将几个ArrayList集合合并在一起，并且不能有重复

如何将几个ArrayList集合合并在一起并且没有重复元素呢？

//查找集合

 ArrayList messlist1 = table_message.findMessageByTitle(messageSearch);        ArrayList messlist2=table_message.findMessageByWriter(messageSearch);        ArrayList messlist3=table_message.findMessageByWritedate(messageSearch);        ArrayList messlist4=table_message.findMessageByContent(messageSearch); //进行连接刷掉重复的        Set set = new HashSet(messlist1);        set.addAll(new HashSet(messlist2));        set.addAll(new HashSet(messlist3));        set.addAll(new HashSet(messlist4));

//进行转换

       ArrayList list=new ArrayList(set);

这样就ok了