actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS) 产生 NullPointerException

Question

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import com.parse.ParseAnalytics;
import com.parse.ParseUser;

import adapter.SectionPargerAdapter;


public class MainActivity extends FragmentActivity implements ActionBar.TabListener
{

    public static final String TAG = MainActivity.class.getSimpleName();

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link android.support.v4.app.FragmentPagerAdapter} derivative, which
     * will keep every loaded fragment in memory. If this becomes too memory
     * intensive, it may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    SectionPargerAdapter mSectionPargerAdapter;
    ViewPager mViewPager;

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

        setContentView(R.layout.activity_main);
        ParseAnalytics.trackAppOpened(getIntent());

        ParseUser currentUser = ParseUser.getCurrentUser();

        if(currentUser == null) {

            navigateToLogin();
        } else {
            Log.i(TAG, currentUser.getUsername());
        }

        final ActionBar actionBar = getActionBar();
//        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);


        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);



        mSectionPargerAdapter = new SectionPargerAdapter(this, getSupportFragmentManager());

        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionPargerAdapter);

        mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener(){

            @Override
            public void onPageSelected(int position) {
                actionBar.setSelectedNavigationItem(position);

                for (int i = 0; i < mSectionPargerAdapter.getCount(); i++) {
                    // Create a tab with text corresponding to the page title defined by
                    // the adapter. Also specify this Activity object, which implements
                    // the TabListener interface, as the callback (listener) for when
                    // this tab is selected.
                    actionBar.addTab(actionBar.newTab()
                            .setText( mSectionPargerAdapter.getPageTitle(i))
                            .setTabListener(MainActivity.this));}
            }
        });

    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        mViewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {

    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {

    }

    private void navigateToLogin() {
        Intent intent = new Intent(this, LoginActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(intent);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int itemId = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (itemId == R.id.action_logout) {
            ParseUser.logOut();
            navigateToLogin();
        }

        return super.onOptionsItemSelected(item);
    }
}

我面对 actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS) 产生了 NullPointerException。我该如何解决这个错误？我不知道如何解决。在教程视频中，这没有错误，但教程与 Eclipse 一起使用。我使用安卓工作室。请告诉我如何解决？非常感谢！

Answer 1

看看这个here。这家伙实现它非常好。但是我不建议使用 actionBar 选项卡，因为如果您想将应用程序升级到更高版本，则不推荐使用此方法。所以你不要使用actionBar，如果你愿意，你可以使用以下内容

YourAdapter mAdapter;
    ViewPager mViewPager;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_collection_demo);

        // ViewPager and its adapters use support library
        // fragments, so use getSupportFragmentManager.
        mAdapter = new YourAdapter(getSupportFragmentManager());
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mAdapter);
    }

适配器：

public class YourAdapter extends FragmentStatePagerAdapter {
    private String[] titles = { "Item 1", "Item 2", "Item 3" };
    public YourAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int i) {
        switch(i){
           case 0:{
              return new FragementA();
           }case 1:{
              return new FragmentB();
           }case 2:{
              return new FragmentC();
           }
        }
    }

    @Override
    public int getCount() {
        return titles.length;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return titles[position];
    }
}

您将返回的片段和onCreateView方法的实现：

public static class FragmentA extends Fragment {
    public static final String ARG_OBJECT = "object";

    @Override
    public View onCreateView(LayoutInflater inflater,
            ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.my_layout_file, container, false);
        //Simple implementation how to target text view in your layout
        TextView tv = (TextView)rootView.findViewById(R.id.my_text_view);
        return rootView;
    }
}

Answer 2

真正的问题在于AppBaseTheme 中的res/values/style.xml 和res/values-v14/style.xml（两种样式）。
如果将清单中的 Theme.AppCompat.Light 和 Theme.AppCompat.Light.DarkActionBar 更改为 Theme.Holo.Light 和 Theme.Holo.Light.DarkActionBar 将 android:minSdkVersion 更改为版本 11，它可以解决问题...
但它会产生另一个问题：你输了AppCompat.Light theme...
但是actionbar.setNavigationmode 中的错误NullExceptionPointer 已解决..