list.add(null);
insert list
--->errorが発生,list.size=1
list.addAll(new list())
insert list
--->error発生しない、登録しない,list.size=1
list.add(null);
insert list
--->errorが発生,list.size=1
list.addAll(new list())
insert list
--->error発生しない、登録しない,list.size=1
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