java - can I access a static Arraylist of Vector of another thread? in this example? -

having problems accessing static vector thread , class in thread. 1 class gui class called lotteryplay, has static vector called packarray, , other class called multithreader , part of shown below, runs in different thread. ideas on wrong?

the thing think of trying access static vector thread. possible?

   @override public void run() {     try {                out = new printwriter(socket.getoutputstream(), true);            in = new bufferedreader(new inputstreamreader(socket.getinputstream()));         system.out.println("streams setup new thread\n");              line = "";               while((line = in.readline())!= null){               this.messagefromclient(line);              if(!(counter > 1)){                 textsplitter(line);                 socketpack = new socketpack(socket, timestamp, address);                 lotteryplay.packarray.add(socketpack);   <<<----null pointer exception                 system.out.println("size of packarray " + lotteryplay.packarray.size());                 system.out.println();                 system.out.println("pakc array "+ lotteryplay.packarray);             }              system.out.println("from client: " + line.trim() + "\n");              }  // end while loop 

exception in thread "thread-3" java.lang.nullpointerexception at.com.lotterygame.multithreader.run(multithreader.java:55) @ java.lang.thread.run(thread.java:662)

the fact you're getting nullpointerexception suggests lotteryplay.packarray null. need make sure it's initialized before use it.

if really want keep static variable, i'd advise make final, e.g.

public static final vector<socketpack> packarray = new vector<socketpack>(); 

however, i'd advise redesign application avoid requiring static variables. think needs data, , pass appropriately. static variables make code harder reason , test.

additionally, note although each operation in vector individually synchronized, doesn't make thread-safe. example, can't safely iterate on vector without synchronization, otherwise 1 thread modify while you're iterating. should consider collections in java.util.concurrent.