Why is an interpreted language considered more portable?

Question

Java is often cited as being more portable than other, say compiled, languages as the executable can be run on any platform with a JVM. But code written in C can be run on any platform with a C compiler.

So, naively, there are two alternatives: make lots of different compilers for lots of different platforms and transfer source code over a network for say an applet, which is compiled client-side; or make lots of different virtual machines to run on lots of different platforms and transfer the same, executable program or applets over networks.

Why is the latter better? I can see how server-side compilation is desirable, but I feel there is more to it than this. I can appreciate that it was less work for Sun Microsystems to create JVMs for many platforms than compilers for many platforms, but this surely wasn't the major motivation.

Answer 1

The perhaps most authoritative answer to why Java was designed to be interpreted may be found in the whitepaper that announced the Java language back in 1995:

1.2.3 Architecture Neutral and Portable

Java technology is designed to support applications that will be deployed into heterogeneous network environments. In such environments, applications must be capable of executing on a variety of hardware architectures. Within this variety of hardware platforms, applications must execute atop a variety of operating systems and interoperate with multiple programming language interfaces. To accommodate the diversity of operating environments, the Java Compiler TM product generates bytecodes--an architecture neutral intermediate format designed to transport code efficiently to multiple hardware and software platforms. The interpreted nature of Java technology solves both the binary distribution problem and the version problem; the same Java programming language byte codes will run on any platform.

Architecture neutrality is just one part of a truly portable system. Java technology takes portability a stage further by being strict in its definition of the basic language. Java technology puts a stake in the ground and specifies the sizes of its basic data types and the behavior of its arithmetic operators. Your programs are the same on every platform--there are no data type incompatibilities across hardware and software architectures.

and

1.2.5 Interpreted, Threaded, and Dynamic

The Java interpreter can execute Java bytecodes directly on any machine to which the interpreter and run-time system have been ported. In an interpreted platform such as Java technology-based system, the link phase of a program is simple, incremental, and lightweight. You benefit from much faster development cycles--prototyping, experimentation, and rapid development are the normal case, versus the traditional heavyweight compile, link, and test cycles.

It is also worth mentioning that the Java API goes far beyond standard libraries for C or C++.

Note that this perspective is somewhat dated. While largely still accurate, a modern take on the trade-off between a priori and runtime compilation whould include the additional optimization oppurtunities afforded by execution time statistics, and probably avoid the use of the word "interpreted" altogether - at least if we are somewhat serious about performance.

Answer 2

But code written in C can be run on any platform with a C compiler.

Not in the same way. You either need to compile it on that machine with that specific compiler, or need a compiler that is capable of cross-compiling. Either way, you have a bigger workload.

Still, there is some C Code that is quite portable. A simple program that just calculates basic arithmetic is quite portable, even in C, if you are willing to compile it to different platforms.

The second big important difference is the platform. As soon as you do I/O or use syscalls, your code becomes platform-specific, just because you need to directly interface with the host system. An interpreted language offers a unified platform. If my programs runs on the JVM, it just runs on it, no matter which system is host to the JVM. If I use "native" calls to the host OS, I have to use the proper ones for each OS - but with Java, my "OS" is the JVM.

Btw, there is so called "portable" C/C++ code, but it also hinges on similar concepts as the JVM. If you use Qt and similar libraries that offer uniform APIs on multiple platforms, then you can create quite portable C/C++ programs.