This is an interesting reading from Wikipedia.

“Write once, run anywhere” (WORA), or sometimes write once, run everywhere (WORE), is a slogan created by Sun Microsystems to illustrate the cross-platform benefits of the Java language. Ideally, this means Java can be developed on any device, compiled into a standard bytecode and be expected to run on any device equipped with a Java virtual machine (JVM). The installation of a JVM or Java interpreter on chips, devices or software packages has become an industry standard practice.

This means a programmer can develop code on a PC and can expect it to run on Java enabled cell phones, as well as on routers and mainframes equipped with Java, without any adjustments. This is intended to save software developers the effort of writing a different version of their software for each platform or operating system they intend to deploy on.

This idea originated as early as in the late 1970s, when the UCSD Pascal system was developed to produce and interpret p-code. UCSD Pascal (along with the Smalltalk virtual machine) was a key influence on the design of the Java virtual machine, as is cited by James Gosling.

The catch is that since there are multiple JVM implementations, on top of a wide variety of different operating systems such as Windows, Linux, Solaris, NetWare, HP-UX, and Mac OS, there can be subtle differences in how a program may execute on each JVM/OS combination, which may require an application to be tested on various target platforms. This has given rise to a joke among Java developers, “Write Once, Debug Everywhere”.

I have tested a simple compiled Java code (.class) on 64-bit Windows 7 platform and copied the file to a Debian ARMv6 (Raspberry Pi) platform. The compiled Java class was unable to run on the ARMv6. It, however, was all right on 64-bit Ubuntu. A better way to handle it probably is to transfer all the source files to the specific platform and compile there. The Raspberry Pi platform is not good enough for a normal Java IDE. You probably have to compile the source code using command lines (still remember how to use javac?).

In comparison, the Squeak Smalltalk programming language and environment, boasts as being, “truly write once run anywhere”, because it “runs bit-identical images across its wide portability base”