java - Using interface callbacks in Android -

i trying write simple chat application on android.

it takes speech, converts text, sends server.

i want able use texttospeech read response server gives.

i keep getting java.lang.nullpointerexception: attempt invoke interface method 'void {myappname}.mycallback.callbackcall()' on null object reference. have looked @ other questions similar answers, conflicting, "the interface should instantiated first" or "you don't instantiate interfaces", etc., , confusing me.

the receiver:

public class mainactivity extends actionbaractivity implements texttospeech.oninitlistener, mycallback {     ...      private void sendtoserver(string msg) {         cthread = new clientthread();         cthread.msg = msg;         thread thread = new thread(cthread);         thread.start();     }      @override     public void callbackcall() {         log.d("callback", cthread.serverresponsesaved);         // speaking here.     } } 

the actual code responsible messages:

interface mycallback {     public void callbackcall(); }  public class clientthread implements runnable {     string address = "xxx.xxx.x.xx";     int port = yyyyy;     boolean connected = false;     string msg = "";     private string serverresponse;     string serverresponsesaved;     mycallback callback;      @override     public void run() {         try {             socket socket = new socket(address, port);             this.connected = true;             while(connected) {                 try {                     printwriter out = new printwriter(new bufferedwriter(new outputstreamwriter(socket.getoutputstream())), true);                     out.println(this.msg);                     bufferedreader in = new bufferedreader(new inputstreamreader(socket.getinputstream()));                         if ((serverresponse = in.readline()) != null) {                             serverresponsesaved = serverresponse;                             log.i("server says", serverresponsesaved);                             callback.callbackcall();                         }                 } catch (exception e) {                     e.printstacktrace();                 }             }         } catch (ioexception e) {             e.printstacktrace();             this.connected = false;         }     } } 

is way i'm implementing interface wrong?

edit:

trying instantiate by: callback = new mycallback(); gives me 'mycallback abstract. cannot instantiated.' error.

the use of interface based on: here

you not setting callback anywhere. define mycallback , give name callback still has nothing assigned it, hence nullpointerexception when hit callback.callbackcall();

i'm assuming want set mainactivity linked, have pass clientthread through constructor like:

public class clientthread implements runnable {      public clientthread(mycallback callback){          this.callback = callback;      } } 

then change:

cthread = new clientthread(); 

to

cthread = new clientthread(this); 

however if callback function wants ui related things, or should run on main thread, have use handler post main thread. if want that, easiest pass in context clientthread , this:

public class clientthread implements runnable {      /*your things*/      context mctx;      public clientthread(context context){          this.mctx = context;          if(context instanceof mainactivity){              this.callback = (mainactivity) callback;          }      } } 

then change callback bit to:

handler mainhandler = new handler(mctx.getmainlooper()); mainhandler.post(new runnable() {             @override             public void run() {                 callback.callbackcall();             }         });