Why in Java incompatibility of types to casting is checked at runtime and type mismatch to converting at compile time?

Question

Let's say there are 2 classes. And Child extends Parent.

public class Parent {}

and

public class Child extends Parent {}

I know that the following code is incorrect:

Child obj = new Parent(); // causes java.lang.Error

or

Child obj = (Child) new Parent(); // causes java.lang.ClassCastException

But I don't understand why I get a compile error in the first case and runtime exception in the second case. After all, a parent can never be convert or cast to child. Why second of these cases is not checked at compile time?

I would be very grateful for a clear and reasonable idea!

Answer 1

Child obj = new Parent();

In this case the compiler tries to check (implicit cast) if Parent object can be set to Child reference and when this casting fails, you get compile time error.

Child obj = (Child) new Parent();

In this case the compiler sees that you are explicitly casting Parent object to Child, so compiler leaves it for the runtime to decide if the casting is valid, in a sense the developer takes the responsibility of the casting. So if this casting fails, you get a runtime error.