What must every Java program contain?

Every Java program must contain a main method because this method serves as the entry point for the Java Virtual Machine (JVM) to start executing the program. The declaration of the main method is specified with the exact signature: public static void main(String[] args). Without this method, the JVM would not know where to begin, and as a result, the program would not run.

While public classes, methods with specific names, or interfaces may have their use in various Java applications, they are not universally required in every Java program. For example, even if a class is not public, the program can still have a main method. Therefore, the presence of the main method is essential for any standalone Java application.