Introduction

The explosion of interest in Java over the last year has been driven largely by its röle in bringing a new generation of interactive pages to the World Wide Web. Undoubtedly various features of the language--compactness, byte-code portability, security, and so on--make it particularly attractive as an implementation language for applets embedded in Web pages. But it is clear that the ambitions of the Java development team go well beyond enhancing the functionality of HTML documents. According to [20]

``Java is designed to meet the challenges of application development in the context of heterogeneous, network-wide distributed environments. Paramount among these challenges is secure delivery of applications that consume the minimum of system resources, can run on any hardware and software platform, and can be extended dynamically.''

Several of these concerns are mirrored in developments in the High Performance Computing world over a number of years. A decade ago the focus of interest in the parallel computing community was on parallel hardware. A parallel computer was typically built from specialized processors integrated through a proprietary high-performance communication switch. If the machine also had to be programmed in a proprietary language, that was an acceptable price for the benefits of using a supercomputer. This attitude was not sustainable as one parallel architecture gave way to another, and the cost of porting software became exorbitant. For several years now, portability across platforms had been a central concern in parallel computing [4, 5, 14, 13, 23].

More fundamentally, the assumption that high performance computing will be done primarily on specialized supercomputers is questioned increasingly. Rapid strides in performance and connectivity of ordinary workstations and PCs make it look equally possible that the future of parallel computing will be on local area networks, or even the Internet [19, 17].

With Java positioned to become a standard programming language on the Internet, and scientific parallel processing edging towards network-based computation, it is natural to ask how these two technologies will interact. How suitable is Java for scientific computing, and do lessons from research in parallel computing have implications for the development of Java?

Popular acclaim aside, there are some reasons to think that Java may be a good language for scientific and parallel programming.

• Java is a descendant of C++. C and C++ are used increasingly in scientific programming; they are already used almost universally by implementers of parallel libraries and compilers. In recent years numerous variations on the theme of C++ for parallel computing have appeared. See, for example [7, 28, 11, 2, 12, 21].
• Java omits various features of C and C++ that are considered ``difficult''--notably, pointers. Poor compiler analysis has often been blamed on these features. The inference is that Java, like Fortran, may be a suitable source language for highly optimizing compilers (although direct evidence for this belief is still lacking).
• Java comes with builtin multithreading. Independent threads may be scheduled on different processors by a suitable runtime. In any case multithreading can be very convenient in explicit message-passing styles of parallel programming [24].

We will return to the question of whether parallel computing may have implications for the development of Java in section 5.

The acronym ``HPJava'' was coined in a draft white paper produced by members of the PCRC consortium in the first half of 1996 [10]. At NPAC we have been experimenting with some of the ideas put forward there.

• Overview of this article.