In a parallel language, it is desirable to have both local variables (like the ones in MPI programming) and global variables (like the ones in HPF programming). The former provide flexibility and are ideal for task parallel programming; the latter are convenient especially for data parallel programming.
In HPJava, variable names are divided into two sets. In general those declared using ordinary Java syntax represent local variables and those declared with [[ ]] represent global variables. The two sectors are independent. In the implementation of HPJava the global variables have special data descriptors associated with them, defining how their components are divided or replicated across processes. The significance of the data descriptor is most obvious when dealing with procedure calls.
Passing array sections to procedure calls is an important component in the array processing facilities of Fortran90 . The data descriptor of Fortran90 will include stride information for each array dimension. One can assume that HPF needs a much more complex kind of data descriptor to allow passing distributed arrays across procedure boundaries. In either case the descriptor is not visible to the programmer. Java has a more explicit data descriptor concept; its arrays are considered as objects, with, for example, a publicly accessible length field. In HPJava, the data descriptors for global data are similar to those used in HPF, but more explicitly exposed to programmers. Inquiry functions such as grp(), rng() have a similar role in global data to the field length in an ordinary Java array.
Keeping two data sectors seems to complicate the language and its syntax. But it provides convenience for both task and data parallel processing. There is no need for things like the LOCAL mechanism in HPF to call a local procedure on the node processor. The descriptors for ordinary Java variables are unchanged in HPJava. On each node processor ordinary Java data will be used as local varables, like in an MPI program.