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

Question

我试图从另一个类中调用一个方法，然后该类调用AlertDialog方法，我尝试了很多方法，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，我在活动类上有一个RecyclerView列表，其中包含被调用的方法。以下是来自具有AlertDialog:的活动类的一些代码

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);
}
}

它总是导致这一行出现空指针异常：

new AlertDialog.Builder(ListClass.this)

并且在ListClass:中调用该方法

showNullFieldNotification();

错误消息：

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)

如果我在ListClassonCreate()或从选项菜单中调用showNullFieldNotification()，它不会有问题，但当从WebSvr类调用时，它总是会导致异常。

关于AlertDialog的上下文，我尝试了ListClass.this、getApplicationContext()、getBaseContext()。。。我尝试将onCreate中的上下文保存为getApplicationContext()并使用它。

我试着使用这个问题的解决方案来获取上下文，但这也不起作用。非常感谢您的帮助。

编辑：忘了提一下，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();
}

Answer 1

您忘记创建Alert Dialog，所以它为null，而不是上下文

安卓简单警报对话框

Answer 2

从Dan Bray那里得到了答案。需要使用静态上下文。

由于您是从活动外部调用的，因此需要将上下文保存在活动类中：

public static Context context;

内部创建：

context = this;

显示通知：

new Handler(Looper.getMainLooper()).post(new Runnable(){
@Override
public void run(){
new AlertDialog.Builder(context)
.setTitle("Warning, some fields are empty")
.setNegativeButton("OK",new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialogInterface,int i){
dialogInterface.cancel();
showNotification=false;
}
}).show();
}
});

活动类中调用的方法也必须是public static。