Implicit interface, class casting in JAVA

Question

I am learning for a JAVA Programmer I certificate and among questions there is one I can't understand:

//Given:

interface I{}
class A implements I{}
class B extends A {}
class C extends B{}

//and

A a = new A();
B b = new B();

Identify options that will compile and run without error.

A. a = (B)(I)b;
B. b = (B)(I)a;
C. a = (I)b;
D. I i = (C)a;

Now I know that the answer is A) but I don't get it, if the class B is a child of class A, then 'a' can be equal to 'b' without casting, why is the answer B) wrong? What does even casting (B)(I) means?

look to it like this all B are A but not all A are B. In this example only B extends A but there might be a new class Z extends A which will be an A but will not be a B — user10386912
– user10386912, Commented Dec 4, 2018 at 20:29

Jason Armstrong · Accepted Answer · 2018-12-04 20:35:23Z

2

B extends A. So you can cast instances of B to A but not the other way around. The code will compile, but will throw a ClassCastException at runtime.

Trying to cast to a descendant class is called downcasting. The cast to 'I' in this case is what would allow that code to compile, but result in the ClassCastException being thrown.

answered Dec 4, 2018 at 20:35

Jason Armstrong

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Comments

Gaurav Pathak · Accepted Answer · 2018-12-05 11:45:40Z

0

Parent reference can be used to hold child objects. Thus below all are correct.
```
A a = new A();
B b = new B();
I i = new A();
a = new B();
b = new C();
```
Below has attempted to cast an object to a subclass of which it is not an instance.Thus below will throw the CCE.
```
b = (B)(I)a;
c = (C)(I)a;
```
You may check the same as System.out.println(a instanceof C);

answered Dec 5, 2018 at 11:45

Gaurav Pathak

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