It’s the realization of cross-platform code that works the same across any system you run it on. Java’s approach to achieving this feat is counterintuitive at first. In a way, Java isn’t compatible with anything but one computer. Stranger still, this computer doesn’t actually exist. The computer that Java code targets is the Java Virtual Machine (JVM). This is a program written by Java’s creators and distributed for practically any computing device you can think of. As long as you have it installed, any Java code you run is handled by this “imaginary” computer living inside your computer. Java code is executed by the JVM, which sends appropriate platform-specific instructions to your computer, so everything works the same on every OS and architecture.
Of course, the method used by Java isn’t really the selling point here. Most users and many developers don’t care how software compatibility is achieved, only that it happens. Many languages promise cross-platform functionality, and usually, that promise is ultimately true, but the journey isn’t always easy. Programming languages must be compiled for their target platforms, scripting languages require platform-specific interpreters, and it’s rare that either can ensure consistent access to low-level system resources. Cross-platform support is getting better and better, with libraries to help with translating paths and environment variables and settings, and some frameworks (notably Qt) do much to bridge the gap for peripheral access. But Java has it and delivers it consistently and reliably.
