HashMap contains 4 elements but only 3 are shown in debug

Question

I have defined an HashMap with the following code:

final Map<OrderItemEntity, OrderItemEntity> savedOrderItems = new HashMap<OrderItemEntity, OrderItemEntity>();
final ListIterator<DiscreteOrderItemEntity> li = ((BundleOrderItemEntity) oi).getDiscreteOrderItems().listIterator();

while (li.hasNext()) {
    final DiscreteOrderItemEntity doi = li.next();
    final DiscreteOrderItemEntity savedDoi = (DiscreteOrderItemEntity) orderItemService.saveOrderItem(doi);
    savedOrderItems.put(doi, savedDoi);
    li.remove();
}

((BundleOrderItemEntity) oi).getDiscreteOrderItems().addAll(doisToAdd);
final BundleOrderItemEntity savedBoi = (BundleOrderItemEntity) orderItemService.saveOrderItem(oi);
savedOrderItems.put(oi, savedBoi);

I put 4 items into the HashMap. When I debug, even if the size is 4, it only shows 3 elements:

This is the list of the elements it contains.

{DiscreteOrderItemEntity@1c29ef3c=DiscreteOrderItemEntity@41949d95, DiscreteOrderItemEntity@2288b93c=DiscreteOrderItemEntity@2288b93c, BundleOrderItemEntity@1b500292=BundleOrderItemEntity@d0f29ce5, DiscreteOrderItemEntity@9203174a=DiscreteOrderItemEntity@9203174a}

What can be the problem?

Answer 1

Hashmaps handle collisions.

Since your HashMap is composed by only 16 buckets, the hash of the element must be reduced to a number that spans between 0 and 15 (e.g. hash % 16). So two elements may be in the same bucket (the same HashMapNode).

You can inspect each HashMapNode to find out which one contains two elements.