我试图从另一个类中调用一个方法,然后再调用一个AlertDialog
方法,我已经尝试了很多事情,但是它总是会导致空指针异常。调用类正在运行Web服务器,活动类上有一个RecyclerView
列表,其中包含要调用的方法。这是活动类中带有AlertDialog
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中的代码:
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);
}
}
它总是在此行上导致空指针异常:
new AlertDialog.Builder(ListClass.this)
以及在ListClass
中调用该方法的位置:
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showNullFieldNotification()
ListClass
中或从选项菜单中调用onCreate()
,则它没有问题,但是从WebSvr
类中调用时,它总是会引起异常。
对于AlertDialog
上的上下文,我尝试过ListClass.this
,getApplicationContext()
,getBaseContext()
...我试图将onCreate
中的上下文另存为getApplicationContext()
,并使用它。
我已经尝试使用this问题中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:忘记提及我也曾尝试以这种方式建立警报:
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();
}
您忘记创建警报对话框,因此它为null,而不是上下文
从here的答案中得到Dan Bray的答案。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;
和onCreate内:
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为public static
。