What are first-class objects in Java and C#?
When I started OO programming many years ago I gained the impression that variables (if that is the right word) were either "primitives" (int, double, etc.) or first-class objects (String, JPane, etc.). This is reinforced by a recent answer on primitives in Java and C# (@Daniel Pryden: Are primitive types different in Java and C#?). However don't know whether C# ValueTypes are primitives, objects or some other beast such as second-class objects. I see that SO has only one use of the first-class tag so maybe it is no longer a useful term.
I did not find the Wikipedia article useful ("This article is in need of attention from an expert on the subject."). I'd be grateful for a taxonomy and current usage of terms, primarily related to Java and C# (though maybe other languages will shed enlightenment).
Clarification: I'd like to understand the term first-class and what its range of use is.
Answer
The notion of "first-class citizen" or "first-class element" in a programming language was introduced by British computer scientist Christopher Strachey in the 1960s in the context of first-class functions. The most famous formulation of this principle is probably in Structure and Interpretation of Computer Programs by Gerald Jay Sussman and Harry Abelson:
- They may be named by variables.
- They may be passed as arguments to procedures.
- They may be returned as the results of procedures.
- They may be included in data structures.
Basically, it means that you can do with this programming language element everything that you can do with all other elements in the programming language.