Java generics list

Question

I know following is true.

List<? extends Number> aNumberSuperList = new ArrayList<>();
List<? extends Integer> aIntegerSuperList = new ArrayList<>();
aNumberSuperList = aIntegerSuperList;

But what type of objects can be added to such a list.

List<? extends Number> aNumberSuperList2 = new ArrayList<>();
aNumberSuperList2.add(???)

Integer,Double ,... they have same super class that is Number. — Fevly Pallar, Nov 18 '14 at 18:40
Nothing can be added without casting, because the generic type is unknown. — khelwood, Nov 18 '14 at 18:41
You have a syntax error in the second block of code: `new ArrayList<>();`, with the final `()`. — Stefano Sanfilippo, Nov 18 '14 at 18:42
The <> is the Java 8 diamond operator, to avoid the redundancy by typing it twice. — Stefan, Nov 18 '14 at 18:49
@Stefan The diamond operator was introduced in Java 7, not 8. — Dawood ibn Kareem, Nov 18 '14 at 19:20

score 7 · Accepted Answer · answered Nov 18 '14 at 18:44

Only null can be added to a List<? extends Number>. This is because the exact type parameter isn't known. A List<Integer> could be assigned to this list, and so could a List<AtomicLong>. The compiler must prevent a call to add with anything but null, to avoid the situation where a Integer might be added to something that's referred to as a List<? extends Number>, but is really a List<Double>, for example. null is the only safe thing to add here, because it can be of any type.

Java generics list

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