在Java中自动装箱时的NPE

Question

I have the following piece of code:

map =  db_.treeMapCreate(validMapName_)
          .keySerializer(Serializer.LONG)
          .valueSerializer(Serializer.LONG)
          .make(); //mapDB
protected void onTaskCompletion(TaskInfo task)
{
   long startBlk = task.blkId;
   long count = task.count;
   for (int i=0; i < count; ++i)
   {
      long blk = startBlk + i;
      Long oldVal = map.get(blk); //NPE here
      ...
      ...
   }
}

How is it possible to get an NPE while autoboxing? I can understand getting an NPE on unboxing ie if I had:

long oldVal = map.get(blk)

then that can throw an NPE.

Edit: Map isn't null. To be more specific, inside the BTreeMap code of mapDB, this line gets executed:

if(key==null) throw new NullPointerException();

In case someone wants to take a look: BtreeMap of mapDB Line: 1024

Partial stacktrace:

java.lang.NullPointerException
        at org.mapdb.BTreeMap.get(BTreeMap.java:1024)
        at org.mapdb.BTreeMap.get(BTreeMap.java:1013)
        at com.service.onTaskCompletion(TaskHandler.java:312)

I'm unable to reproduce it so I cannot give a minimal, verifiable example. I have tried to run it so that I perform a get() for a key that doesn't exist, and it DOESN'T give an NPE.

Answer 1

I think the problem is that you are looking at the wrong version of the BTreeMap code. If you go back to the previous commit of this class , line 1024 is not:

if(key==null) throw new NullPointerException();

but:

while(!A.isLeaf()) {...}

where A is set by:

BNode A = engine.get(current, nodeSerializer);

This makes much more sense. Basically, null is returned from engine.get . How that is possible is beyond my understanding, but this could very well be a bug of mapdb itself.

Answer 2

The NPE could be due to any of the following:

1. Map being null
2. Passing a null value to get method (not the case)
3. Passing a key to get() that does not exist in MapDB

I am more concerned about number 3 in here because, if you pass a key that does not exist then null is returned and trying to store null in Long oldVal cause the exception (I believing that something as follow happens):

Long l = new Long(null); //NPE

To find one whether it is #3 do the following

//if key don't exist MapDB returns null
if(map.get(blk) != null) { //not null } else { //yes null}

To find out if map is null obviously as pointed out by others, you do as follow

//invoking get on a null can cause NPE too, so another reason
if(map != null) { // not null } else { //yes null}

To support the fact that #3 could be causing the NPE, see this post with a similiar issue .

Looking into the docs for BTreeMap the get(Object) below

public V get(Object key)
Specified by:
get in interface Map<K,V>
Overrides:
get in class AbstractMap<K,V>

Looking at Map and AbstractMap the get(Object key) is specified as follow:

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

get(Object) of Map<K,V> and A bstractMap<K,V> returns null if no mapping is found. It is also capable of throwing NullPointerException . So #3 could still be the case that there is no entry for the key passed to get method in map.

Answer 3

NPE is because map object is null and you are referring to get() method of null,thereby NPE. Do check of map for null and you will find your solution.

if(map!=null){
long oldVal = map.get(blk);}
 else {
System.out.println("Map is null");
}

Answer 4

I see only one possibility: The variable "map" is null.

Your code sample is incomplete, as you didn't show and where "map" is really initialized.