unbounded-wildcard

I am mainly a C# developer and I was teaching Data Structures to my friend and they use Java in their University and I saw such an expression in Java: void printCollection(Collection<?> c) { for (Object e : c) { System.out.println(e); } } I haven't seen such a thing in C# so I wonder what's the difference between Collection<T> and Collection<?> in Java? void printCollection(Collection<T> c) { for (Object e : c) { System.out.println(e); } } I think it could have been written in the way above too. The guy in the documentation was comparing Collection<Object> and Collection<T> though. Examples

A raw list converts to List<?> just fine. Why can't a list of raw lists convert to a list of List<?> ? { // works List raw = null; List<?> wild = raw; } { // Type mismatch: cannot convert from List<List> to List<List<?>> List<List> raw = null; List<List<?>> wild = raw; } Backstory (to mitigate the xy problem ): An API I'm using returns List<JAXBElement> . I happen to know that it is always List<JAXBElement<String>> . I plan to loop and build my own List<String> , but I was trying to fix (but not suppress) the raw type compiler warning when I write List<JAXBElement> raw = api(); . I tried: List

How come one must use the generic type Map<?, ? extends List<?>> instead of a simpler Map<?, List<?>> for the following test() method? public static void main(String[] args) { Map<Integer, List<String>> mappy = new HashMap<Integer, List<String>>(); test(mappy); } public static void test(Map<?, ? extends List<?>> m) {} // Doesn't compile // public static void test(Map<?, List<?>> m) {} Noting that the following works, and that the three methods have the same erased type anyways. public static <E> void test(Map<?, List<E>> m) {} Radiodef Fundamentally, List<List<?>> and List<? extends List<?>>

I was reading about generics and I did not understand the need for unbound wildcards and how it differs from raw type. I read this question but still did not get it clearly. In the Java tutorial page for unbound wildcard I got below two points and I did not understood first point: If you are writing a method that can be implemented using functionality provided in the Object class. When the code is using methods in the generic class that don't depend on the type parameter. For example, List.size() or List.clear() . In fact, Class<?> is so often used because most of the methods in Class<T> do