What mechanisms in Java ensure the ability to write code that works on different operating systems?

Java ensures code portability across operating systems through the following mechanisms:

• Java Virtual Machine (JVM) — abstracts the hardware and system platform, allowing the same bytecode to run on different OSes.
• Java Standard Library (Java API) — provides a unified interface for working with files, networks, streams, and other resources, hiding OS differences.
• Absence of direct system calls — code interacts with the OS through JVM, ensuring independence.

Example: a program written in Java can run on Windows, Linux, and macOS without changes to the source code thanks to JVM and standard APIs.