How is the ability to run Java programs on different operating systems implemented without changes in the code?

Java programs run on various operating systems without changes to the code thanks to the use of JVM (Java Virtual Machine). Java code is compiled into bytecode — an intermediate format that is platform-independent. The JVM installed on a specific OS interprets or compiles the bytecode into machine code understandable by that system. Thus, the same program can run on Windows, Linux, macOS, and other OSes without modifications.

Example:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, world!");
    }
}

This code is compiled into bytecode and can be run on any OS with a installed JVM.