Java dynamic class cast

Question

I'm aware that this could be a possible duplicate, but the other threads haven't solved my problem completely.

Let's say I have the following classes A and B which extend BasicClass.

public abstract class BasicClass {
   // ...
}

public class A extends BasicClass {
   // ...
}

public class B extends BasicClass {
   // ...
}

And now I want to cast class A to B dynamically.

a.getClass().cast(bClass.newInstance);

but every time I get an Exception.

java.lang.ClassCastException
at java.lang.Class.cast(Unknown Source)

Can you tell me why?

Sotirios Delimanolis · Accepted Answer · 2014-11-19 03:39:44Z

4

Because, in your inheritance hierarchy, a B is not an A. You cannot cast one to the other.

answered Nov 19, 2014 at 3:39

Sotirios Delimanolis

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JavaRocker · Accepted Answer · 2014-11-19 04:09:48Z

1

Answer:

Because A is not extending B. (technicaly: A IS not B)

Explanation:

JVM performs "IS-A" test before it casts. If "IS-A" test fails, you will get "ClassCastException".(Just like you are getting now).

IS-A : IS-A test is to check whether a class(to be casted, in your case A) is in inheritance tree of the class(casted to, in your case B).

IS-A is the functional term, syntax to check IS-A test is instanceof". e.g a instanceof b : explanation of instance of:
LHS and RHS of the instance of must be the object. thus a and b are obeject. Their respective classes are checked for inheritance.

In your example following result will come.

a instanceof b : False

a instanceof basicObject : True

b instanceof a : False

b instanceof basicObject : True

basicObject instanceof a : False

basicObject instanceof b : False

You can only cast where result of instanceof is true.

answered Nov 19, 2014 at 4:09

JavaRocker

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2 Comments

user2948795 Over a year ago

Thanks for the explanation. I got it now. Would it be possible to cast A to B in two steps: A -> BasicClass -> B ?

Sotirios Delimanolis Over a year ago

@user2948795 A cast does not change the type of the object. Nothing can do that. Casting lets you view a reference of one type as a reference of another type.

August · Accepted Answer · 2014-11-19 03:42:37Z

0

Your hierarchy looks like this:

   BasicClass
      / \
     /   \
    A     B

A is not an ancestor of B, so it's impossible to cast an instance of B to A.

answered Nov 19, 2014 at 3:42

August

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Java dynamic class cast

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Explanation:

In your example following result will come.

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Answer:

Explanation:

In your example following result will come.

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