For the following code sample:
public static class Abc<X> { }
public static class Def<Y> { }
public static class Ghi<Z> { }
public void doThis() {
List<?> listOne;
List<Abc<?>> listTwo;
List<Abc<Def<?>>> listThree;
List<Abc<Def<Ghi<?>>>> listFour;
List<Abc<Def<Ghi<String>>>> listFive;
Abc<Def<Ghi<String>>> abcdef;
abcdef = new Abc<Def<Ghi<String>>>();
listOne.add(abcdef); // line 1
listTwo.add(abcdef); // line 2
listThree.add(abcdef); // line 3
listFour.add(abcdef); // line 4
listFive.add(abcdef); // line 5
}
Lines 1, 3, and 4 do not compile:
(line 1)
The method add(capture#1-of ?) in the type List<capture#1-of ?> is not applicable for the arguments (Abc<Def<Ghi<String>>>)
(line 3)
The method add(Abc<Def<?>>) in the type List<Abc<Def<?>>> is not applicable for the arguments (Abc<Def<Ghi<String>>>)
(line 4)
The method add(Abc<Def<Ghi<?>>>) in the type List<Abc<Def<Ghi<?>>>> is not applicable for the arguments (Abc<Def<Ghi<String>>>)
Lines 2 and 5, however, compile.
Could anyone explain why lines 1, 3, and 4 are not legal assignments? And if wildcard parameters cannot be used in that way on those lines, then why is the assignment on line 2 legal?
null
. – Reuter