I am trying to code a 1-many mapping inside a List element like so:

<list name="inputProcessSteps_Hibernate">
<key column="cmpnt_typ_c"/>
<index column="STEP_ID_N"/>
<one-to-many class="com.nppc.mes.productsupply.productdefinition.InputProcessStep"/>
</list>

When I check the parent object, it contains a list with the 5 elements I expect, plus 10 nulls, which are not expected.

When I change the mapping to use set:

<set name="inputProcessSteps_Hibernate">
<key column="cmpnt_typ_c"/>
<one-to-many class="com.nppc.mes.productsupply.productdefinition.InputProcessStep"/>
</set>

I get a collection with the 5 expected elements only.

Why does List generate the unexpected nulls?

If the index value set has gaps in the sequence (think array indexes) then each gap will result in a null entry in the collection.