不能抛出私有成员类的实例？ - Java

Question

What does this error mean, and why does it apply? I can't find much info with Google about member classes and static contexts, or what those mean, in a case that seems relevant to my situation.

Here's the error I'm getting:

non-static variable this cannot be referenced from a static context

It points to this line, and at the new operator:

throw new ParenthesisException();

ParenthesisException is a private member class of the main class. I think the problem is probably related to that, but that's about all I can figure.

This is my definition of ParenthesisException. It is inside the main class definition: (I am sorry if the formatting is not very good)

private class ParenthesisException extends Throwable
{
    public ParenthesisException(){}
    public String strErrMsg()
    {
        return "ERROR: Every '(' needs a matching ')'";
    }
}

I find the error message rather cryptic. I would appreciate a brief explanation of "static contexts" and why the new operator isn't working for my member class, and how I can throw an instance of a private member class.

Answer 1

If I had to guess what's going on based on the code fragment you've posted, the error is probably caused because you're trying to throw a ParenthesisException out of a static method.

In Java, classes defined inside of another class automatically store a pointer to the object inside which they were created. That is ParenthesisException has an implicit pointer back to the enclosing class inside of which it was created with new . This means that, in particular, you cannot construct a new ParenthesisException inside of a static method, because there is no this pointer that can be used to refer to the containing class.

To fix this, you should make ParenthesisException a static inner class like this:

private static class ParenthesisException extends Throwable
{
    public ParenthesisException(){}
    public String strErrMsg()
    {
        return "ERROR: Every '(' needs a matching ')'";
    }
}

This static after the private says that ParenthesisException does not hold a reference back to an enclosing object, which is probably what you wanted anyway. It also means that you can create new ParenthesisException s inside of static methods.

Hope this guess is correct, and hope this helps!

Answer 2

The error is a bit confusing when you haven't seen it before, but it's exactly what it says: You can't use a non-static variable from a non-static context. Your main method and most likely any method in the class that contains your main method is static, so you can't use a non-static variable from it.

ParenthesisException is non-static because you haven't declared it as static, yet it is declared within your main class and called within a static method.

Don't declare this as an inner-class -- make your a new public class. You'll then be able to instantiate this exception.

Answer 3

Your main function is 'static context'. You can only call static function and there is not 'this'. When you try to create a new exception you effectively call this.new ParenthesisException() , because the exception is a inner class. But you can't do that. You need to create an istance of your main class and then create the exception object on that instance.

MainClass m = new MainClass();
throw m.new ParenthesisException();

Another solution is to define your exception as static class:

private static class ParenthesisException extends Throwable

Then you can instantiate it from static contexts, so you don't need an instance of the main class.

Answer 4

Without code this is strictly a guess, but based on the error message I believe this would work. If your main class is Animal then you need to instantiate Animal so inside of main if you do a simple...

new Animal(); as the only line in main

then make a new method....

public Animal(){
    /* put code that used to be in main */
}

This will most likely fix your problem....again without full code this is strictly a guess, but based on what you said and the error I believe this will solve the problem.