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David Pollard
4,324 PointsJava - Typecasting. Counterintuitive that upcasting is always legal ?
Craig mentioned in his excellent video that upcasting an object class is always legal. But, I'm struggling to wrap my head around how that can be ?
A child class may (probably) have certain properties that the parent doesn't. Using the infamous bicycle/mountain bike (parent/child) class analogy. A mountain bike may have a property of seat height.
If it gets upcasted, what happens to seat height, does it just get lost ?
Conversely, downcasting should always be possible as mountain bike would have ALL the properties of its parent "bicycle".
Can anyone help me get my head around why that upcasting is ALWAYS legal, and downcasting isn't. It seems it should be the other way around !?
Thank you
dp
2 Answers
Steven Parker
231,269 PointsYou're right, any specialized properties would get lost. If you need to access them later, you could just temporarily cast the value in an expression but retain the original reference.
On the other hand, casting a "bicycle" as a "mountain bike" could be a problem as it would then be expected to have a "seat height" property and of course it does not.
Does it make more sense now?
David Pollard
4,324 PointsThat's brilliant Steven.
Yes, makes perfect sense now, thank you !
:-)
dp