Catching java exceptions FileNotFound and IOExcept

问题:

Is the FileNotFoundException somehow a "sub-exception" of the IOException?

This is my code opening an input stream to a file at the given path:

   method(){
        FileInputStream fs;
        try {
            fs = new FileInputStream(path);
            //
            fs.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

How come I can neglect the FileNotFound and just catch the IOException instead? Is the FNFException a part of the IOException?

Instead of catching the exceptions, what if I throw an IOException in my method?

    method() throws IOException{

        FileInputStream fs;
        fs = new FileInputStream(path);
        //
        fs.close();

    }

Can I proceed to catch a FileNotFoundException at the invoking method like this?

    try {

         method();

    }catch (FileNotFoundException e1) {}

Thanks for any help you might be able to provide!

回答1:

Is the FileNotFoundException somehow a "sub-exception" of the IOException?

Yes, FileNotFoundException extends IOException:

java.lang.Object
    java.lang.Throwable
        java.lang.Exception
            java.io.IOException
                java.io.FileNotFoundException

How come I can neglect the FileNotFound and just catch the IOException instead?

Catching a base class of the exception being thrown will catch the exception, unless there is a more specific catch clause available.

Can I proceed to catch a FileNotFoundException at the invoking method like this?

Absolutely! In fact, this is a good thing to do: your code should handle only the exceptions with which it knows how to deal, and let all other exceptions propagate to a place where it could be dealt with in a better way.

回答2:

Yes, it is.

If you look at inheritance FileNotFoundException is a sub-class of IOException. By catching the super class you also catch anything that extends it.

You can catch the more specific one first as in your first example if you need to handle it differently.

回答3:

From Java 7 onwards you can do:

catch(ExceptionType1 | ExceptionType2 e) to do multiple-exception catches. However, in your case, you can just catch the IOException, as you suggest.

回答4:

Is the FileNotFoundException somehow a "sub-exception" of the IOException?

Yes it is.

Instead of catching the exceptions, what if I throw an IOException in my method?

You can do so. When a method throws an excpetion, it can throw this exception in particular or any exception inheriting it.

Can I proceed to catch a FileNotFoundException at the invoking method like this?

Yes. If a method declares to throw A, you can catch B if B inherits A (even if B isn't thrown by A explicitly).

Side note:

• you don't close your resources correctly;
• if you use Java 7, use the new Files API instead.

The first point is very important. You .close() in your try block; if you can successfully open the file but fail to read from it or whatever, .close() will not be called. You should do:

FileInputStream in = ...;
try {
    // operate on "in"
} finally {
    in.close();
}

For Java 7:

try (
    FileInputStream in = ...;
) {
    // operate on "in"
}
// "in" is closed for you here

回答5:

Yes, as the javadoc shows it, FileNotFoundException is a subclass of IOException.

If you really wantFileNotFoundException, you must catch only this execption, otherwise catching IOException will also catch any exception subclassing it, like FileNotFoundException any many others.