Can you find all classes in a package using reflection?

Question

Is it possible to find all classes or interfaces in a given package? (Quickly looking at e.g. Package, it would seem like no.)

Answer 1

The most robust mechanism for listing all classes in a given package is currently ClassGraph, because it handles the widest possible array of classpath specification mechanisms, including the new JPMS module system. (I am the author.)

List classNames = new ArrayList<>();
try (ScanResult scanResult = new ClassGraph().acceptPackages("my.package")
        .enableClassInfo().scan()) {
    classNames.addAll(scanResult.getAllClasses().getNames());
}