CSG17 Chapter 9 Unbounded Wildcards question

[Ondrej Ducy]

Hi there,

in CSG17 Ch9 there is a code sample:

List<?> x1 = new ArrayList<>();
var x2 = new ArrayList<>();

Book says that both x1 and x2 returns Object when get() method is called.
I want to test it so I am trying to fill it first.
f.e. this code is just fine:

x2.add(new Object());

but when I try this:

x1.add(new Object());

I get an error:
Generics.java:30: error: no suitable method found for add(Object)  x1.add(new Object());    ^    method List.add(CAP#1) is not applicable      (argument mismatch; Object cannot be converted to CAP#1)    method List.add(int,CAP#1) is not applicable      (actual and formal argument lists differ in length)  where CAP#1 is a fresh type-variable:    CAP#1 extends Object from capture of ? 1 error error: compilation failed
So my question is: Why am I not able to fill the first list?
Thanks

[Ondrej Ducy]

Ok, I have found answer in next section. It seems that unbound and upperbound wildcards creates logically immutable lists.
Sorry for spam.

[Stephan van Hulst]

Consider this:

List<?> x1 = new ArrayList<Tiger>(); // this is legal

Object object = new Elephant();      // this is legal

x1.add(object);                      // this is NOT legal

If you could add an Object to a List<?>, it would also allow you to add an Elephant to a List<Tiger>, which obviously isn't a good idea.

[Stephan van Hulst]

Ondrej Ducy wrote:Ok, I have found answer in next section. It seems that unbound and upperbound wildcards creates logically immutable lists.
Sorry for spam.

This is not exactly true.

The list itself is not immutable. You just can't add anything to it using that particular variable. If you have a different reference to the same list, it might be possible to modify the list through that reference.

[Ondrej Ducy]

Stephan van Hulst wrote:If you could add an Object to a List<?>, it would also allow you to add an Elephant to a List<Tiger>, which obviously isn't a good idea.

It was not obvious to me at first time, but when I read that in next section of book, it made sense.
Thanks for help. ;-)

[Ondrej Ducy]

Stephan van Hulst wrote: If you have a different reference to the same list, it might be possible to modify the list through that reference.

Ok, thats interesting, but how would I get reference to that list?
I tried this and it doesnt work:

  List<Object> y1 = x1;
  y1.add(new Object());

It seems to me that only possible soulution would be to have a reference to the list before assigning to x1 variable.
Or is there another way?

[Stephan van Hulst]

Ondrej Ducy wrote:It seems to me that only possible soulution would be to have a reference to the list before assigning to x1 variable.
Or is there another way?

No, that's exactly right:

var objects = new List<Object>();

List<?> x1 = objects;

[Ondrej Ducy]

Stephan van Hulst wrote:

Ondrej Ducy wrote:It seems to me that only possible soulution would be to have a reference to the list before assigning to x1 variable.
Or is there another way?

No, that's exactly right:

var objects = new List<Object>();

List<?> x1 = objects;

Ok, now its clear to me. Thank you!