java - Retrieving data from firebase returning NULL -

when call listpost(), return null. guess doesn't wait listener fetch post firebase. how can wait fetch post firebase before arraypost returned?

public post[] listpost() {     arraylist<post> list = new arraylist<post>();      // fetch post firebase     postref.addvalueeventlistener(new valueeventlistener() {          @override         public void ondatachange(datasnapshot snapshot) {             for(datasnapshot child : snapshot.getchildren()) {                 string id = child.getkey();                 string title = child.child("title").getvalue().tostring();                 string content = child.child("content").getvalue().tostring();                 string date = child.child("date").getvalue().tostring();                 string status = child.child("status").getvalue().tostring();                  post post = new post();                 post.setid(id);                 post.settitle(title);                 post.setcontent(content);                 post.setdate(date);                 post.setstatus(status);                 list.add(post);             }         }          @override         public void oncancelled(firebaseerror error) {             system.out.println("the read failed: " + error.getmessage());         }     });      // convert arraylist array      post[] arraypost = new post[list.size()];      list.toarray(arraypost);     return arraypost; } 

while can turn listpost() synchronous method using semaphores, not way firebase intended work. example, every time call addvalueeventlistener() add new listener called whenever data changes, once every time called listpost().

if purpose of listpost() method update state somewhere (e.g. ui), can instead update state directly ondatachanged() method. ensure add 1 value event listener , updates in data reflected in current state without needing refresh.

// setup once when app loads postref.addvalueeventlistener(new valueeventlistener() {      @override     public void ondatachange(datasnapshot snapshot) {         arraylist<post> list = new arraylist<post>();         for(datasnapshot child : snapshot.getchildren()) {             string id = child.getkey();             string title = child.child("title").getvalue().tostring();             string content = child.child("content").getvalue().tostring();             string date = child.child("date").getvalue().tostring();             string status = child.child("status").getvalue().tostring();              post post = new post();             post.setid(id);             post.settitle(title);             post.setcontent(content);             post.setdate(date);             post.setstatus(status);             list.add(post);         }         // list of posts here         updatesomething(list);     }      @override     public void oncancelled(firebaseerror error) {         system.out.println("the read failed: " + error.getmessage());     } }); 

since have list of children here can use childeventlistener here instead , react child added, child changed , child removed events update state or ui.