Multiple catch statement in java

Question

I am trying to understand how multiple catch statement works in java. In the documentation, they say that you are not allowed to have related types in catch statement.

Here I have a little sample that I am playing with:

class a extends Exception{}

class b extends RuntimeException{}

public class test{
    public static void main(String[] args){
        try {
            System.out.println(new a() instanceof Exception);
            System.out.println(new b() instanceof Exception);
            throw new a();
        }catch(a | b e) {
            System.out.println(e.getStackTrace());
        }
    }
}

Both classes are instance of Exception. First, one is obvious way and second inherit from RuntimeException and RuntimeExeption inherit Exception.

So why is this code compiling? Shouldn't exception a cover also exception b? Thank you

Answer 1

By related types I believe the relationship they are referring to is the extends (or subclass) relationship. So in your case you have:

Exception 
├── a
└── RuntimeException
    └── b

a is a subclass of Exception

b is a subclass of RuntimeException which is a subclass of Exception

a is not a subclass of b AND b is not a subclass of a

Therefore the enhanced catch block can catch a and b.