I'm trying to call a method from another class which then calls an AlertDialog method, and I've tried many things but it always causes a null pointer exception. The calling class is running a web server, and I have a RecyclerView list on the activity class containing the method being called. here's some code from the activity class with 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;
}
...
}
And here's the code from my web server class:
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);
}
}
It always causes null pointer exception on this line:
new AlertDialog.Builder(ListClass.this)
and where the method is called in ListClass:
showNullFieldNotification();
Error message:
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)
If I call showNullFieldNotification() in ListClass onCreate() or from the options menu it works no problem, but when called from the WebSvr class it always causes the exception.
For context on the AlertDialog I've tried ListClass.this, getApplicationContext(), getBaseContext()... I've tried to save a context in onCreate as getApplicationContext() and use that.
I've tried to use the solution from this question to get context but that didn't work either. Would really appreciate any help.
EDIT: Forgot to mention that I had also tried to build the alert in this way:
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();
}
