Re: inheriting BufferedImage and using ImageIO.read()
mx wrote:
So, I have a SubImage class which extends BufferedImage and I would like=
to use ImageIO.read() to create a SubImage, but since it returns a
BufferedImage, I can't find a way of doing that.
Should I be using another design? or is there a way to get past this
problem?
I would also add that right now my SubImage simply adds methods to the
BufferedImage class. Yet downcasting causes an exception.
Since you absolutely show no code whatsoever, much less a Simple, Self-Cont=
ained Compilable Example (SSCCE http://sscce.org/), I'll guess, but the lac=
k of information you provide may cause me to miss the mark. (Follow the ad=
vice in http://sscce.org/.)
Subclassing 'BufferedImage' is almost certainly a mistake, but without know=
ing your reasons or what changes you made it's hard to be 100% certain. St=
ill, I'd take 10:1 that you should "favor composition over inheritance". (h=
ttp://java.sun.com/docs/books/effective/) That is, use a class that has a =
member reference to a 'BufferedImage' rather than extending it.
As for the downcast, I'll take 100:1 odds that you created a 'BufferedImage=
' that was not a 'SubImage' then tried to downcast it. You can only downca=
st a reference if it points to an instance of the subtype. Otherwise you g=
et a 'ClassCastException'. You did get a 'ClassCastException', right? You=
didn't even bother to tell us that much, much less what the message said o=
r what code it applied to.
To answer your questions: Yes, you should be using another design. Maybe =
even if you give us more information we could help you figure out what that=
other design should be. Like, what are you trying to accomplish, and why =
did you imaging subclassing 'BufferedImage' would help, and *WHAT IS YOUR C=
ODE?* (http://sscce.org/) (Note the "Self-Contained" in that initialism's=
expansion!)
The way to get around your problems (there are more than one), read the Ora=
cle tutorials about inheritance and casting in Java. Buy and study (repeat=
edly) the book /Effective Java/, 2nd ed., by Joshua Bloch, referenced above=
..
Somebody have the FAQ for asking smart questions handy? Between that and h=
ttp://sscce.org/ we should be able to help this guy if he follows that advi=
ce.
So: Don't extend 'BufferedImage'. Don't try to downcast unless you know t=
he instance is of the proper subtype. Do tell us what you want to accompli=
sh, rather than merely how you imagined you should accomplish it. Do study=
diligently the tutorials, /Effective Java/ and http://sscce.org/.
--
Lew