NullPointerException

This sentence:

receivers.addAll( lookupMap.get(it.next() ) );

/* --------------------------------------------------------*/

Please input the name of pathway:

aae00010

Connecting to KEGG...

Connected successfully.

java.lang.NullPointerException

at java.util.AbstractCollection.addAll(AbstractCollection.java:303)

at org.tingting.bn.mn.graphbuilder.test.PathGraphBuilder.main(PathGraphBuilder.java:161)

/*---------------------------------------------------------*/

Now I've found why this problem occurred. NPE(NullPointerExcetion) is caused by Null point of the input variable. Here it is because lookupMap.get( ) returns a Null. I disabled the paragraph to generate the lookupMap for the sake of easy debugging, but I don't notice that this lookupMap is used when enzymerelationshipMap is generated.