CODEWITHSUNDEEP
javaeclipseexception
Tomer Amir
Tomer Amir

Reputation: 1585

Why do I need "throws Exception"?

This is my code:

public static void main(String[] args) throws Exception {
    Scanner s = new Scanner(new File("story.txt"));
    someFunction(s);
}

The story.txt is in the the project root, but without the throws Exception the code doesn't run. Why?

Upvotes: 0

Views: 155

Answers (5)

supercat
supercat

Reputation: 81159

If a method or constructor either throws an exception which isn't derived from RuntimeException, or calls any method whose declaration indicates it must do so, then it must either catch that exception or declare that the exception may be thrown from it. This language feature is almost a good thing, but in its present form has been widely acknowledged as a mistake. Nonetheless, it is sufficiently firmly established as part of the Java language that it's not going anywhere.

Conceptually, it's good to make a distinction between exceptions which are likely to have special meaning to a method's immediate caller and those which would not. Unfortunately, the fact that an exception would have a special meaning to its immediate caller doesn't mean the immediate caller is going to be interested in it. The way Java implements checked exceptions, if Foo is declared as throwing a checked exception and Bar calls Foo but isn't prepared to handle that exception, it must either declare itself as throwing that exception (even though it's unlikely to have any special meaning to its caller), or else catch the exception it has no hope of handling (perhaps rethrowing as a type derived from RuntimeException). Catching and rethrowing as a type derived from RuntimeException is semantically the best approach, but ends up being the most verbose, especially if one wants to avoid making RuntimeException objects.

Note also that because there's no way a chained constructor call can be wrapped in a try block, any constructor which is chained through a constructor that is declared as throwing any exceptions is required to declare itself as throwing those same exceptions. Things are implemented that way for a reason, but it unfortunately makes it difficult for a class constructor to distinguish exceptions which got thrown as part of the base-class construction process and those which got thrown in the construction of the derived class.

Upvotes: 1

Tomer Amir
Tomer Amir

Reputation: 1585

I asked my professor, and he explained that Java makes you handle exceptions.
You must be ready for them if a function might throw them.
You can either use:
try {...
} catch { ...
}
to handle exceptions yourself, or you can use:
throws Exceptions to "pass them to the next level" (handle them later).

Thanks for the help!

Upvotes: 1

Masudul
Masudul

Reputation: 21961

Because, Scanner s = new Scanner(new File("story.txt")); throws checked FileNotFoundException. You should throws or catch checked exception for compilation.

Upvotes: 0

Daniel Larsson
Daniel Larsson

Reputation: 6394

You should throw a FileNotFoundException, because the Scanner constructor can throw one. Since it is a checked exception, you must either pass it further up the stack or handle it somehow.

Upvotes: 2

Marko Topolnik
Marko Topolnik

Reputation: 200158

You don't need throws Exception, but it won't hurt any if you leave it like that.

What you do need is a throws FileNotFoundException because the Scanner constructor you are using is declared to throw that exception, and you don't catch any exceptions.

Upvotes: 6

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