无法将BasicDBList转换为数组（java）

Question

I have a MongoDB collection where documents contain several arrays. I"m retrieving those documents and storing their data into POJOs.

In some instances, I can do this:

BasicDBList kws = (BasicDBList)obj.get(Constants.KEYWORDS_STR);
if(!kws.isEmpty())  
    tb.setKeywords(kws.toArray(new String[0]));

However, I have one array that's stumping me. I thought it contained Longs, but toArray(new Long[0]) throws java.lang.ArrayStoreException. I figured ok, for some reason they are stored as Strings, and tried converting to an array of Strings, and still got a java.lang.ArrayStoreException.

How can I tell what class the BasicDBList thinks it has stored in it?

Answer 1

通过获取BasicDBList的迭代器并查看对象，我发现它持有Integers（这很奇怪，因为它应该是Longs，但这是另一个问题的主题）。

Answer 2

Since BasicDBList extends ArrayList<Object> , (note - generics are not used), so there's no guarantee that kws will contain homogeneous types within it. You should be able to change new String[0] to new Object[0] and avoid the exception.

There's no way to easily interrogate the array without some kind of statements:

if (instanceof ){...} 
else if (instanceof) {...}

Your data model should be mapped out well enough that this pattern is not required, or at least well enough that you know the finite set of what to expect.