What is necessary for a program written in Java to work?

To run a Java program, the following are required:

1. JRE (Java Runtime Environment): An environment for executing Java code, containing the JVM (Java Virtual Machine) and standard class libraries. It is the JVM that executes the compiled bytecode.
2. Compiled bytecode: The source .java file must be compiled into .class files using the javac compiler (part of the JDK). The JVM executes the bytecode, not the source code.
3. Main class with a main method: The compiled bytecode must include a class containing a static main method with the signature public static void main(String[] args). The JVM starts executing the program from this method.

For example, to run a compiled class MyProgram:

java MyProgram

The JVM will find and execute the main method in the MyProgram class.