Regarding upcasting and downcasting in java im trying to come

Question

Regarding upcasting and downcasting in java im trying to come

up with some examples & causes of different kinds of errors regarding casting and polymorphism

given B extends A

((B) new A())
will cause a runtime error, right?

and say C doesn't extend anything
(C(new A()))
B ba = new A()
will cause a compile time error

so
are there any more cases like this im not thinking about that would result in compile/runtime errors?

also if u can provide an intuition behind those that'd be appreciated thanks

0

20.02.2022

7 ответов

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dankey Автор вопроса

isn't there a scenerio in which this'll cause an error?

0

20.02.2022

Nader Jafari

dankey
isn't there a scenerio in which this'll cause an e...

If B didn't extend A Java would've thrown a ClassCastException which is a RuntimeException

0

20.02.2022

dankey Автор вопроса

Nader Jafari
If B didn't extend A Java would've thrown a ClassC...

i was sure im missing something. sure there aren't any small annoyances regarding casting in between classes with is a relation between them?

0

20.02.2022

Nader Jafari

dankey
i was sure im missing something. sure there aren'...

I don't think so. Primitives are all numeral types so it's just a matter of widening and narrowing. As for references, classes with parent-(grand)child relations can be casted, you lose fields and methods upcasting but since children can't make inherited fields and methods less visible it just can't go wrong. The only slightly problematic one is downcasting which you can prevent at compile time using instanceof

0

20.02.2022

dankey Автор вопроса

Nader Jafari
I don't think so. Primitives are all numeral types...

umm can you give an example for such problematic downcasting?

0

20.02.2022

Nader Jafari

Nader Jafari
If B didn't extend A Java would've thrown a ClassC...

.

0

20.02.2022