android - can't get values out of ondatachange method -

i'm developing android app i'm using firebase database when in got variable in ondatachange method , assign them global variables got null variables when call variables in ondatachange method not null.

public class positionatemarkertask extends asynctask {     public arraylist<location> arraylist= new arraylist<>();     public void connect() {         //setting connexion parameter         final firebase ref = new firebase("https://test.firebaseio.com/test");         query query = ref.orderbychild("longitude");          //get data db         query.addlistenerforsinglevalueevent(new valueeventlistener() {              @override             public void ondatachange(datasnapshot datasnapshot) {                 //checking if user exist                 if(datasnapshot.exists()){                     (datasnapshot usersnapshot : datasnapshot.getchildren()) {                         //get each user has target username                         location location =usersnapshot.getvalue(location.class);                             arraylist.add(location);                         //if password true , data storaged in sharedpreferences file , home activity launched                     }                 }                 else{                     system.out.println("not found");                 }             }              @override             public void oncancelled(firebaseerror firebaseerror) {                   system.out.println("problem ");              }         });     }      @override     protected object doinbackground(object[] params) {         connect();         return null;     }      @override     protected void onpostexecute(object o) {         super.onpostexecute(o);          system.out.println("the firs long is"+arraylist.get(0).getlongitude());      } } 

welcome asynchronous programming, messes thought true. :-)

firebase retrieves/synchronizes database automatically in background. work happens on separate thread, don't need , asynctask. unfortunately means can't wait data.

i typically recommend reframe code "first a, b" "whenever a, b it".

in case, want data , print longitude of first item. reframed is: whenever receive data, print longitude of first item.

query query = ref.orderbychild("longitude");  query.addlistenerforsinglevalueevent(new valueeventlistener() {     @override     public void ondatachange(datasnapshot datasnapshot) {         if(datasnapshot.exists()){             (datasnapshot usersnapshot : datasnapshot.getchildren()) {                 location location =usersnapshot.getvalue(location.class);                 arraylist.add(location);             }             system.out.println("the first long is"+arraylist.get(0).getlongitude());                }         else{             system.out.println("not found");         }     }      @override     public void oncancelled(firebaseerror firebaseerror) {           system.out.println("problem ");     } }); 

a few things note here:

1. if you're interested in first item, can limit query 1 item: query = ref.orderbychild("longitude").limittofirst(1). retrieve less data.

2. i recommend using addvalueeventlistener() instead of addlistenerforsinglevalueevent(). former keep synchronizing data. means if insert/change longitude of item in list, code automatically retriggered , print (potentially) new first item.