去年在实习的时候，看电子版的《OReilly.Java.Enterprise.in.a.Nutshell.3rd.Edition.Nov.2005》时，对于其中讲hibernate的类映射（Class Mappings）问题的时候，提及的一个概念，first-class，迷惑不解。科班的教育根本没有向我提及这个词在计算机科学里的意义，纯粹的字面解释让人在上下文的语境中真的有点摸不清什么意思。
So far, we've been working with what Hibernate calls "first-class" objects.
其实这篇日志早已在去年8月16日写下，不过是在其他地方，现在觉得有必要转过来。
下面的这个解释是别人查wikipedia然后留在blog上的，借用下来，看了之后应该再不晕了吧。
First-class object
In computing, a first-class object (also -value, -entity, -citizen), in the context of a particular programming language, is an entity which can be used in programs without restriction (when compared to other kinds of objects in the same language). Depending on the language, this can imply:
being expressible as an anonymous literal value
being storable in variables
being storable in data structures
having an intrinsic identity (independent of any given name)
being comparable for equality with other entities
being passable as a parameter to a procedure/function
being returnable as the result of a procedure/function
being constructable at runtime
For example, in C, it is not possible to create new functions at runtime, whereas other kinds of object can be created at runtime. So functions in C are not first-class objects; sometimes they are called second-class objects because they can still be manipulated in most of the above fashions (via function pointers). Similarly, strings are not first class objects in FORTRAN 66 as it is not possible to assign them to variables, whereas numbers can be so assigned.
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First-class function
In computer science, a programming language is said to support first-class functions if it treats functions as first-class objects. Specifically, this means that functions can be created during the execution of a program, stored in data structures, passed as arguments to other functions, and returned as the values of other functions.
