Custom exceptions

Definition

A custom exception is your own exception class, named after the problem, such as InsufficientFundsException. It can carry extra data so the caller knows exactly what went wrong.

Example code

An exception that carries the missing amount

class InsufficientFundsException extends Exception {
    private final long missing;

    InsufficientFundsException(long missing) {
        super("Missing " + missing + " to complete the payment");
        this.missing = missing;
    }

    long missing() { return missing; }
}

class Wallet {
    private long balance = 500;

    void pay(long amount) throws InsufficientFundsException {
        if (amount > balance) throw new InsufficientFundsException(amount - balance);
        balance -= amount;
    }
}

class Demo {
    public static void main(String[] args) {
        Wallet wallet = new Wallet();
        try {
            wallet.pay(800);
        } catch (InsufficientFundsException e) {
            System.out.println(e.getMessage());
            System.out.println("Top up at least " + e.missing());
        }
    }
}

// Output:
// Missing 300 to complete the payment
// Top up at least 300