Generic Method, How parameter type is determined

Question

For method
<T> void fromArrayToCollection(T[] a, Collection<T> c) { ... } Why does
fromArrayToCollection(sa, co);passes and
fromArrayToCollection(oa, cs); fails.
oa - Object Array, Object[]
cs - Collection of String, Collection<String>
sa - String Array, String[]
co - Collection of Object, Collection<Object>
What are the rules governing the type of T inferred by compiler?

Answer 1

epiphanetic wrote:
I think you still haven't fully understood the issue.
I suggest, you also read the same generics tutorial by Gilad Bracha, section 6 from where I found this issue :). Ha! But I think it's misleading that that section uses arrays.
In his words "It will generally infer the most specific type argument that will make the call type-correct." Its also mentioned that collection parameter type has to be supertype of Array parameter type but no reason is given. I wonder why it fails to infer correct type in second case.Assume you passed in an array of Objects, and a Collection of Strings, and it was possible that T would then be Object. Using Bracha's example implementation:
static <T> void fromArrayToCollection(T[] a, Collection<T> c) {
   for (T o : a) {
      c.add(o); // correct
}Now imagine you had this code making use of it:
Object[] objects = {Integer.valueOf(1), "hi", new Object()};
Collection<String> strings = new LinkedList<String>();
fromArrayToCollection(objects, strings);
String string = strings.iterator().next(); //get first String, which is actually an IntegerTrying to get the first String would give a ClassCastException. So clearly that method cannot not be safely invoked.
The reason I think he's confusing things by using the array is because you might get it in your head that this would be OK:
static <T> void fromCollectionToCollection(Collection<T> one, Collection<T> two) {
   for ( T t : one ) {
      two.add(t);
Collection<Object> col1; Collection<String> col2;
doSomething(col1, col2);When clearly it's unsafe, as now your Collection of Strings might have a non-String in it! That's why I said this is more a nuance of generic arrays than of type inference proper.