java - Android Java：从另一个类调用的AlertDialog方法导致空指针异常

问题描述

我试图从另一个类调用一个方法，然后调用一个AlertDialog方法，我尝试了很多东西，但它总是会导致空指针异常。调用类正在运行一个 Web 服务器，我RecyclerView在活动类上有一个列表，其中包含被调用的方法。这是来自活动类的一些代码AlertDialog：

public class ListClass extends AppCompatActivity {

    private WebSvr server;
    private static final int PORT = 8080;
    private androidx.appcompat.widget.Toolbar toolbar;
    private ListView lv;
    private CustomAdapter customAdapter;
    public ArrayList<EditModel> editModelArrayList;
    public static String[] newList = new String[20];

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_server_screen);
        toolbar = (androidx.appcompat.widget.Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        startWebServer();

        customAdapter = new CustomAdapter(this, editModelArrayList);
        lv.setAdapter(customAdapter);
    }

    public static boolean listUpdated;

    public void updateList() {
        listUpdated = false;
        Runnable r = new Runnable() {
            @Override
            public void run() {

                for (int i = 0; i < 20; i++) {
                    if (CustomAdapter.editModelArrayList.get(i).getEditTextValue() != "") {
                        newList[i] = CustomAdapter.editModelArrayList.get(i).getEditTextValue();
                    } else {
                        if (!showNotification) {

                            runOnUiThread(new Runnable() {
                                @Override
                                public void run() {
                                    showNullFieldNotification(); // <------------- CALLING THIS 
                                }
                            });
                            showNotification = true;
                        }
                    }
                }
                listUpdated = true;
            }
        };
        Thread thread = new Thread(r);
        thread.start();
    }

    boolean showNotification = false;

    public void showNullFieldNotification() {
        new AlertDialog.Builder(ListClass.this)
                .setTitle("Warning, some fields are empty")
                .setNegativeButton("OK", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        dialogInterface.cancel();
                        showNotification = false;
                    }
                }).show();
    }

    public static boolean returnListUpdated() {
        return listUpdated;
    }

...

}

这是我的网络服务器类的代码：

public class WebSvr extends NanoHTTPD {

    public ListClass ListClass = new ListClass();

    public WebSvr(int port){
        super(port);
    }

    @Override
    public Response serve(String uri, Method method,
                          Map<String, String> header,
                          Map<String, String> parameters,
                          Map<String, String> files) {


        ListClass.updateList(); // <--------------- THE METHOD CALLED

        while (!ListClass.returnListUpdated()) {
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        String strResponse = Arrays.toString(ListClass.returnString());

        return newFixedLengthResponse(strResponse);
    }
}

它总是在这一行导致空指针异常：

new AlertDialog.Builder(ListClass.this)

以及在哪里调用该方法ListClass：

showNullFieldNotification();

错误信息：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.espressif.iot_esptouch_demo, PID: 18217
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:152)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
        at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:224)
        at android.app.AlertDialog$Builder.<init>(AlertDialog.java:454)
        at com.myProject.ListClass.showNullFieldNotification(ListClass.java:177)
        at com.myProject.ListClass$4.run(ListClass.java:193)
        at android.os.Handler.handleCallback(Handler.java:789)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6665)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:772)

如果我调用或从选项菜单调用它没有问题，但是当从类调用时showNullFieldNotification()它总是会导致异常。ListClass onCreate()WebSvr

对于AlertDialog我尝试过的上下文ListClass.this，getApplicationContext()...getBaseContext()我尝试将上下文保存在onCreateas 中getApplicationContext()并使用它。

我试图使用这个问题的解决方案来获取上下文，但这也不起作用。非常感谢任何帮助。

编辑：忘了提到我也尝试过以这种方式构建警报：

public void showNullFieldNotification2() {
        AlertDialog.Builder alert = new AlertDialog.Builder(ListClass.this);
                alert.setTitle("Warning, some fields detected as being empty or having \"0\" as value");
                alert.setNegativeButton("OK", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        dialogInterface.cancel();
                        showNotification = false;
                    }
                });
                AlertDialog dialog = alert.create();
                dialog.show();
    }

标签： javaandroidnullpointerexceptionandroid-alertdialog

java - Android Java：从另一个类调用的AlertDialog方法导致空指针异常

问题描述

解决方案

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