问题
When java can implicitly do up casting , why does not it implicitly do down casting ?Please explain with some simple example?
回答1:
The point is that upcasting will always succeed, so it's safe - whereas downcasting can fail:
String x = getStringFromSomewhere();
Object y = x; // This will *always* work
But:
Object x = getObjectFromSomewhere();
String y = (String) x; // This might fail with an exception
Because it's a "dangerous" operation, the language forces you to do it explicitly - you're basically saying to the compiler "I know more than you do at this point!"
来源：https://stackoverflow.com/questions/23042236/implicit-upcasting-and-explicit-downcasting-in-java