獲取顯示java.lang.NullPointerException

Question

我試圖來存儲所有的消息對象接受了一分鐘，一棵樹地圖和之後的一個分鐘的車一過，序列化和字節[]返回到另一個階級同時清除地圖，並開始儲存在下一分鐘收到的消息等等。

public class StoreMessage extends Thread implements Serializable{ 

    public static byte[] b=null; 
    public static Map <Long,Message> map1=Collections.synchronizedMap(new TreeMap<Long,Message>()); 
    public static Calendar c1=Calendar.getInstance(); 
    public static int year=c1.get(Calendar.YEAR); 
    public static int month=c1.get(Calendar.MONTH); 
    public static int day=c1.get(Calendar.DAY_OF_MONTH); 
    public static int hour=c1.get(Calendar.HOUR_OF_DAY); 
    public static int min=c1.get(Calendar.MINUTE); 
    public static GregorianCalendar gc = new GregorianCalendar(year, month, day, hour, min); 
    public static Date d=gc.getTime(); 
    public static long time1=(d.getTime())/60000; //precision till minute of the time elapsed since 1 Jan 1970 
    public static long time2=time1+1; 
    public static byte[] store(Message message)throws Exception{ 

    while(true) 
     { 
      if(time1<time2) 
      { 
       long preciseTime=TimeUnit.MILLISECONDS.toNanos(System.currentTimeMillis())+(System.nanoTime()-startNanotime); 
       map1.put(preciseTime, message); 
      } 

      else 
      { 
       b=Serializer.serialize(map1); 
       map1.clear(); 
       time1=time2; 
       time2++; 
          return b; 
      } 

      } 
     } 
    }  

爲什麼這個代碼給我空指針異常突出INT LEN = b.length個;的另一個類，它被稱爲返回值？

public static void record(Message message){ 
     try{ 
      //storing the serialized message in byte[] 
      byte[] b =StoreMessage.store(message); 
      int len=b.length; //<--highlights this line for null pointer exception 

即使在進行修改後（即將返回放在else塊內），它也不會將控件返回給調用類。另外，沒有SOP語句（添加時）打印在else塊內部。爲什麼？

The Serializer class 
    public class Serializer { 
     //serializes an object and returns a byte array 
     public static byte[] serialize(Object map) throws IOException 
      { 
      ByteArrayOutputStream b = new ByteArrayOutputStream(); 
      ObjectOutputStream o = new ObjectOutputStream(b); 
      o.writeObject(map); 
      return b.toByteArray(); 
      } 

     //de-serialization of the byte array and returns an object 
     public static Object toObject (byte[] bytes) 
     { 
      Object obj = null; 
      try 
      { 
      ByteArrayInputStream bis = new ByteArrayInputStream (bytes); 
      ObjectInputStream ois = new ObjectInputStream (bis); 
      obj = ois.readObject(); 
      } 
      catch (Exception ex) { } 
      return obj; 
     } 
    } 

Answer 1

這是因爲你的b變量是null。在這裏看到：

您輸入方法，使檢查if(time1<time2)。這是true。因此你不會進入其他地方，也不要初始化b。 Afterwords後，你將返回b的值。它是null。如果你問我，你已經錯誤地列出了退貨聲明。放置在else語句返回的，所以你要確保循環被終止，還是你會在返回前初始化數組：

while(true) { 
    if(time1<time2) { 
     long preciseTime=TimeUnit.MILLISECONDS.toNanos(System.currentTimeMillis())+(System.nanoTime()-startNanotime); 
     map1.put(preciseTime, message); 
     } else { 
     b=Serializer.serialize(map1); 
     map1.clear(); 
     time1=time2; 
     time2++; 
     return b; 
     } 
} 
}