当我在菜单项上实现可见性时，我得到了java.lang.NullPointerException

Question

我已经在这几天工作，但没有运气。我正在根据facebook和我的应用程序状态变化的会话实现菜单项的可见性。

我只需要在我的登录活动中隐藏菜单项，然后一旦访问被验证，它当然会显示菜单本身。

我只参加了一项活动：

用于菜单代码：

 @SuppressLint({ "InlinedApi", "NewApi" })
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        super.onCreateOptionsMenu(menu);
        getMenuInflater().inflate(R.menu.main, menu);


   menu.findItem(R.id.action_previous).setEnabled(viewPager.getCurrentItem() > 0);

        // Add either a "next" or "finish" button to the action bar, depending on which page
        // is currently selected.
   MenuItem item = menu.add(Menu.NONE, R.id.action_next, Menu.NONE,    (viewPager.getCurrentItem() == adapter.getCount() - 1)
                        ? R.string.action_finish
                        : R.string.action_next);

        item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM | MenuItem.SHOW_AS_ACTION_WITH_TEXT);

      menu1 = menu;
        return true;

    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int itemId = item.getItemId();
        if (itemId == R.id.action_previous) {
            // Go to the previous step in the wizard. If there is no previous step,
            // setCurrentItem will do nothing.
            viewPager.setCurrentItem(viewPager.getCurrentItem() - 1);
            return true;
        } else if (itemId == R.id.action_next) {
            // Advance to the next step in the wizard. If there is no next step, setCurrentItem
            // will do nothing.
            viewPager.setCurrentItem(viewPager.getCurrentItem() + 1);
            return true;
        }

        return super.onOptionsItemSelected(item);
    }



 *************************

  ViewPager viewPager;
 PagerAdapter adapter;
//  public int mState = 1;
 Menu menu1;



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



    int[] bookCover = new int[] { 

        R.drawable.image1, 
        R.drawable.image2,
        R.drawable.image3, 

    };

    // Locate the ViewPager in viewpager_main.xml
    viewPager = (ViewPager) findViewById(R.id.pager);
    // Pass results to ViewPagerAdapter Class
    adapter = new ViewPagerAdapter(this, bookCover);
    // Binds the Adapter to the ViewPager
    viewPager.setAdapter(adapter);
    viewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {

        @SuppressLint("NewApi")
        @Override
        public void onPageSelected(int position) {
         // When changing pages, reset the action bar actions since they are dependent
         // on which page is currently active. An alternative approach is to have each
         // fragment expose actions itself (rather than the activity exposing actions),
         // but for simplicity, the activity provides the actions in this sample.
            //mState = 1;
        invalidateOptionsMenu();
        }

    });




    Button facebook_connect = (Button) findViewById(R.id.facebook_login);
    facebook_connect.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View arg0) {
            if (isFacebookConnected()) {
                disConnectFacebook();
            } else {
                connectFacebook();
            }

        }
    });
}


 ********************
 facebook onState:

 private void onSessionStateChange(Session session, SessionState state, Exception exception) {
        // Check if the user is authenticated and
        // a deep link needs to be handled.
        if (state.isOpened()) {
            MenuItem item = menu1.findItem(R.id.action_next);
            item.setVisible(true);
            viewPager.setVisibility(View.VISIBLE);

        } else if (state.isClosed()) {

            // Make the recipe list hidden
 //
//              MenuItem item = menu.findItem(R.menu.main);
 //             item.setVisible(false);
//              viewPager.setVisibility(View.INVISIBLE);
        }
    }

Answer 1

在您的代码中，此字段未初始化

MenuItem menu; 

所以当你试图调用setVisible（）时，onSessionStateChange（）中有空指针异常