Question

只有在我开始在项目中使用片段

后才会出现错误

这是我的代码..

public class MainActivity extends AppCompatActivity {

private Toolbar toolbar;
private ViewPager mPager;
private SlidingTabLayout mTabs;
private MyPagerAdapter adapter;

public static final int product_result = 0;

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

    toolbar = (Toolbar) findViewById(R.id.app_bar);

    setSupportActionBar(toolbar);
    assert getSupportActionBar() != null;
    getSupportActionBar().setDisplayShowHomeEnabled(true);

    NavigationDrawerFragment navigationDrawerFragment = (NavigationDrawerFragment)
            getSupportFragmentManager().findFragmentById(R.id.fragment_nav_drawer);
    navigationDrawerFragment.setUp(R.id.fragment_nav_drawer, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar);

    mPager = (ViewPager) findViewById(R.id.pager);
    adapter = new MyPagerAdapter(getSupportFragmentManager());
    mPager.setAdapter(adapter);

    mTabs = (SlidingTabLayout) findViewById(R.id.tabs);
    mTabs.setDistributeEvenly(true);
    mTabs.setCustomTabView(R.layout.custom_tab_view, R.id.tabText);
    int bgColor = ContextCompat.getColor(this, R.color.colorPrimary);
    mTabs.setBackgroundColor(bgColor);
    mTabs.setSelectedIndicatorColors(ContextCompat.getColor(MainActivity.this, R.color.colorAccent));
    mTabs.invalidate();
    mTabs.setViewPager(mPager);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();
    if (id == R.id.action_settings) {

        return true;
    }
    if (id == R.id.navigate) {
        startActivity(new Intent(this, SubActivity.class));
    }
    if (id == R.id.TabActivity) {
        startActivity(new Intent(this, ActivityUsingTabLibrary.class));
    }
    if (id == R.id.VectorTest) {
        startActivity(new Intent(this, VectorTestActivity.class));
    }
    return super.onOptionsItemSelected(item);
}


class MyPagerAdapter extends FragmentStatePagerAdapter {
    int icons[] = {R.drawable.home, R.drawable.hot_article, R.drawable.dizzy_person};

    public MyPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {

        Fragment fragment = null;
        switch(position){
            case product_result:
                fragment = FragmentProduct.newInstance("","");
                break;
        }
        return fragment;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        Drawable drawable = ResourcesCompat.getDrawable(getResources(), icons[position], null);
        drawable.setBounds(0, 0, 36, 36);
        ImageSpan imageSpan = new ImageSpan(drawable);
        SpannableString spannableString = new SpannableString(" ");
        spannableString.setSpan(imageSpan, 0, spannableString.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
        return spannableString;
    }

    @Override
    public int getCount() {
        return 3;
    }
}

}

这是我的fragment.class，我传递了jsonObject toast。

public class FragmentProduct extends Fragment {

public static final String product_url = "http://oasisgroups.com/oApp/product.php";

private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";

private String mParam1;
private String mParam2;
private VolleySingleton volleySingleton;
private ImageLoader imageLoader;
private RequestQueue requestQueue;

public FragmentProduct() {
    // Required empty public constructor
}

public static FragmentProduct newInstance(String param1, String param2) {
    FragmentProduct fragment = new FragmentProduct();
    Bundle args = new Bundle();
    args.putString(ARG_PARAM1, param1);
    args.putString(ARG_PARAM2, param2);
    fragment.setArguments(args);
    return fragment;
}

public static String getRequestUrl(int limit) {
    return product_url + "&limit=" + limit;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (getArguments() != null) {
        mParam1 = getArguments().getString(ARG_PARAM1);
        mParam2 = getArguments().getString(ARG_PARAM2);
    }

    volleySingleton = VolleySingleton.getInstance();
    requestQueue = volleySingleton.getmRequestQueue();
    JsonObjectRequest request = new JsonObjectRequest(Request.Method.GET, getRequestUrl(10),
            new Response.Listener<JSONObject>() {
                @Override
                public void onResponse(JSONObject response) {
                    L.t(getActivity(), response.toString());
                }
            }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {

        }
    });
    requestQueue.add(request);

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_product, container, false);
}

}

栈跟踪

02-29 12:10:01.047 17100-17100/com.mydesign.rockstar.materialdesignexample E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.mydesign.rockstar.materialdesignexample, PID: 17100
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.app.Fragment.setMenuVisibility(boolean)' on a null object reference
    at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:116)
    at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:870)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:1086)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:952)
    at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1474)
    at android.view.View.measure(View.java:18804)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:901)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
    at android.view.View.measure(View.java:18804)
    at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:940)
    at android.view.View.measure(View.java:18804)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
    at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
    at android.view.View.measure(View.java:18804)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
    at android.view.View.measure(View.java:18804)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
    at android.view.View.measure(View.java:18804)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
    at android.view.View.measure(View.java:18804)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
    at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
    at android.view.View.measure(View.java:18804)
    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2112)
    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1228)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1464)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1119)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6060)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
    at android.view.Choreographer.doCallbacks(Choreographer.java:670)
    at android.view.Choreographer.doFrame(Choreographer.java:606)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
    at android.os.Handler.handleCallback(Handler.java:746)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5443)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

我在这link中也看到了同样的问题，我应用他的解决方案，但它对我不起作用..我仍然卡住了，帮助我。所以检查我的代码并建议我在哪里缺少。

Answer 1

哦，是的..我终于找到了我的错误..首先，我的代码中没有错误。一切都是对的。

我有三个标签布局，我只传递一个tablayout，其他两个都是null，所以我必须应用三个switch case语句。

 public Fragment getItem(int position) {

    Fragment fragment = null;
    switch(position){
        case product_result:
            fragment = FragmentProduct.newInstance("","");
            break;
    }
    return fragment;
}

而不是

  public Fragment getItem(int num) {
        Fragment fragment = null;
        switch (num) {
            case TAB_PRODUCT_RESULT:
                fragment = FragmentProduct.newInstance("", "");
                break;
            case TAB_SECOND:
                fragment = MyFragment.getInstance(num);
                break;
            case TAB_THIRD:
                fragment = MyFragment.getInstance(num);
                break;
        }
        return fragment;
    }

多数民众赞成......

Answer 2

我认为问题出在您public Fragment getItem(int position)的{{1}}方法中。您下面的代码段正在返回MyPagerAdapter个片段。

null

您只能获得位置Fragment fragment = null; switch(position){ case product_result: fragment = FragmentProduct.newInstance("",""); break; } return fragment;的真实片段，因为0已初始化为product_result。在所有其他情况下，它返回null。修改上面的代码片段以始终返回非空的片段实例。

Answer 3

我有类似的问题，我试图删除空片段，这会导致错误。空检查解决了问题。检查您的片段交易。你不能添加空片段。

Fragment fragment = null;
if (fragment != null)
   getSupportFragmentManager().beginTransaction()
           .add(R.id.container, fragment, "id").commit();

Answer 4

首先，您必须像这样在您的活动中添加适配器

-------这是根据数据大小添加选项卡布局的数量------------

    for (FirstModel firstModel : list_editdata) {
        tabLayout.addTab(tabLayout.newTab().setText(firstModel.getTitle()));
    }

     **/////Set an Adapter** 

SectionAdapter  mSectionAdapter   = new SectionAdapter(getSupportFragmentManager());
    viewPager.setAdapter(mSectionAdapter);

现在在“活动”中添加适配器：我取一个片段并反复用于切换情况下。这将根据您在“活动”中的标签布局编号自动重新生成

public class SectionAdapter  extends FragmentPagerAdapter {

    private String[] tabTitles = new String[]{"", "", "", ""};


    public SectionAdapter (FragmentManager fm) {
        super(fm);
    }


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


    @Override
    public Fragment getItem(int position) {


        switch (position) {

            case 0:
                Tab1 tab1 = new Tab1();
                return tab1 ;
            case 1:
                Tab1 tab2 = new Tab1();
                return tab2 ;
            case 2:
                Tab1 tab3 = new Tab1();
                return tab3 ;
            case 3:
                Tab1 tab4 = new Tab1();
                return tab4 ;

        }


        return null;


    }

void android.support.v4.app.Fragment.setMenuVisibility（boolean）＆＃39;在null对象引用上

4 个答案: