A question on Generics

[Suman Sarker]

Hi,
Can anyone please explain why the following code works?

ArrayList<? super IOException> exceptions = new ArrayList<IOException>();
FileNotFoundException e1 = new FileNotFoundException("FNF");
EOFException e2 = new EOFException("EOF");
exceptions.add(e1);
exceptions.add(e2);

Something strange is going on here. I don't really understand how FileNotFoundException or EOFException can be added to the list even though they are not super type of IOException.

I appreciate your help.

[Dieter Quickfend]

the ArrayList reference can take any generic type that is a supertype of IOException.

So the reference could be an ArrayList<Exception>, and it could be an ArrayList<IOException>, and it could be an ArrayList<Object>

So if you assume that the Generic type of the ArrayList MUST be IOException or a supertype of IOException...
which types is this ArrayList guaranteed to be able to take as parameters?

[Suman Sarker]

Dieter Quickfend wrote:So if you assume that the Generic type of the ArrayList MUST be IOException or a supertype of IOException...
which types is this ArrayList guaranteed to be able to take as parameters?

Thanks for your reply. I would expect it to take an Exception or even an Object as its parameters but not somthing that's subtype of IOException. I thought the super wildcard is there to impose a restriction on the list to not allow any sub types of IOException. FileNotFoundException and EOFException are subtypes of IOException. Aren't they?

For example, the following code doesn't work and I cmpletely understand why it doesn't.

List<? super Integer> ints = new ArrayList<Integer>();
Float f = 1.0f;
Double d = 2.0;
Long l = 3l;

ints.add(f);
ints.add(d);
ints.add(l);

So, I still fail to understand why the code in my first post works.

[Dieter Quickfend]

The super wildcard is there to guarantee that you do not pass a List with a Generic type that is a subtype of IOException.


Which means you could not do this:

List<? super IOException> arrayList = new ArrayList<FileNotFoundException>();

this will not work.
Now, this means the compiler just made sure that the generic type of the list you're passing is IOException or a superclass of IOException
And since we know that we can assign any object to a reference of its supertype in Java (for instance, a List<Exception> can take as a parameter in its add() method an Exception or any of its subclasses)...
What did the compiler just make sure?

[Suman Sarker]

Got ya. Thanks once again for your quick reply.

[Dieter Quickfend]

Got another one for you;

What could you pass to the List if the reference type was "List<? extends IOException>"?

[Suman Sarker]

Dieter Quickfend wrote:Got another one for you;

What could you pass to the List if the reference type was "List<? extends IOException>"?

A List with any class that that extends IOException?

For example, ArrayList<FileNotFoundException> should work this time.

[Dieter Quickfend]

And what could you add to that list?

[Suman Sarker]

Dieter Quickfend wrote:And what could you add to that list?

I don't think extends wildcard would let you add "anything" to the list. That's something I already know ;)

[Dieter Quickfend]

Gotta love that null reference

Good luck on the exam!

[Abimaran Kugathasan]

Good help Dieter!

[Suman Sarker]

Thanks