而循环Java将程序正在读取的PDU存储在ArrayList中

Question

我有一个名为receivePdu()的Java方法，它将侦听通过网络发送的任何PDU，并在控制台中打印实体ID和模拟中每个PDU的位置。 我正在使用一个while循环来实现这一点，这样我的代码在运行的过程中就一直在监听pdu

现在我想修改我的代码，这样除了在控制台中打印关于每个单独PDU的信息外，它还将该信息存储在ArrayList中，以便我的程序中的另一个Java类可以使用它

我的receivePdu()方法当前看起来如下所示：

public static void receivePdu(){

    try{
        /*Specify the socket to receive the data */
        socket = new MulticastSocket(EspduSender.PORT);
        address = InetAddress.getByName(EspduSender.DEFAULT_MULTICAST_GROUP);
        socket.joinGroup(address); 

        /*Loop infinitely, receiving datagrams */
        while(true){
            byte buffer[] = new byte[MAX_PDU_SIZE];
            packet = new DatagramPacket(buffer, buffer.length);

            socket.receive(packet);

            Pdu pdu = pduFactory.createPdu(packet.getData()); /*    Commented on 15/04/2014 @ 09:15 */

            if(pdu != null){
                System.out.print("Got PDU of type: " + pdu.getClass().getName());
                if(pdu instanceof EntityStatePdu){
                    EntityID eid = ((EntityStatePdu)pdu).getEntityID();
                    Vector3Double position = ((EntityStatePdu)pdu).getEntityLocation();
                    System.out.print(" EID:[" + eid.getSite() + ", " + eid.getApplication() + ", " + eid.getEntity() + "] ");
                    System.out.print(" Location in DIS coordinates: [" + position.getX() + ", " + position.getY() + ", " + position.getZ() + "]");
                } else if(!(pdu instanceof EntityStatePdu)){
                    System.out.println("There are no PDUs currently being received.");
                }
                System.out.println();
            }
        } /*end while */
    } /*end try */
    catch(Exception e){
        System.out.println(e);
        e.printStackTrace();
        System.out.println("This is where the error is being generated");
        /*09/04/2014 @ 17:100
         * If this exception gets called, presumably it either means that pdu is not an instance of EntityStatePdu, or
         * that pdu does not actually hold a packet.  */
    }
}

现在，我已经在类中添加了一个ArrayList（在receivePdu()方法之外），使用行：public static ArrayList<pdu> espdu;并修改了我的while循环，将每个PDU添加到ArrayList：

/*Loop infinitely, receiving datagrams */
while(true){
    byte buffer[] = new byte[MAX_PDU_SIZE];
    packet = new DatagramPacket(buffer, buffer.length);

    socket.receive(packet);

    Pdu pdu = pduFactory.createPdu(packet.getData()); /*    Commented on 15/04/2014 @ 09:15 */

    if(pdu != null){
    System.out.print("Got PDU of type: " + pdu.getClass().getName());
    if(pdu instanceof EntityStatePdu){
    EntityID eid = ((EntityStatePdu)pdu).getEntityID();
    Vector3Double position = ((EntityStatePdu)pdu).getEntityLocation();
    System.out.print(" EID:[" + eid.getSite() + ", " + eid.getApplication() + ", " + eid.getEntity() + "] ");
    System.out.print(" Location in DIS coordinates: [" + position.getX() + ", " + position.getY() + ", " + position.getZ() + "]");
    espdu.add(pdu);
} else if(!(pdu instanceof EntityStatePdu)){
        System.out.println("There are no PDUs currently being received.");
}
System.out.println();
    }
} /*end while */

然而，当我现在运行我的代码时，尽管显示了接收到的第一个PDU，但我得到一个java.lang.NullPointerException，其中调用了espdu.add(pdu);行，它正在抱怨这一行：espdu.add(pdu);

我不明白为什么会出现空指针异常-有人有什么想法吗？我以上面提到的方式声明了ArrayList，其余变量位于类的顶部。在此方面的任何帮助或指导都将不胜感激