How to throw exception in static method, both in same class

Question

Example1Exception and Example1Method belong together in the same file. It would not make sense to put them in separate files.

public class Example1
{
    public class Example1Exception extends Exception
    {
        public Example1Exception(String message)
        {
            super(message);
        }
    }

    public static void Example1Method() throws Example1Exception
    {
        throw new Example1Exception("hello"); //error: non-static variable this cannot be referenced from a static context
    }
}

How can I throw Example1Exception in Example1Method?

Use an IDE like Eclipse or NetBean. That will help a lot with the java syntax. — K.C., Jan 17 '14 at 13:56
It would really help if you would post code that you've compiled, so that the error you're claiming matches the real one. Your declaration of Example1Exception isn't a proper class declaration, and your Example1Method method declaration is missing a return type and parameter list. This sort of thing distracts from the real issue. — Jon Skeet, Jan 17 '14 at 14:08
@cja: I didn't say you needed to post your original code. It's *great* that you tried to write a short but complete example demonstrating the problem. It's not great that you didn't try to compile it before posting it, to make sure that it actually *did* demonstrate the same problem you're facing in your real code. — Jon Skeet, Jan 17 '14 at 15:51

Jon Skeet · Answer 1 · 2014-01-17T14:07:12.350

11

(Assuming you actually declare Example1Exception using a class declaration..., and that the method declaration is fixed too...)

Example1Exception is an inner class - it needs a reference to an enclosing instance of the outer class.

Options:

Provide it with a reference (but why?)
Make it a nested (but non-inner) class by changing the declaration to include static
Make it a top-level class

Personally I'd usually go for the last option - why do you want it to be a nested class anyway? Why would it not make sense to put them in separate files? What do you gain by having it as a nested class, other than a bunch of complexity? Do you really want people to declare catch (Example1.Example1Exception ex) { ... }

If you really want it to be nested, you probably just want it to be a non-inner class - you're not using the implicitly-required reference to an instance of Example1.

edited Jan 17 '14 at 14:07

answered Jan 17 '14 at 13:55

Jon Skeet

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I think exception classes should be public. A file can contain just one public class (according to http://stackoverflow.com/questions/2336692/java-multiple-class-declarations-in-one-file). So I can't declare several exception classes in one file. So either I end up with a load of small files each containing one exception or I go for the first or second option. – cja Jan 17 '14 at 15:17
@cja: What is the *harm* in having several small files each containing a single exception? Are you really declaring *so* many different exceptions that it's a problem for you? If so, you should consider whether or not that's really helpful... – Jon Skeet Jan 17 '14 at 15:20
Reduced browsability. You're reading a newspaper. You want every article on its own page or does it make sense to group all the brief minor crime reports together? – cja Jan 17 '14 at 15:47
@cja: I would say you're reducing the readability for your *users* by nesting classes just on the grounds that you don't like having multiple small files. I also don't think it reduces the readability for your code either... Classes are not (primarily) intended for the purpose of grouping other types: packages are. Anyway, I've given my opinion - if you want to make them all nested (non-inner) classes then that's your choice. – Jon Skeet Jan 17 '14 at 15:49

score 0 · Accepted Answer · answered Jan 17 '14 at 13:59

0

Make the exception class static:

public static class Example1Exception extends Exception
{
    public Example1Exception(String message)
    {
        super(message);
    }
}

answered Jan 17 '14 at 13:59

Tim B

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An exception class can be static? – cja Jan 17 '14 at 14:29
Any inner class can be static. Exceptions are just classes like any other, except that you can throw them. – Tim B Jan 17 '14 at 14:42

K.C. · Answer 3 · 2014-01-17T14:10:28.307

If you really want to do it like that, this is code :

public class Example1 {
    public class Example1Exception extends Exception {
        public Example1Exception(String message) {
            super(message);
        }
    }

    public static void Example1Method() throws Example1Exception {
        throw new Example1().new Example1Exception("hello");
    }
}

But like someone else already said: it makes not much sense to declare the Exception as an inner class.

How to throw exception in static method, both in same class

3 Answers3