Downcasting Exception

Question

I made this code:

protected Lala lala;
private Oyeha oyeha;

public void setLala(Lala lala) {
    this.lala = lala;
}


this.oyeha = (Oyeha) this.lala;
            executeHostBean = this.oyeha.updateMethod(a, b, c, d, e);

Lala is the parrent class.

Oyeha is the child which extends Lala.

But when i run this code, i got error:

java.lang.ClassCastException: $Proxy201

I can't find where my mistake is. Can somebody help me solve this problem, please? Thank you. :)

score 0 · Answer 1 · answered Jul 24 '12 at 04:00

Your problem is probably that this.lala is getting set to an instance of Lala which is not an instance of Oyeha. Since Oyeha is a subclass of Lala, it's possible to call public void setLala with an argument that is an instance of Oyeha, like this:

Lala childInstance = new Oyeha();
setLala(childInstance);

but it's also possible to call that method with an argument that is just an instance of Lala, like this:

Lala parentInstance = new Lala();
setLala(parentInstance);

In the first case, this.lala is an Oyeha, so you can cast it to Oyeha. But in the latter case,this.lala is just a Lala, and your cast of this.lala to Oyeha will fail, because a Lala is not an Oyeha.

score 0 · Answer 2 · answered Jul 24 '12 at 04:08

0

If you have something like Spring injecting the Lala object, it could be creating a dynamic proxy object (that's what the error message suggests).

If downcasting is absolutely necessary in your design, you might want to rethink the design.

answered Jul 24 '12 at 04:08

Judge Mental

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Dennis Meng · Answer 3 · 2012-07-24T04:34:07.247

As an addition to the other answers (they've basically nailed it already):

Suppose you could downcast like that, such that the line this.oyeha = (Oyeha) this.lala worked, and there was some field in Oyeha, say, test.

If you then tried to use this.oyeha.test somewhere, how would it know where test is? Even worse, what if instances of Oyeha take substantially more memory, and it arbitrarily picks an address outside what was put aside for the instance of Lala?

I imagine that the inventors of Java foresaw a situation like that, and thus made it so that you cannot take an instance of a type and cast down to one of its subclasses.

(I would have put this in a comment, but there's too much)

Downcasting Exception

3 Answers3