捕获多个异常的处理程序？

Question

I am experimenting with exceptions and i want to ask when it is possible to handle multiple exceptions in one handler and when it is not?

For example i wrote the following code which combines two exceptions ( FileNotFoundException OutOfMemoryError ) and the program runs properly without any error. Al thought the handling is not so relevant with the functionality of the code i chose them just to see when i can combine multiple exceptions in on handler :

import java.io.FileNotFoundException;
import java.lang.OutOfMemoryError;

public class exceptionTest {
    public static void main(String[] args) throws Exception {
        int help = 5;

        try {
            foo(help);
        } catch (FileNotFoundException | OutOfMemoryError e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public static boolean foo(int var) throws Exception {
        if (var > 6)
            throw new Exception("You variable bigger than 6");
        else
            return true;
    }
}

But when i choose different type of exceptions the compiler gives me error . For example when i choose IOException and Exception i have the error the exception is already handled " :

import java.io.IOException;
import java.lang.Exception;



public class exceptionTest {
    public static void main(String[] args) throws Exception {
        int help = 5;

        try {
            foo(help);
        } catch (IOException | Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public static boolean foo(int var) throws Exception {
        if (var > 6)
            throw new Exception("You variable bigger than 6");
        else
            return true;
    }

}

So why is this happening ? Why in one occasion i can use multiple exception in handler and in the other not ? Thank you in advance.

Answer 1

You are getting the message because IOException is a subclass of Exception . Therefore, if an IOException were thrown, it would be caught by a catch (Exception e) statement, so catching it as an IOException is redundant.
The first example works because neither FileNotFoundException nor OutOfMemoryError is a subclass the other.

However, you can catch sub-classed exceptions using the separate catch statement:

try{
    // code that might throw IOException or another Exception
} catch (IOException e) {
    // code here will execute if an IOException is thrown
} catch (Exception e) {
    // code here will execute with an Exception that is not an IOException
}

If you do this, please note that the subclass must come first.