Generics <? super A> doesn't allow superTypes of A to be added to the list
I am using wildcard and lower bound generics on list and yet compiler is throwing error.
Code:
int intStart = 0;
Number num = new Integer(2);
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With
List, I should have been allowed to add any objects of type Integer or its superTypes e.g. Number or Object.That's incorrect.
Listdoesn't mean a list that can hold any supertype of Integer. It means a list whose generic type may be some specific supertype of Integer. So it might actually beList,ListorList. All of those list types may contain anInteger, but the same can't be said forNumber. ANumbermight be aDoubleor aLong, which isn't allowed in aList.To make it a little more concrete, consider this snippet:
Long longValue = 2L; ListlistOfIntegers = new ArrayList<>(); // listOfIntegers.add(longValue); The commented line obviously shouldn't compile, and doesn't. But what if we added this:
Number longAsNumber = longValue; List listOfSuper = listOfIntegers; listOfSuper.add(longAsNumber);This is equivalent to the snippet in your question. Should the final line compile? If it would, it would violate the generic invariant of
List.
In response to your comment,
Listis indeed unnecessary in your example. A more common case would be to add flexibility to a method parameter. For example, a methodvoid addInt(Listlist) { list.add(1); } would only be able to accept
List, which would be unnecessarily restrictive. Changing the parameter type toListwould allow the method to acceptListas well asList, either of which can contain anIntegervalue.This only works if the list is consuming the generic type in question. If the method tried to produce values from the list, it would be forced to assume they're of type
Object, since we don't have a definite supertype. For more information, see What is PECS (Producer Extends Consumer Super)?
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