Abstract: "First proposed as a mechanism for enhancing Web content, Java has taken off as a serious general-purpose programming language. Industry and academia alike have expressed great interest in using Java as a programming language for scientific and engineering computations. Applications in these domains are characterized by intensive numerical computing, and often have very high performance requirements. In this paper we discuss programming techniques that lead to Java numerical codes with performance comparable to Fortran or C, the more traditional languages for this field. The techniques are centered around the use of high-performance numerical libraries, written entirely in Java, and on compiler technologies. Proper use of this package, and of other appropriate tools for compiling and running a Java application, results in code that is clean, portable, and fast. We illustrate the programming and performance issues through case studies in data mining and electromagnetism."