从Set里面取出有序的记录
Set里面的记录是无序的，如果想使用Set，然后又想里面的记录是有序的，就可以使用TreeSet，而不是HashSet，在使用TreeSet的时候，里面的元素必须是实现了Comparable接口的，TreeSet在进行排序的时候就是通过比较它们的Comparable接口的实现！
此外，还讲一点，TreeSet是在以前的老版本的jdk里面定义的，大概1.4以前的吧，是线程安全的，由于是线程安全的，所以，在并发环境下极大的影响了程序的速度！而HashSet是非线程安全的，是为了满足Set的使用和高速的程序访问出现的！
下面是HashSet的无序和TreeSet的有序的比较：
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Test类：
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User类：
-----------------HashSet Start------------------ id = 3, username = uname3, password = pswd3 id = 4, username = uname4, password = pswd4 id = 1, username = uname1, password = pswd1 id = 2, username = uname2, password = pswd2 id = 7, username = uname7, password = pswd7 id = 8, username = uname8, password = pswd8 id = 5, username = uname5, password = pswd5 id = 6, username = uname6, password = pswd6 id = 9, username = uname9, password = pswd9 id = 10, username = uname10, password = pswd10------------------HashSet End---------------------------------------TreeSet Start------------------ id = 1, username = uname1, password = pswd1 id = 2, username = uname2, password = pswd2 id = 3, username = uname3, password = pswd3 id = 4, username = uname4, password = pswd4 id = 5, username = uname5, password = pswd5 id = 6, username = uname6, password = pswd6 id = 7, username = uname7, password = pswd7 id = 8, username = uname8, password = pswd8 id = 9, username = uname9, password = pswd9 id = 10, username = uname10, password = pswd10------------------TreeSet End----------------------?
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