Alex Rivera | Logout

Why is List<Number> not a sub-type of List<Object>?

Asked 2009-08-06T17:29:37.757
9
public void wahey(List<Object> list) {}

wahey(new LinkedList<Number>());

The call to the method will not type-check. I can't even cast the parameter as follows:

wahey((List<Object>) new LinkedList<Number>());

From my research, I have gathered that the reason for not allowing this is type-safety. If we were allowed to do the above, then we could have the following:

List<Double> ld;
wahey(ld);

Inside the method wahey, we could add some Strings to the input list (as the parameter maintains a List<Object> reference). Now, after the method call, ld refers to a list with a type List<Double>, but the actual list contains some String objects!

This seems different to the normal way Java works without generics. For instance:

Object o;
Double d;
String s;

o = s;
d = (Double) o;

What we are doing here is essentially the same thing, except this will pass compile-time checks and only fail at run-time. The version with Lists won't compile.

This leads me to believe this is purely a design decision with regards to the type restrictions on generics. I was hoping to get some comments on this decision?

java generics casting covariance
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"What we are doing here is essentially the same thing, except this will pass compile-time checks and only fail at run-time. The version with Lists won't compile."

What you're observing makes perfect sense when you consider that the main purpose of Java generics is to get type incompatibilities to fail at compile time instead of run time.

From java.sun.com

Generics provides a way for you to communicate the type of a collection to the compiler, so that it can be checked. Once the compiler knows the element type of the collection, the compiler can check that you have used the collection consistently and can insert the correct casts on values being taken out of the collection.

answered 2009-08-06T19:11:14.727

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