What is an unbounded wildcard (List<?>), and when is it more appropriate to use than a bounded generic method with a type parameter <T>?

List<?> ('list of unknown type') represents a list of SOME specific but unknown type, useful when writing a method that operates on the list generically without needing to know or use its specific element type (like printing every element via toString(), or checking the list's size) -- when the method DOES need to establish a relationship between the wildcard type and something else (like ensuring two parameters share the same element type, or returning a value dependent on the element type), a generic type parameter <T> is required instead, since a wildcard's specific captured type can't be referenced or related to anything else within the method's own signature.

// Unbounded wildcard: fine when the method genuinely doesn't care about the specific type
public static void printAll(List<?> list) {
    for (Object item : list) { System.out.println(item); }  // only Object-level operations needed
}

// Generic type parameter: needed when relating type across parameters/return value
public static <T> void copyFirst(List<T> source, List<T> destination) {
    destination.add(source.get(0));  // T ties source and destination together meaningfully
}