java - Timed functions throw null pointer exception -

hi creating class in can pass down functions called either periodicity or once off. . problem functions passing down gets called immediately, after when suppose called following error:

java.lang.nullpointerexception @ timedfunction$2.run(timedfunction.java:41) @ java.util.timerthread.mainloop(unknown source) @ java.util.timerthread.run(unknown source) 

the base class follow (only relivant sections):

import java.util.concurrent.callable; ...      public timedfunction () {}      public void addsingleevent (callable func, int seconds){         //convert seconds miliseconds                 int time = seconds * 1000;                  //create new timer                 new java.util.timer().schedule(                          new java.util.timertask() {                             @override                             public void run() {                                 try {                                     func.call();                                 } catch (exception e) {                                     // todo auto-generated catch block                                     e.printstacktrace();                                 }                             }                         },                          time                  );     } 

i testing class looks this:

public static void main(string[] args) {     // todo auto-generated method stub     timedfunction tm = new timedfunction();      tm.addsingleevent(helloworld(), 5);     tm.addrepeatedevent(dataworld(), 1); }  private static callable helloworld() {     system.out.print("hello world!");     system.out.print(" ");     return null; } 

check helloworld method - it's returning null!

i think needs like

private static callable helloworld() {     return new callable<string>() {         return "hello world";     } } 

if had guess, think you're getting confused how use lambas. if that's case, syntax wrong, should this:

tm.addsingleevent(() -> "hello world", 5); tm.addrepeatedevent(() -> "data world", 1);