meet it

Make your own exception, and Java makes callers face it.

Some problems are nobody's fault: a shop runs out of stock, or a file goes missing. Java calls these checked exceptions. They extend Exception but not RuntimeException, and Java checks that every caller deals with them before the program can run.

Make a by extending Exception, with a constructor that hands the message to super. A method that can throw it says so with after its brackets. Each caller must then catch it, or declare it in turn.

public class Main {
    static class OutOfStockException extends Exception {
        OutOfStockException(String message) { super(message); }
    }
    static void buy(String cake, int left) throws OutOfStockException {
        if (left == 0) {
            throw new OutOfStockException("Sorry, no " + cake + " left");
        }
        System.out.println("One " + cake + ", coming up!");
    }
    public static void main(String[] args) {
        try {
            buy("cupcake", 3);
            buy("eclair", 0);
        } catch (OutOfStockException e) {
            System.out.println(e.getMessage());
        }
    }
}

Name an exception after its problem, and end the name with Exception.

guess first

Nobody dealt with it. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

public class Main {
    static class TooColdException extends Exception {
        TooColdException(String message) { super(message); }
    }
    static void swim(int degrees) throws TooColdException {
        if (degrees < 15) {
            throw new TooColdException("Brr! Only " + degrees + " degrees");
        }
        System.out.println("Splash!");
    }
    public static void main(String[] args) {
        swim(20);
    }
}

your turn

Your turn. Make your own exception.

On line 2, make NoSeatsException a checked exception by extending the right class. On line 5, warn that book can throw it. Put your name in on line 13, then press run.

Your code

output

press run to see what happens

fix it

The treasure chest never warned anyone. Fix it!

Broken code to fix

output

press run to see what happens

Java says unreported exception WrongCodeException on line 7. A checked exception is thrown, but who was warned?
A method that can throw a checked exception must say so after its brackets.
Change line 5 to static void openChest(int code) throws WrongCodeException {, then press run.

nailed it

you made an exception of your own. :D

Extend Exception for a checked exception named after the problem.
throws warns that a method can throw it.
Every caller must catch it or declare it too.

new words in your collection

checked exception
throws
custom exception

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