Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference

Question

I'm new to this and have the following error when trying to add a Navigation Drawer to an activity with Fragments:

java.lang.RuntimeException: Unable to start activity ComponentInfo{uk.co.XXXX.EditFactFind}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
        at android.app.ActivityThread.access$800(ActivityThread.java:151)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5257)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
        at uk.co.XXXX.NavigationDrawerFragment.setUp(NavigationDrawerFragment.java:135)
        at uk.co.XXXX.EditFactFind.onCreate(EditFactFind.java:69)
        at android.app.Activity.performCreate(Activity.java:5990)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)

My 1st Java file is:

package XXXX;

import android.support.v4.app.FragmentActivity;
import android.support.v7.app.AppCompatActivity;
import android.app.Activity;
import android.app.ActionBar;
import android.support.v4.app.Fragment;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.content.SharedPreferences;
import android.content.res.Configuration;
import android.os.Bundle;
import android.preference.PreferenceManager;
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.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

/**
 * Fragment used for managing interactions for and presentation of a navigation drawer.
 * See the <a href="https://developer.android.com/design/patterns/navigation-drawer.html#Interaction">
 * design guidelines</a> for a complete explanation of the behaviors implemented here.
 */
public class NavigationDrawerFragment extends Fragment {

    /**
     * Remember the position of the selected item.
     */
    private static final String STATE_SELECTED_POSITION = "selected_navigation_drawer_position";

    /**
     * Per the design guidelines, you should show the drawer on launch until the user manually
     * expands it. This shared preference tracks this.
     */
    private static final String PREF_USER_LEARNED_DRAWER = "navigation_drawer_learned";

    /**
     * A pointer to the current callbacks instance (the Activity).
     */
    private NavigationDrawerCallbacks mCallbacks;

    /**
     * Helper component that ties the action bar to the navigation drawer.
     */
    private ActionBarDrawerToggle mDrawerToggle;

    private DrawerLayout mDrawerLayout;
    private ListView mDrawerListView;
    private View mFragmentContainerView;

    private int mCurrentSelectedPosition = 0;
    private boolean mFromSavedInstanceState;
    private boolean mUserLearnedDrawer;

    public NavigationDrawerFragment() {
    }

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

        // Read in the flag indicating whether or not the user has demonstrated awareness of the
        // drawer. See PREF_USER_LEARNED_DRAWER for details.
        SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(getActivity());
        mUserLearnedDrawer = sp.getBoolean(PREF_USER_LEARNED_DRAWER, false);

        if (savedInstanceState != null) {
            mCurrentSelectedPosition = savedInstanceState.getInt(STATE_SELECTED_POSITION);
            mFromSavedInstanceState = true;
        }

        // Select either the default item (0) or the last selected item.
        selectItem(mCurrentSelectedPosition);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        // Indicate that this fragment would like to influence the set of actions in the action bar.
        setHasOptionsMenu(true);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        mDrawerListView = (ListView) inflater.inflate(
                R.layout.fragment_navigation_drawer, container, false);
        mDrawerListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                selectItem(position);
            }
        });
        mDrawerListView.setAdapter(new ArrayAdapter<String>(
                getActivity(),
                android.R.layout.simple_list_item_activated_1,
                android.R.id.text1,
                new String[]{
                        getString(R.string.title_section1),
                        getString(R.string.title_section2),
                        getString(R.string.title_section3),
                        "Assets",
                        "ID",
                }));
        mDrawerListView.setItemChecked(mCurrentSelectedPosition, true);
        return mDrawerListView;
    }

    public boolean isDrawerOpen() {
        return mDrawerLayout != null && mDrawerLayout.isDrawerOpen(mFragmentContainerView);
    }

    /**
     * Users of this fragment must call this method to set up the navigation drawer interactions.
     *
     * @param fragmentId   The android:id of this fragment in its activity's layout.
     * @param drawerLayout The DrawerLayout containing this fragment's UI.
     */
    public void setUp(int fragmentId, DrawerLayout drawerLayout) {
        mFragmentContainerView = getActivity().findViewById(fragmentId);
        mDrawerLayout = drawerLayout;

        // set a custom shadow that overlays the main content when the drawer opens
//        mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);
        // set up the drawer's list view with items and click listener

        ActionBar actionBar = getActionBar();
        actionBar.setDisplayHomeAsUpEnabled(true);
        actionBar.setHomeButtonEnabled(true);

        // ActionBarDrawerToggle ties together the the proper interactions
        // between the navigation drawer and the action bar app icon.
        mDrawerToggle = new ActionBarDrawerToggle(
                getActivity(),                    /* host Activity */
                mDrawerLayout,                    /* DrawerLayout object */
                R.drawable.ic_drawer,             /* nav drawer image to replace 'Up' caret */
                R.string.navigation_drawer_open,  /* "open drawer" description for accessibility */
                R.string.navigation_drawer_close  /* "close drawer" description for accessibility */
        ) {
            @Override
            public void onDrawerClosed(View drawerView) {
                super.onDrawerClosed(drawerView);
                if (!isAdded()) {
                    return;
                }

                getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
            }

            @Override
            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                if (!isAdded()) {
                    return;
                }

                if (!mUserLearnedDrawer) {
                    // The user manually opened the drawer; store this flag to prevent auto-showing
                    // the navigation drawer automatically in the future.
                    mUserLearnedDrawer = true;
                    SharedPreferences sp = PreferenceManager
                            .getDefaultSharedPreferences(getActivity());
                    sp.edit().putBoolean(PREF_USER_LEARNED_DRAWER, true).apply();
                }

                getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
            }
        };

        // If the user hasn't 'learned' about the drawer, open it to introduce them to the drawer,
        // per the navigation drawer design guidelines.
        if (!mUserLearnedDrawer && !mFromSavedInstanceState) {
            mDrawerLayout.openDrawer(mFragmentContainerView);
        }

        // Defer code dependent on restoration of previous instance state.
        mDrawerLayout.post(new Runnable() {
            @Override
            public void run() {
                mDrawerToggle.syncState();
            }
        });

        mDrawerLayout.setDrawerListener(mDrawerToggle);
    }

    private void selectItem(int position) {
        mCurrentSelectedPosition = position;
        if (mDrawerListView != null) {
            mDrawerListView.setItemChecked(position, true);
        }
        if (mDrawerLayout != null) {
            mDrawerLayout.closeDrawer(mFragmentContainerView);
        }
        if (mCallbacks != null) {
            mCallbacks.onNavigationDrawerItemSelected(position);
        }
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mCallbacks = (NavigationDrawerCallbacks) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException("Activity must implement NavigationDrawerCallbacks.");
        }
    }

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

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt(STATE_SELECTED_POSITION, mCurrentSelectedPosition);
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        // Forward the new configuration the drawer toggle component.
        mDrawerToggle.onConfigurationChanged(newConfig);
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // If the drawer is open, show the global app actions in the action bar. See also
        // showGlobalContextActionBar, which controls the top-left area of the action bar.
        if (mDrawerLayout != null && isDrawerOpen()) {
            inflater.inflate(R.menu.global, menu);
            showGlobalContextActionBar();
        }
        super.onCreateOptionsMenu(menu, inflater);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }

        if (item.getItemId() == R.id.action_example) {
            Toast.makeText(getActivity(), "Example action.", Toast.LENGTH_SHORT).show();
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    /**
     * Per the navigation drawer design guidelines, updates the action bar to show the global app
     * 'context', rather than just what's in the current screen.
     */
    private void showGlobalContextActionBar() {
        ActionBar actionBar = getActionBar();
        actionBar.setDisplayShowTitleEnabled(true);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
        actionBar.setTitle(R.string.app_name);
    }

    private android.app.ActionBar getActionBar() {
         return ((FragmentActivity) getActivity()).getActionBar();

    }

    /**
     * Callbacks interface that all activities using this fragment must implement.
     */
    public static interface NavigationDrawerCallbacks {
        /**
         * Called when an item in the navigation drawer is selected.
         */
        void onNavigationDrawerItemSelected(int position);
    }
}

My 2nd is:

package XXXX;


import java.io.Serializable;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;


import android.app.AlertDialog;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.widget.DrawerLayout;
import android.app.ActionBar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class EditFactFind extends FragmentActivity implements NavigationDrawerFragment.NavigationDrawerCallbacks {

    public static final int RESULT_DELETE = -500;
    private boolean isInEditMode = true;
    private boolean isAddingFactFind = true;
    private NavigationDrawerFragment mNavigationDrawerFragment;
    private CharSequence mTitle;

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

        final Button saveButton = (Button)findViewById(R.id.saveButton);
        final Button cancelButton = (Button)findViewById(R.id.cancelButton);
        final EditText titleEditText = (EditText)findViewById(R.id.titleEditText);
        //final EditText factFindEditText = (EditText)findViewById(R.id.factFindEditText);
        final TextView dateTextView = (TextView)findViewById(R.id.dateTextView);
        final Button nextButton =(Button) findViewById(R.id.nextButton);

        //Create fragment and give it an argument for the selected article
        secA_pg1 iniSecFrag = new secA_pg1();
        Bundle args = new Bundle();
        args.putInt(secA_pg1.ARG_INDEX, 1);
        iniSecFrag.setArguments(args);

        FragmentTransaction initialTransaction = getFragmentManager().beginTransaction();

        // Replace whatever is in the fragment_container view with this fragment,
        // and add the transaction to the back stack so the user can navigate back
        initialTransaction.replace(R.id.fragment_container, iniSecFrag);
        initialTransaction.addToBackStack(null);

        //Commit the transaction
        initialTransaction.commit();

        //// TODO: 10/09/2015  
        mNavigationDrawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
        mTitle = getTitle();

        // Set up the drawer.
        mNavigationDrawerFragment.setUp(
                R.id.navigation_drawer,
                (DrawerLayout) findViewById(R.id.drawer_layout));

        Serializable extra = getIntent().getSerializableExtra("FactFind");
        if(extra != null)
        {
            FactFind factFind = (FactFind) extra;
            titleEditText.setText(factFind.getTitle());
          //  factFindEditText.setText(factFind.getFactFindTitle());

            DateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss");
            String date = dateFormat.format(factFind.getDate());

            dateTextView.setText(date);

            isInEditMode = false;
            titleEditText.setEnabled(false);
          //  factFindEditText.setEnabled(false);
            saveButton.setText("Edit");

            isAddingFactFind = false;

        }

        cancelButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                setResult(RESULT_CANCELED, new Intent());
                finish();
            }
        });

        nextButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                //secA_pg1 secFrag = (secA_pg1) getFragmentManager().findFragmentById(R.id.Sec_A_pg1_fragment);
                //Create fragment and give it an argument for the selected article
                secA_pg2 newSecFrag = new secA_pg2();
                Bundle args = new Bundle();
                args.putInt(secA_pg2.ARG_INDEX, 2);
                newSecFrag.setArguments(args);

                FragmentTransaction transaction = getFragmentManager().beginTransaction();

                // Replace whatever is in the fragment_container view with this fragment,
                // and add the transaction to the back stack so the user can navigate back
                transaction.replace(R.id.fragment_container, newSecFrag);
                transaction.addToBackStack(null);

                //Commit the transaction
                transaction.commit();
            }
        });

        saveButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {


                if(isInEditMode)
                {
                    Intent returnIntent = new Intent();
                    FactFind factFind = new FactFind(titleEditText.getText().toString(),Calendar.getInstance().getTime());
                    returnIntent.putExtra("FactFind", factFind);
                    setResult(RESULT_OK, returnIntent);
                    finish();

                }
                else
                {
                    isInEditMode = true;
                    saveButton.setText("Save");
                    titleEditText.setEnabled(true);
                   // factFindEditText.setEnabled(true);
                }

            }
        });
    }


    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setMessage(R.string.are_you_sure_you_want_to_delete_this_fact_find_it_can_t_be_undone_);
        builder.setTitle("Confirm Delete");
        builder.setPositiveButton("Delete", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                Intent returnIntent = new Intent();

                setResult(RESULT_DELETE, returnIntent);
                finish();

            }
        });
        builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub
            }
        });

        builder.create().show();
        return true;

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        if (!mNavigationDrawerFragment.isDrawerOpen()) {
            // Only show items in the action bar relevant to this screen
            // if the drawer is not showing. Otherwise, let the drawer
            // decide what to show in the action bar.
            getMenuInflater().inflate(R.menu.nav_drawer, menu);
            restoreActionBar();
            return true;
        }
        else {
            //return super.onCreateOptionsMenu(menu);
            getMenuInflater().inflate(R.menu.action_menu, menu);
            return true;
        }
    }
    //@Override
    public void onSelectedFragChanged(int index) {
        FragmentManager fragmentManager = getFragmentManager();
        secA_pg1 secA_pg1 = (secA_pg1) fragmentManager.findFragmentById(R.id.Sec_A_pg1_fragment);

        secA_pg1.setSectionTitle(index);
    }

    public void restoreActionBar() {
        ActionBar actionBar = getActionBar();
        //actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
        actionBar.setDisplayShowTitleEnabled(true);
        actionBar.setTitle(mTitle);
    }

    @Override
    public void onNavigationDrawerItemSelected(int position) {

    }
}

I read through a few other articles that mentioned the ActionBar needs to be SupportActionBar but that threw up even more problems when I changed the imports over on both Java files. Quite possible that I've made a mistake there as well though.

Answer 1

Error:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference

you are getting this error because your ActionBar is null.

You should change

public class EditFactFind extends FragmentActivity 

to

public class EditFactFind extends ActionBarActivity

or

public class EditFactFind extends AppCompatActivity

Use AppCompatActivity if you are using latest appcompact library.

and also change ActionBar actionBar = getSupportActionBar();