Enthuware

Hi Admin

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HashMap<?, List<String>> box = new HashMap<?, List<String>>();

You cannot have a '?' on the right hand side. I.e. you must provide a type name while instantiating a typed class.

This compiles:
I bring this up only because otherwise one might be tempted to read your statement as "any appearance of ? on the RHS of = is illegal syntax and will not compile."

Thanks.

I'm looking for some clarification on this, because I am someone who DID (briefly) interpret the statement as "any appearance of ? on the RHS of = is illegal syntax and will not compile." and I'm trying to make sure I understand correctly.

Question: Would it be correct to say the following:

When instantiating a generic type, you must specify a class name. However, if that class name is itself a generic type, this generic type itself may use a wildcard in its declaration.

This means: is invalid, because the instantiation of new HashMap must specify a type for its key.

But it also means: is valid, because in this context the HashMap is instantiated to map Integer to List of 'unknown' type. This works because although HashMap is being instantiated, neither Integer nor List<?> are being instantiated at this time, they are placeholders.

Is this a correct summary of the pertinent rules?

Thank you for your time in responding!

Yes, you are right. The actual instantiation requires a type.