小编Sim*_*tta的帖子

我想在我的应用程序中显示后退箭头按钮,但当我将其放入我的代码时应用程序崩溃:

getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setDisplayShowHomeEnabled(true);

style.xml

 <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
    <item name="colorPrimary">@color/PrimaryColor</item>
    <item name="colorPrimaryDark">@color/PrimaryColorDark</item>
    <item name="colorAccent">@color/PrimaryColor</item>
    <item name="colorControlHighlight">@color/ControlHighlight</item>
    <item name="android:statusBarColor" tools:targetApi="21">@android:color/transparent</item>
</style>


<style name="ActionBarPopupThemeOverlay" parent="ThemeOverlay.AppCompat.Light" >
    <item name="android:textColor">#000000</item>
</style>


<style name="MyCustomTabLayout" parent="Widget.Design.TabLayout">
    <item name="tabIndicatorColor">#FFFFFF</item>
    <item name="tabIndicatorHeight">3dp</item>
    <item name="tabTextAppearance">@style/MyCustomTabTextAppearance</item>
</style>


<style name="MyCustomTabTextAppearance" parent="TextAppearance.Design.Tab">
    <item name="android:textColor">#FFFFFF</item>
</style>

toolbar.xml

<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_height="@dimen/abc_action_bar_default_height_material"
android:elevation="0dp"
android:layout_width="match_parent"
android:id="@+id/toolbar"
android:background="@color/PrimaryColor"
app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
app:popupTheme="@style/ActionBarPopupThemeOverlay"/>

类(不是片段):

public class WebViewCanale extends AppCompatActivity {

    Toolbar toolbar;
    TextView textView;
    WebView webView;
    ProgressBar progressBar;

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

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setDisplayShowHomeEnabled(true);

        //Toolbar …

在我的应用程序中有2个选项卡(Android设计支持库)这是带有ViewPagerAdapter的片段:

public class ListaCanali extends Fragment {

public static TabLayout tabLayout;
public static ViewPager viewPager;
public static int int_items = 2 ;

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

    //Titolo Fragment
    ((MainActivity) getActivity()).setActionBarTitle("Lista Canali");

    //FindViewById
    tabLayout = (TabLayout) v.findViewById(R.id.tabs);
    viewPager = (ViewPager) v.findViewById(R.id.viewpager);

    //Funzioni Tab
    viewPager.setAdapter(new MyAdapter(getChildFragmentManager()));
    tabLayout.post(new Runnable() {
        @Override
        public void run() {
            tabLayout.setupWithViewPager(viewPager);
        }
    });

    return v;
}


//Adattatore Tab
class MyAdapter extends FragmentPagerAdapter {

    public MyAdapter(FragmentManager fm) …

我有一个带按钮的片段，单击该按钮应该更改我应用程序中的主题，但是当我打开该类时，我只会收到此错误：这是 logcat

08-14 21:32:45.914  29624-29624/addid E/AndroidRuntime? FATAL EXCEPTION: main
Process: appid, PID: 29624
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at appid.Fragment.GuidaTv.InitView(GuidaTv.java:75)
        at appid.Fragment.GuidaTv.onCreateView(GuidaTv.java:41)
        at android.support.v4.app.Fragment.performCreateView(Fragment.java:1789)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:955)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1138)
        at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:740)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1501)
        at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:458)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

我的课：

public class GuidaTv extends Fragment implements View.OnClickListener{

private Button FirstThemButton;
private Button SecondThemButton;
private Button ThirdThemButton;
private SharedPreferences sharePrefences;
private …