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Kawa: Compiling Scheme to Java

1.2 Objects and Values

Java J. Gosling, B. Joy, G.~Steele: The Java Language Specification, Addison-Wesley, 1996 has primitive types (such as 32-bit int) as well reference types. If a variable has a reference type, it means that it can contains references (essentially pointers) to objects of a class, or it can contain references to objects of classes that "extend" (inherit from) the named class. The inheritance graph is "rooted" (like Smalltalk and unlike C++); this means that all classes inherit from a distinguished class java.lang.Object (or just Object for short).

Standard Scheme W. Clinger and J. Rees (editors): Revised 4 report on the algorithmic language Scheme. LISP Pointers, 4(3):1--55, 1991. has a fixed set of types, with no way of creating new types. It has run-time typing, which means that types are not declared, and a variable can contain values of different types at different times. The most natural type of a Java variable that can contain any Scheme value is therefore Object, and all Scheme values must be implemented using some class that inherits from Object.

The task then is to map each Scheme type into a Java class. Whether to use a standard Java class, or to write our own is a tradeoff. Using standard Java classes simplifies the passing of values between Scheme functions and existing Java methods. On the other hand, even when Java has suitable built-in classes, they usually lack functionality needed for Scheme, or are not organized in any kind of class hierarchy as in Smalltalk or Dylan. Since Java lacks standard classes corresponding to pairs, symbols, or procedures, we have to write some new classes, so we might as well write new classes whenever the existing classes lack functionality. One extreme would be to define a new SchemeObject class, and derive from it classes for all Scheme values. While this might make implementing Scheme easier, Kawa does not go that far, partly because we want to allow Scheme variables to contain arbitrary Java objects.

The Scheme boolean type is one where we use a standard Java type, in this case Boolean (strictly speaking java.lang.Boolean). The Scheme constants `#f' and `#t' are mapped into static fields (i.e. constants) Scheme.falseObject and Scheme.trueObject. These are initialized to new Boolean(false) and new Boolean(true), respectively, when Kawa starts up.

On the other hand, numbers and collections are reasonably organized into class hierarchies, which Java does not do well. So Kawa has its own classes for those, discussed in the following sections.