Upcasting (from a subtype to a supertype) is always safe, so no explicit cast is needed.
Downcasting (from a supertype to a subtype) has the potential to be incorrect, so an explicit cast is required.
For example, consider... In this case, a Cat is always an Animal. So if you have a reference of type Cat, it's always safe to upcast that reference to type Animal. There's no way to get into trouble, so an explicit cast is not needed.
On the other hand, an Animal is not always a Cat. (It might be a Goat, for example.) So if you have a reference of type Animal, it's not always safe to downcast that that reference to type Cat, so an explicit cast is needed.
The cast is a way for the programmer to tell the compiler, "Trust me, I know what I'm doing, and you can treat this instance as a Cat." Now, if the programmer turns out to be wrong and the instance is not really a Cat, then the code will compile (because of the explicit cast), but a ClassCastException will be thrown at runtime.
Note that because Cat extends Animal, it's perfectly reasonable that a variable of type Animal might reference an instance of Cat. So when the programmer uses an explict cast to say, "Trust me...," the compiler says, "Okay, that's reasonable, so I guess you know what you're doing." On the other hand, if the types were unrelated, the compiler would say, "Sorry, I know you're the programmer, but I just can't believe that a variable of type Animal actually references an instance of ToyBoat." So if there is no relationship, then the code will not compile even with an explicit cast.
It's important to keep in mind that casting only changes what type the reference is treated as. It does not change the instance itself. The true type of an instance is determined by the constructor that's called with "new." For example, if you call "new Animal," then the instance -- the object created -- is an Animal. You cannot turn this into a Cat by downcasting it. [ December 18, 2006: Message edited by: marc weber ]
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subject: didn't understant class castings between parent and child (all cases)