HPJava adds to Java a concept of multi-dimensional arrays called ``multiarrays'' (consistent with proposals of the Java Grande Forum). To support parallel programming, these multiarrays are extended to ``distributed arrays'', very closely modelled on the arrays of High Performance Fortran. The new distributed data structures are cleanly integrated into the syntax of the language (in a way that doesn't interfere with the existing syntax and semantics of Java--for example ordinary Java arrays are left unaffected).
In the current implementation, the source HPJava program is translated to an intermediate standard Java file. The preprocessor that performs this task is reasonably sophisticated. For example it performs a complete static semantic check of the source program, following rules that include all the static rules of the Java Language Specification . So it shouldn't normally happen that a program accepted by the HPJava preprocessor would be rejected by the backend Java compiler. The translation scheme depends on type information, so we were essentially forced to do a complete type analysis for HPJava (which is a superset of standard Java). Moreover we wanted to produce a practical tool, and we felt users would not accept a simpler preprocessor that did not do full checking.
The current version of the preprocessor also works hard to preserve line-numbering in the conversion from HPJava to Java. This means that the line numbers in run-time exception messages accurately refer back to the HPJava source. Clearly this is very important for easy debugging.
A translated and compiled HPJava program is a standard Java class file, ready for execution on a distributed collection of JIT-enabled Java Virtual Machines. All externally visible attributes of an HPJava class--e.g. existence of distributed-array-valued fields or method arguments--can be transparently reconstructed from Java signatures stored in the class file. This makes it possible to build libraries operating on distributed arrays, while maintaining the usual portability and compatibility features of Java. The libraries themselves can be implemented in HPJava, or in standard Java, or as JNI interfaces to other languages. The HPJava language specification documents the mapping between distributed arrays and the standard-Java components they translate to.
Currently HPJava is supplied with one library for parallel computing--a Java version of the Adlib library of collective operations on distributed arrays . A version of the mpiJava  binding of MPI can also be called directly from HPJava programs. Of course we would hope to see other libraries made available in the future.