Casting and dynamic vs static type in Java

Question

I'm learning about static vs dynamic types, and I am to the point of understanding it for the most part, but this case still eludes me.

If class B extends A, and I have:

A x = new B();

Is the following allowed?:

B y = x;

Or is explicit casting required?:

B y = (B)x;

Thanks!

Answer 1

Explicit casting is required, and will succeed.

The reason why it's required is because it doesn't always succeed: a variable declared as A x can refer to instances that aren't instanceof B.

{
Object o = "Ha!";
String s = o; // DOESN'T COMPILE
// Type mismatch: cannot convert from Object to String
}

{
Object o = "Ha!";
String s = (String) o; // compiles fine, cast succeeds at run-time
}

{
Object o = Boolean.FALSE;
String s = (String) o; // compiles fine, throws ClassCastException at run-time
}

Whether or not a cast is required is determined only by the declared types of the variables involved, NOT by the types of the objects that they are referring to at run-time. This is true even if the references can be resolved at compile-time.

{
final Object o = "Ha!";
String s = o; // STILL doesn't compile!!!
}

Here, even though the final variable o will always refer to an instanceof String, its declared type is still Object, and therefore an explicit (String) cast is still required to compile.