无需引发异常即可捕获，但IOException可以

Question

Why does the following code compile fine, but the method being called does not need to throw Exception ? Isn't Exception a checked exception and not an unchecked exception? Please clarify.

class App {
    public static void main(String[] args) {
        try {
            amethod();
            System.out.println("try ");
        } catch (Exception e) {
            System.out.print("catch ");
        } finally {
            System.out.print("finally ");
        }
        System.out.print("out ");
    }
    public static void amethod() { }
}

If I want to use a try catch with IOexception (a checked exception), the method being called needs to throw IOException . I get this.

import java.io.IOException;

class App {
    public static void main(String[] args) {
        try {
            amethod();
            System.out.println("try ");
        } catch (IOException e) {
            System.out.print("catch ");
        } finally {
            System.out.print("finally ");
        }
        System.out.print("out ");
    }
    public static void amethod() throws IOException { }
}

Answer 1

Isn't 'Exception' a checked exception and not an unchecked exception?

Yes, it is.

But even if we know the method doesn't throw Exception itself, the code catch(Exception e){ could still execute. The code in the try block could still throw something that inherits from Exception . That includes RuntimeException and its subclasses, which are unchecked.

catch(IOException e){ , on the other hand, can only catch checked exceptions. (Java doesn't allow multiple inheritance, so anything that's a subclass of IOException can't possibly be a subclass of RuntimeException .) The compiler can fairly easily figure out that none of the code in the try block can possibly throw an IOException (since any method throwing a checked exception must explicitly say so) which allows it to flag the code.

Answer 2

The behavior you're observing comes from the fact that the Java Language Specification treats Exception specially in this circumstance. According to §11.2.3 :

It is a compile-time error if a catch clause can catch checked exception class E ₁ and it is not the case that the try block corresponding to the catch clause can throw a checked exception class that is a subclass or superclass of E ₁ , unless E ₁ is Exception or a superclass of Exception .

This is reasonable because Exception (and its superclass Throwable ) can be used to catch exceptions that extend RuntimeException also. Since runtime exceptions are always possible, the compiler always allows Exception to appear in a catch clause regardless of the presence of checked exceptions.