把四个集合里面的内容都存入一个集合（最后的集合已经有了四个集合的set，get方法了）
怎么四变一
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addAll() doesn’t work...
Are the generics of these four collections the same?
If the generics are consistent, use the addall() method to add it to a new List. If you need to remove duplicates, use Set’s addall ()
Write a wrapper collection class and delegate to the corresponding sub-collection as needed. Usually just inherit AbstractList, and the constructor puts four or more sub-collections in.
It is recommended that you go to Google's guava toolkit... There are ready-made ones...
addAll() is enough
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addAll() doesn’t work...
Are the generics of these four collections the same?
If the generics are consistent, use the addall() method to add it to a new List. If you need to remove duplicates, use Set’s addall ()
Write a wrapper collection class and delegate to the corresponding sub-collection as needed.
Usually just inherit AbstractList, and the constructor puts four or more sub-collections in.
It is recommended that you go to Google's guava toolkit... There are ready-made ones...
addAll() is enough