错误：java.lang.nullpointerexception

Question

I'm stucked with this problem, I don't know what's wrong with my code, please help me. I got error java.lang.nullpointerexception on this code:

List<DataPoint> listPoints;
if((listPoints = hashMap.get(h)) == null) {
    listPoints = new ArrayList<DataPoint>();
    DataPoint point = new DataPoint((int)songId, i);
    listPoints.add(point);
    hashMap.put(h, listPoints);
}

Answer 1

if you are loading HashMap from other thread it might be still null at the point of using it. Also h may be null too. A little bit more detail will be good

Answer 2

if((listPoints = hashMap.get(h)) == null)
{
    listPoints = new ArrayList<DataPoint>();
    DataPoint point = new DataPoint((int)songId, i);
    listPoints.add(point);
    hashMap.put(h, listPoints);
}

From that snippet, the only lines you'd be getting NullPointerException is in the hashMap.get() and hashMap.put() calls. My bet is that hashMap was not initialized and hence is null . So, when you call hashMap.get() , you get your exception.

Answer 3

您正在使用的HashMap的实现（例如ConcurrentHashMap）可能不接受空键-如果h为空，则将导致NullPointerException。