how to make sure a variable only takes on certain values

[Bernd Stransky]

Hello,
I have a String variable that is supposed to only have certain values, e.g.
"up"
"down"
"unknown"
etc.
I can certain check in my method if the String parameter passed by the object caller has one of these values.
Question: How can I enforce that the caller only can set the value to one of the required ones?
Any info is appreciated,
Thanks
Bernd

[Ernest Friedman-Hill]

You can't, really. What you can do is define a
to represent only the legal values:
public class Direction {
// Private constructor is important!
private Direction() {}
public static final Direction UP = new Direction();
public static final Direction DOWN = new Direction();
// etc.
}

Now you redefine your method to accept a Direction as an argument; the callers can only pass in one of the defined public static values!

[Michael Morris]

You can also enforce it using Exceptions. For example:

public void someMethod(String direction) {
    if (!direction.equals("up") && !direction.equals("down") && (!direction.equals("unknown") {
        throw new IllegalArgumentException("Direction is invalid: " + direction);
    }
    // Do the cool stuff with direction
}

You may decide to define your own Exception since IllegalArgumentException is not a checked Exception. Ordinarily though you prefer a RuntimeException in these situations since it generally indicates an error in program logic.

[Ernest Friedman-Hill]

The OP said

I can certain check in my method if the String parameter passed by the object caller has one of these values.

and wanted to know how to get static checking instead.

[Bear Bibeault]

The OP said ...

This is true, but many times posters ask for something without realizing that there might be alternative ways to do things, or that a way they thought wasn't optimal may actually be the accepted practice.
I don't think pointing out such alternatives is a bad thing. Do you?
bear

[Bear Bibeault]

P.S. That said, I would also have suggested the Type-Safe Enum pattern as a possible solution, but you beat me to it!
bear

[Bear Bibeault]

P.P.S. Welcome to the Ranch!

[Bernd Stransky]

Thanks for your ideas.
Bernd

[Ernest Friedman-Hill]

Bear:

I don't think pointing out such alternatives is a bad thing. Do you?

No, not at all. Sorry, I didn't mean to sound rude.

[Joel McNary]

Just to throw in my $.02, Java 1.5 will provide the ability to declare enumerations. This doesn't help you now, but it would down the road....

[Jim Yingst]

Actually it can help you now if you install the pre-release generics compiler frm Sun. Though it's a bit of a pain to use, and doesn't work well with IDEs (who don't know about the new features). And if you try to do serious development with it, there's a significant risk that the eventual release will not behave exactly the same, so you may get in trouble. But you can certainly play around with it...

[Michael Morris]

Originally posted by Ernest Friedman-Hill:
The OP said

and wanted to know how to get static checking instead.

Dr. Friedman-Hill,
First, let me say that it is an honor to have someone of your caliber answering questions here at JavaRanch. Second, I meant no disrespect nor did I mean to suggest that your solution was not optimal, but as Bear mentioned sometimes poster haven't considered other possibilites.
I would like to ask a favor of you. Would you start a thread in JIG Advanced on rule based systems. That is a subject that I and I know many others here are interested in, yet I know very little about the theory. As soon as your book is available I plan to get a copy but it would be nice to know a little bit in advance.