异常处理-捕获多个异常无法正常工作

Question

I have a catch block with multiple exceptions - Arithmetic and NullPointer and one catch block with Exception. I am calling a method from catch block, but it is not find the instance of exception correctly.

try {
        int a = 10/0;
    } catch (ArithmeticException | NullPointerException e) {
        Exce(e);
    } catch (Exception e) {
        Exce(e);
    }

public static void Exce(ArithmeticException ex) {
    System.out.println("Arithmetic");
}

public static void Exce(Exception ex) {
    System.out.println("Exception");
}

But i am getting output as "Exception"..I am not sure why Arithmetic is not getting displayed

When i have separate catch block for Arithmetic and Null pointer..I am able to print "Arithmetic"..But catch with multiple exceptions not working...

Answer 1

If you split the ArithmeticException and NullPointerException into 2 catch blocks, it works as you expect. I assume this is because variable e is declared of type Exception to be able to hold ArithmeticException and NullPointerException.

public static void main(String[] args) {
    try {
        int a = 10 / 0;
    } catch (ArithmeticException e) {
        Exce(e);
    } catch (NullPointerException e) {
        Exce(e);            
    } catch (Exception e) {
        Exce(e);
    }
}

Answer 2

} catch (ArithmeticException | NullPointerException e) {
    Exce(e);
}

The compile time type of e in the above is the union of ArithmeticException and NullPointerException . So when the compiler tries to match the type of e against the Exce methods:

• The Exce(ArithmeticException) overload is not applicable because at runtime e could be a NullPointerException .

• The Exce(Exception) overload is applicable because any value of e that matches the union of ArithmeticException and NullPointerException is an Exception as well.

Unfortunately, you cannot declare a method overload for the ArithmeticException | NullPointerException ArithmeticException | NullPointerException ... even if you wanted to:

• If you want to handle ArithmeticException | NullPointerException ArithmeticException | NullPointerException together like this, you could declare an overload like this:

   public static void Exce(RuntimeException ex) { System.out.println("RuntimeException"); } 

• Alternatively, catch ArithmeticException and NullPointerException separately.

• It is also possible to catch those two exception in one catch clause, and then discriminate them using (say) instanceof and type casts. But it is a lot more code to do that. (And a bad idea for other reasons.)