Java - Why the subclass of ArithmeticException class is not called?

I want to modify the ArithmeticException output message. So, for that i did few experiments. I extended the ArithmeticException class byExtenderClass class. The point of this question is not only to find the solutions to modify the ArithmeticException exception message but also to tell why some of cases below are working as expected but some are not? Following are the cases along with their outputs:

Case 1:

// Both the classes are in the same file 'MyClass.java'
class MyClass{
    public static void main(String args[]){

        int a,b,c;
        a = 1;
        b = 0;

        try{
            c = a / b;
        }catch(ArithmeticException e){
            System.out.println("I caught: " + e);
        }

    }
}

class ExtenderClass extends ArithmeticException{
    // ...
}

Output:

I caught: java.lang.ArithmeticException: / by zero

Result: Works fine as expected.

---

Case 2:

// Both the classes are in the same file 'MyClass.java'
class MyClass{
    public static void main(String args[]){

        int a,b,c;
        a = 1;
        b = 0;

        try{
            c = a / b;
        }catch(ExtenderClass e){
            System.out.println("I caught: " + e);
        }

    }
}

class ExtenderClass extends ArithmeticException{
    // ...
}

Output:

Exception in thread "main" java.lang.ArithmeticException: / by zero
    at MyClass.main(MyClass.java:9)

Result: Means that the throw/catch is not fired. Why the ExtenderClass is not fired? In fact its extending the ArithmeticException class?

---

Case 3:

// Both the classes are in the same file 'MyClass.java'
class MyClass{
    public static void main(String args[]){

        int a,b,c;
        a = 1;
        b = 0;

        try{
            c = a / b;
            throw new ArithmeticException();
        }catch(ArithmeticException e){
            System.out.println("I caught: " + e);
        }

    }
}

class ExtenderClass extends ArithmeticException{
    // ...
}

Output:

I caught: java.lang.ArithmeticException: / by zero

Result: Works fine as expected.

---

Case 4:

// Both the classes are in the same file 'MyClass.java'
class MyClass{
    public static void main(String args[]){

        int a,b,c;
        a = 1;
        b = 0;

        try{
            c = a / b;
            throw new ExtenderClass();
        }catch(ExtenderClass e){
            System.out.println("I caught: " + e);
        }

    }
}

class ExtenderClass extends ArithmeticException{
    // ...
}

Output:

Exception in thread "main" java.lang.ArithmeticException: / by zero
    at MyClass.main(MyClass.java:9)

Result: Means that the throw/catch is not fired. Why the ExtenderClass is not fired? In fact its extending the ArithmeticException class?

---

Why the ExtenderClass class that extends the ArithmeticException is not fired? But when i use the ArithmeticException directly, it gets fired.

While you have declared a custom exception as a subclass of ArithmeticException, there is no way that you can get a / b to throw your custom exception. The JLS specifies that (integer) division by zero will throw ArithmeticException; see JLS 15.17.2 paragraph 3.

Since the exception that is being thrown is ArithmeticException, you won't be able to catch it as your custom exception.

try {
   c = a / b;      
} catch (ExtenderClass ex) {
   ...
}

will catch ExtenderClass and subclasses of ExtenderClass. ArithmeticException is not a subclass of ExtenderClass, so the above won't catch it.

---

Your reason for creating ExtenderClass is ...

I want to modify the ArithmeticException output message.

You would be better off writing some special-case code to substitute a different message for the "/ zero" message when you print it out.

Or ....

try {
   c = a / b;      
} catch (ArithmeticException ex) {
   thrown new ExtenderClass("Division by zero is cool!, ex);
}