我有两个ArrayLists.
ArrayList A包含
['2009-05-18','2009-05-19','2009-05-21']
ArrayList B包含 ['2009-05-18','2009-05-18','2009-05-19','2009-05-19','2009-05-20','2009-05-21','2009-05-21','2009-05-22']
我必须比较ArrayLst A和ArrayLst B. 结果ArrayList应该包含ArrayList中不存在的List .AllList结果应该是
[ '2009-05-20', '2009-05-22']
怎么比较？