Question

So I have this relevant code…

public class PokemonTrainer {
    private Pokemon p = new Squirtle();
    private String name;

    public PokemonTrainer(String name) {
        this.name = name;
    }

    public static void main(String[] args) {
        PokemonTrainer pt = new PokemonTrainer("Ash");
        try {pt.fightGary();}

        catch (Charmander c) {
            System.out.println("You were roasted by a Charmander.");
        }

        catch (Squirtle s) {
            System.out.println("You were drowned by a Squirtle.");
        }

        catch (Bulbasaur b) {
            System.out.println("You were strangled by a Bulbasaur.");
        }

        catch (Pokemon p) {
            System.out.println("You survived!");
        }
    }

    public void fightGary() throws Pokemon {
        throw p;
    }

public class Pokemon extends Exception {}
public class Bulbasaur extends Pokemon {}
public class Squirtle extends Pokemon {}
public class Charmander extends Pokemon {}

Why is it that this prints “You were drowned by a Squirtle”?

In my reasoning, “catch” is a method, and when an object is passed into a method, the method evaluates based on the object’s STATIC TYPE — that is, “Pokemon”, in this case — this is demonstrated in the below short example:

public class PokemonTrainer {
    private Pokemon p = new Squirtle();
    private String name;

    public PokemonTrainer(String name) {
        this.name = name;
    }

    public static void main(String[] args) {
        PokemonTrainer pt = new PokemonTrainer("Ash");
        pt.fightGary(pt.p); // ------------ Prints "Pokemon!!!"
    }

    public void fightGary(Pokemon p) {
        System.out.println("Pokemon!!!");
    }

    public void fightGary(Squirtle s) {
        System.out.println("Squirtle!!!");
    }
}

So what’s different about these two examples? Why does the first example print what it does?

Thanks!

Answer 1

In my reasoning, “catch” is a method

That’s the first mistake. catch isn’t a method, and trying to think of it as if it were is going to cause you problems.

It’s a language construct with its own rules. You should read the language specification section 14.20 or the exceptions tutorial for more information.

In short, when you have a sequence of catch blocks, the first one which matches the execution time type of the thrown exception is executed. (The compiler will prevent you from catching a more-general exception before a more-specific one.)

So if you have:

try {
  doSomething();
} catch (FileNotFoundException e) {
  ...
} catch (IOException e) {
  ...
}

then if doSomething() throws a FileNotFoundException it will execute the first catch block (and only that one) – whereas if it throws any other kinds of IOException, it will execute the second catch block. Any other exceptions will propagate up the stack.