fragment;

public class CrimeListActivity extends SingleFragmentActivity
    implements CrimeListFragment.Callbacks, CrimeFragment.Callbacks {

    @Override
    protected Fragment createFragment() {
        return new CrimeListFragment();
    }

    @Override
    protected int getLayoutResId() {
        return R.layout.activity_masterdetail;
    }

    @Override
    public void onCrimeSelected(Crime crime) {
        if (findViewById(R.id.detail_fragment_container) == null) { // phone
            Intent intent = CrimePagerActivity.newIntent(this, crime.getID());
            startActivity(intent);
        } else { // tablet
            Fragment newDetail = CrimeFragment.newInstace(crime.getID());

            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.detail_fragment_container, newDetail)
                    .commit();
        }
    }

    @Override
    public void onCrimeDeleted(Crime crime) {
        if (findViewById(R.id.detail_fragment_container) == null) { // phone

        } else { // tablet
            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.detail_fragment_container, null)
                    .commit();
        }
    }

    @Override
    public void onCrimeUpdated(Crime crime) {
        CrimeListFragment listFragment = (CrimeListFragment)
                getSupportFragmentManager()
                .findFragmentById(R.id.fragment_container);
        listFragment.updateUI();
    }

}
CrimeListFragment.java 
package com.bignerdranch.android.criminalintent;

import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.helper.ItemTouchHelper;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.TextView;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.List;

public class CrimeListFragment extends Fragment {
    private int clickedCrimePosition;
    private static final String CLICKED_CRIME_POSITION_ID = "clicked_crime_position_id";
    private static final String SAVED_SUBTITLE_VISIBLE = "subtitle";

    private static final int REQUEST_CRIME = 1;

    private RecyclerView mCrimeRecyclerView;
    private CrimeAdapter mAdapter;
    private boolean mSubtitleVisible;

    private LinearLayout mEmptyCrimeListLinearLayout;
    private Button mEmptyCrimeListAddButton;
    private Callbacks mCallbacks;

    /**
     * Required interface for hosting activities
     */
    public interface Callbacks {
        void onCrimeSelected(Crime crime);
        void onCrimeDeleted(Crime crime);
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        mCallbacks = (Callbacks) context;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_crime_list, container, false);

        mCrimeRecyclerView = (RecyclerView) view.findViewById(R.id.crime_recycler_view);
        mCrimeRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));

        //Set up ItemTouchHelper for Swipe Right to delete
        ItemTouchHelper mIth = new ItemTouchHelper(
                new ItemTouchHelper.SimpleCallback(ItemTouchHelper.UP | ItemTouchHelper.DOWN,
                        ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT) {
                    public boolean onMove(RecyclerView recyclerView,
                                          RecyclerView.ViewHolder viewHolder,
                                          RecyclerView.ViewHolder target) {
                        return false;
                    }
                    public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
                        final int pos = viewHolder.getAdapterPosition();
                        Crime crime = mAdapter.getCrimes().get(pos);
                        if (direction==ItemTouchHelper.RIGHT) {
                            mCallbacks.onCrimeDeleted(crime);
                        }
                    }
                });
        mIth.attachToRecyclerView(mCrimeRecyclerView); // Attach ItemTouchHelper to RecyclerView

        mEmptyCrimeListLinearLayout = (LinearLayout) view.findViewById(R.id.empty_crime_list_layout);
        mEmptyCrimeListAddButton = (Button) view.findViewById(R.id.add_crime_button);

        if (savedInstanceState != null) {
            clickedCrimePosition = savedInstanceState.getInt(CLICKED_CRIME_POSITION_ID);
            mSubtitleVisible = savedInstanceState.getBoolean(SAVED_SUBTITLE_VISIBLE);
        }

        updateUI();

        return view;
    }

    @Override
    public void onResume() {
        super.onResume();
        updateUI();
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        super.onCreateOptionsMenu(menu, inflater);
        inflater.inflate(R.menu.fragment_crime_list, menu);

        MenuItem subtitleItem = menu.findItem(R.id.show_subtitle);
        if (mSubtitleVisible) {
            subtitleItem.setTitle(R.string.hide_subtitle);
        } else {
            subtitleItem.setTitle(R.string.show_subtitle);
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.new_crime:
                addCrime();
                return true;
            case R.id.show_subtitle:
                mSubtitleVisible = !mSubtitleVisible;
                getActivity().invalidateOptionsMenu();
                updateSubtitle();
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }

    private void addCrime() {
        Crime crime = new Crime();
        CrimeLab.get(getActivity()).addCrime(crime);
        updateUI();
        mCallbacks.onCrimeSelected(crime);
    }

    private void updateSubtitle() {
        CrimeLab crimeLab = CrimeLab.get(getActivity());
        int crimeCount = crimeLab.getCrimes().size();
        String subtitle = getResources()
                .getQuantityString(R.plurals.subtitle_plural, crimeCount, crimeCount);

        if (!mSubtitleVisible) {
            subtitle = null;
        }

        AppCompatActivity activity = (AppCompatActivity) getActivity();
        activity.getSupportActionBar().setSubtitle(subtitle);
    }

    public void updateUI() {
        CrimeLab crimeLab = CrimeLab.get(getActivity());
        List<Crime> crimes = crimeLab.getCrimes();

        if (mAdapter == null) {
            mAdapter = new CrimeAdapter(crimes);
            mCrimeRecyclerView.setAdapter(mAdapter);
        } else {
            mAdapter.setCrimes(crimes);
            mAdapter.notifyDataSetChanged();
        }

        if (crimes.size() > 0) {
            mEmptyCrimeListLinearLayout.setVisibility(View.GONE);
            mCrimeRecyclerView.setVisibility(View.VISIBLE);
        } else {
            mEmptyCrimeListLinearLayout.setVisibility(View.VISIBLE);
            mCrimeRecyclerView.setVisibility(View.GONE);

            mEmptyCrimeListAddButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    addCrime();
                }
            });
        }

        updateSubtitle();
    }

    private class CrimeHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
        private TextView mTitleTextView;
        private TextView mDateTextView;
        private ImageView mSolvedImageView;
        private Crime mCrime;

        public CrimeHolder(LayoutInflater inflater, ViewGroup parent) {
            super(inflater.inflate(R.layout.list_item_crime, parent, false));
            itemView.setOnClickListener(this);

            mTitleTextView = (TextView) itemView.findViewById(R.id.crime_title);
            mDateTextView = (TextView) itemView.findViewById(R.id.crime_date);
            mSolvedImageView = (ImageView) itemView.findViewById(R.id.crime_solved);
        }

        public void bind(Crime crime) {
            mCrime = crime;
            mTitleTextView.setText(mCrime.getTitle());
            DateFormat dateFormat = new SimpleDateFormat(CrimeFragment.DATE_FORMAT);
            DateFormat timeFormat = new SimpleDateFormat(CrimeFragment.TIME_FORMAT);
            mDateTextView.setText(dateFormat.format(mCrime.getDate()) + " " + timeFormat.format(mCrime.getTime()));
            mSolvedImageView.setVisibility(crime.isSolved() ? View.VISIBLE : View.GONE);
        }

        @Override
        public void onClick(View v) {
            clickedCrimePosition = getAdapterPosition();
            mCallbacks.onCrimeSelected(mCrime);
        }
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == REQUEST_CRIME) {
            // Handle result
        }
    }

    private class CrimeAdapter extends RecyclerView.Adapter<CrimeHolder> {
        private List<Crime> mCrimes;

        public CrimeAdapter(List<Crime> crimes) {
            mCrimes = crimes;
        }

        @Override
        public CrimeHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            LayoutInflater layoutInflater = LayoutInflater.from(getActivity());
            return new CrimeHolder(layoutInflater, parent);
        }

        @Override
        public void onBindViewHolder(CrimeHolder holder, int position) {
            Crime crime = mCrimes.get(position);
            holder.bind(crime);
        }

        @Override
        public int getItemCount() {
            return mCrimes.size();
        }

        public void setCrimes(List<Crime> crimes) {
            mCrimes = crimes;
        }

        public List<Crime> getCrimes() {
            return mCrimes;
        }
    }

    @Override
    public void onSaveInstanceState(Bundle onSavedInstanceState) {
        super.onSaveInstanceState(onSavedInstanceState);
        onSavedInstanceState.putSerializable(CLICKED_CRIME_POSITION_ID, clickedCrimePosition);
        onSavedInstanceState.putBoolean(SAVED_SUBTITLE_VISIBLE, mSubtitleVisible);
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mCallbacks = null;
    }
}
2

That line in the stacktrace is pointing us to line 39 of your CrimeListActivity.
I can’t tell exactly which line that is, since they’re not numbered in the listing, however, I am guessing that it is the following line:

That line looks like it is attempting to replace an item with a null.
I’m just guessing but that may be causing the error because at the top of the StackTrace the system is trying to tell us that it is stumbling upon a null object reference.

Maybe this will get you a bit closer to discovering the issue. Again, this is just a guess. Good luck.

3

I agree with @raddevus. The replace call in your fragment transaction seems like the issue.

Try using the remove method on FragmentTransaction instead.

4

Thanks for pointing out the problem @cstewart/@raddevus,
If I use the remove method on FragmentTransaction it asks for a Fragment object to remove. How do I reference in CrimeListActivity, the CrimeFragment object being displayed in the detail_fragment_container shown on tablets?

5

Use the findFragmentById method on FragmentManager. Similar to what happens in the SingleFragmentActivity.

6

Thank you. That was very helpful