How to set value for a custom attribute type workItemList using plain Java API

hi,
I want to create a workItem using plain Java API with some custom attribute, however, when I set value for a attribute type workItemList, it give error indication "$Proxy20 incompatible with java.util.List", I really don't know what this type meaning for.
Could anybody tell me how to set value for the attribute of this type-workItemList.
BR,
Michael
I want to create a workItem using plain Java API with some custom attribute, however, when I set value for a attribute type workItemList, it give error indication "$Proxy20 incompatible with java.util.List", I really don't know what this type meaning for.
Could anybody tell me how to set value for the attribute of this type-workItemList.
BR,
Michael