Java Generics

[Kim Ming Yap]

Supposedly we have a class :

Node <E> {}

For subtype covariance ..

why Node<Integer> is a subtype of Node <? extends Integer> and not the other way round?

Since Node<? extends Integer> has the max upper bound of Integer so all subtypes (which is ? extends Integer> ) will always be subtype of Integer?

So confusing

[Ankit Garg]

Kim Ming Yap wrote:why Node<Integer> is a subtype of Node <? extends Integer> and not the other way round?

I think you are talking about this

List<? extends Integer> wildcardList = null;
List<Integer> integerList = wildcardList;  //error

List<Integer> integerList2 = null;
List<? extends Integer> wildcardList2 = integerList2;  //OK

The first one doesn't compile because there is a chance that type safety is broken by that assignment. Let's take a different example to understand this as Integer is a final class so cannot be sub classed

List<? extends InputStream> wildcardList = null;
List<InputStream> integerList = wildcardList;      //error

The InputStream class has many sub-classes. Let's consider FileInputStream and PipedInputStream. Now if my code is like this

List<FileInputStream> fisList = new ArrayList<FileInputStream>();
fisList.add(new FileInputStream("file.txt"));
List<? extends InputStream> wildcardList = fisList;
List<InputStream> inputStreamList = wildcardList;    //error

Now if the 4th line in the above code was allowed, then it can break type safety as follows

List<FileInputStream> fisList = new ArrayList<FileInputStream>();
fisList.add(new FileInputStream("file.txt"));
List<? extends InputStream> wildcardList = fisList;
List<InputStream> inputStreamList = wildcardList;  //if there was no error here
inputStreamList.add(new PipedInputStream());       //Oops, I added a PipedInputStream to a list originally of type FileInputStream
                                                   //as actually inputStreamList points to a list of type <FileInputStream>
FileInputStream fis = fisList.get(0);              //ClassCastException, that breaks type safety

I hope you understand it now...