When Sun Microsystems introduced Java in 1995, it promoted the language with the slogan “Write Once, Run Anywhere,” often shortened to WORA. The phrase summarized Java’s central selling point: a program written and compiled one time should run unchanged on any computer that provides a Java runtime.
The substance behind the slogan is documented in Oracle’s own Java materials. The Java Virtual Machine Specification states that “programmers can write a program once, and it will run on any machine supplying a Java run-time environment,” and that the virtual machine “does not assume any particular implementation technology, host hardware, or host operating system.”
The mechanism is bytecode. Oracle’s Java tutorial explains that source code is compiled into class files containing bytecode rather than processor-native code, and that because the Java Virtual Machine runs on many systems, “the same .class files are capable of running on Microsoft Windows, the Solaris Operating System, Linux, or Mac OS.” Compile to bytecode once, and the same file runs everywhere a JVM exists.
In practice, subtle differences between runtime versions and operating systems meant portability was never perfect, which is why programmers sometimes joked about “write once, debug anywhere.” Even so, WORA captured a genuine shift: Java made cross-platform software far more practical than it had been when each program had to be rebuilt for each kind of machine.