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Collections.rotate()用于旋转列表元素，正数距离向右移动，负数向左。例如list=["a","b","c","d","e"]，rotate(list,2)结果为["d","e","a","b","c"]，rotate(list,-2)结果为["c","d","e","a","b"]。412 收藏 -
HashSet通过hashCode和equals方法实现去重，添加元素时先比较哈希值再判断相等性，自定义类需重写这两个方法；String等内置类型已默认实现，可直接去重；注意HashSet无序、允许一个null、非线程安全，大量数据去重性能优越。414 收藏
