What is the purpose to write ? ? ?

UserDao dao = new UserDaoXmlImpl ();
UserDao which is the interface name , UserDaoXmlImpl is to achieve UserDao interface class name.
Why not write : UserDaoXmlImpl dao = new UserDaoXmlImpl ();
began as written , what use is it special ? ? ? Best new hope advice elaborate ah
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multi Aspect multi- state of knowledge !
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multi Aspect polymorphic knowledge !
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In order to better safeguard