java中的隐式向上转换和显式向下转换

Question

当java可以隐式地进行转换时，为什么不隐式地进行转换？请用一些简单的例子来解释一下？

Answer 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!"