Question

我正在为ProgressBar创建常见的java类，我收到如下错误

java.lang.NullPointerException: Attempt to invoke virtual method 'void app.bridgecenterandstaffmanagament.com.bridgecenterandstaffmanagament.CommonClass.Singletonclass_obj.showProgressBar(android.content.Context)' on a null object reference
                                                                                                                           at app.bridgecenterandstaffmanagament.com.bridgecenterandstaffmanagament.Fragments.NotificationFragment.onCreateView(NotificationFragment.java:41)

片段

public class NotificationFragment extends Fragment {


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

    RecyclerView recyclerView;



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment

        View v = inflater.inflate(R.layout.fragment_notification, container, false);


        recyclerView = v.findViewById(R.id.recylierview_notification);



       // Singletonclass_obj.getInstance().showProgressBar(getActivity());
        Singletonclass_obj.getInstance().showProgressBar(getContext());
       // print();

        return v;
    }


}

Java 类

public class Singletonclass_obj extends Application{
    private  ProgressBar progressBar;
    private static Singletonclass_obj singletonclassobj;



    public static Singletonclass_obj getInstance() {
        return singletonclassobj;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        singletonclassobj = this;
    }

    public void showProgressBar(Context context)
    {
      //  Toast.makeText(context, "welcometoindia", Toast.LENGTH_SHORT).show();
        progressBar = new ProgressBar(context, null, android.R.attr.progressBarStyleSmall);
    }

    public void hideProgressBar()
    {
        if (progressBar !=null && progressBar.isShown())
            progressBar.setVisibility(View.INVISIBLE);
     }

}

相同的代码工作正常我的上一个Project..please帮助我任何一个。

Answer 1

根据我的问题above comment

尝试将Singletonclass_obj添加到application tag like below code

中的清单文件中

<application
        android:name=".Singletonclass_obj"<!--add here your Singletonclass_obj class -->
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">

Answer 2

问题是您没有正确初始化应用程序类。基本上，系统不了解您的课程，因此不会创建。您需要在<application>的{{1}}标记中添加以下内容，它才有效：

Manifest

并将其称为：android:name=".Singletonclass_obj"

但是，正如我在评论中所说，您不需要将应用程序类创建为单例，因为它已经是。所以你的应用程序类将是这样的：

((Singletonclass_obj)getActivity().getApplication()).showProgressBar(getContext());

即使这样可以解决您关于NPE的问题，但您仍然无法使用public class Singletonclass_obj extends Application { private ProgressBar progressBar; @Override public void onCreate() { super.onCreate(); } public void showProgressBar(Context context) { // Toast.makeText(context, "welcometoindia", Toast.LENGTH_SHORT).show(); progressBar = new ProgressBar(context, null, android.R.attr.progressBarStyleSmall); } public void hideProgressBar() { if (progressBar !=null && progressBar.isShown()) progressBar.setVisibility(View.INVISIBLE); } }，因为您没有添加视图（该应用程序没有＆＃39;有观点）。

如何将getContext传递给Java类不起作用？

2 个答案: