有关铸造的Java 803考试

Question

in preparation for the 803 exam, I came across a question that really stumped me since I hadn't seen this been done until then. The answer to the question confused me even more - can anyone kindly explain exactly what is happening here?

Here is the question (the answer is 1 apparently):

---

Consider the following classes :

interface I{
}
class A implements I{
}

class B extends A {
} 

class C extends B{
}

And the following declarations:

A a = new A();

B b = new B(); 

Identify option that will compile and run without error:

1. a = (B)(I)b;

2. b = (B)(I) a;

3. a = (I) b;

4. I i = (C) a;

Thanks in advance for your help! :)

Answer 1

1 is fine, but when run separately 2, 3 and 4 error.

From the code, we can note the following relationships, I have used -> to mean extends or implements (that is, they are castable in the direction of the arrow)

C -> B -> A -> I

variable a is of type A
variable b is of type B

Explicit casting using brackets is a runtime check. Implicit casting, as happens when the left and right hand sides of the assignment do not match is checked at compile time.

Using the information above we can look at each of the statements and make the following conclusions:

1. a = (B)(I)b;    // OK
  The target assignment is of type A.  b is runtime castable to I,
  I is runtime castable to B and B is compile time castable to A.

2. b = (B)(I) a;   // RUNTIME ERROR
  The target assignment is of type B.  a is runtime castable to I, but
  A is not runtime castable to B.

3. a = (I) b;      // COMPILE ERROR
  The target assignment is of type A.  b is runtime castable to I but I cannot 
  be cast at compile time to A.

4. I i = (C) a;    // RUNTIME ERROR
  The target assignment is of type I.  a is not runtime castable to C but C 
  is compile time castable to I.