How to list models in a dataset?

[Site Guo]

Is there an API function that lists all models (graphs) within a given dataset? 

There is a function listNames() in Dataset. I am not sure if it is used to 
return all named graphs. When I tried to use it like this:

Iterator<String> models = ds.listNames(); //ds is a dataset object that has 
been created and connected.
while (models.hasNext())
{ 
     System.out.println(models.next());
}

 Java gave me the following error:


Exception in thread "main" java.lang.NullPointerException
    at com.hp.hpl.jena.sparql.util.NodeUtils$1.accept(NodeUtils.java:94)
    at com.hp.hpl.jena.sparql.util.NodeUtils$1.accept(NodeUtils.java:90)
    at 
com.hp.hpl.jena.util.iterator.MapFilterIterator.hasNext(MapFilterIterator.java:55)

I am using TDB.

Could anyone help me figure that out ?
Thanks in advance.

Site (Victor) Guo