Why can't I downcast this object in Java?

Question

I have this method:

    private Message getMessage(DataInputStream in) throws IOException {
    CommandEnum caption = CommandEnum.valueOf(in.readUTF());
    BasicMessage inputMessage;
    if (caption.equals(CommandEnum.BEGIN) || caption.equals(CommandEnum.FILEADDRESS)
            || caption.equals(CommandEnum.CONTAINNAMESERVER))
        inputMessage =  new AddressMessage(caption,in.readUTF(),in.readInt());
    if (caption.equals(CommandEnum.CONTAINFILE) || caption.equals(CommandEnum.DONTCONTAINFILE)
            || caption.equals(CommandEnum.WANTFILE))
        inputMessage =  new FileMessage(caption,in.readUTF());
    else
        inputMessage = new BasicMessage(caption);
    in.readUTF();
    return inputMessage;
}

The signature states that the method return Message, an object I've created. Notice that in some cases it returns AddressMessage, another object of mine that extends Message.

My problem - in case i return new AddressMessage(arguments) I can't downcast it later to be an AddressMessage object [writing (AddressMessage) object]

What do you mean with "I can't downcast it later"? Why not? What happens when you try? — Joachim Sauer, May 12 '11 at 11:46

score 2 · Accepted Answer · answered May 12 '11 at 11:51

I agree, if what you have is an AddressMessage, then it can be always be downcasted as such.

What I expect is more a logic mistake. That you actually try to downcast an instance of another class. Just try to debug that place or print obj.getClass() just to verify what class you are actually trying to downcast.

You can also use if (my_message instanceof AddressMessage) ... or something like this.

score 0 · Answer 2 · edited May 23 '17 at 12:12

0

if (object.instanceOf(AddressMessage))... something like this? look here: Downcasting in Java

edited May 23 '17 at 12:12

Community

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answered May 12 '11 at 11:50

Adrian Ancuta

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score 0 · Answer 3 · answered May 12 '11 at 12:18

You should really use brackets in your if-else constructs, otherwise only the next statement is used as "then" clause, which can be confusing.

This is what your code looks like with brackets:

BasicMessage inputMessage;
if (SNIP1) {
    inputMessage =  new AddressMessage(caption,in.readUTF(),in.readInt());
}
if (SNIP2) {
    inputMessage =  new FileMessage(caption,in.readUTF());
} else {
    inputMessage = new BasicMessage(caption);
}
in.readUTF();
return inputMessage;

you'll either get a FileMessage (SNIP2 = TRUE) or a BasicMessage (SNIP2 = FALSE), nothing else.

Edit: an "else if (SNIP2)" instead of "if (SNIP2)" will solve your problem, but you should really use brackets

Why can't I downcast this object in Java?

3 Answers3