An unchecked exception that carries data
A custom exception is a class you write that extends Exception (checked) or RuntimeException (unchecked). It gives a failure a meaningful name, can carry extra data, and should keep the original exception as its cause.
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Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Write an unchecked InvalidPinException with a field attemptsLeft. A method checkPin(String pin) accepts "1234" and otherwise throws it with the attempts left (start at 3). Try "0000", "1111" and "1234", printing what happens.
Write a checked ParseException-style class CsvException with a line field. A method int sumColumn(String[] lines) parses the second comma-separated value of each line and wraps any NumberFormatException with the line number (starting at 1) as the cause. Run it on {"a,1", "b,2", "c,x"}.
Write a method Throwable rootCause(Throwable t) that follows getCause() to the end. Test it on a three-level chain and print the root's class and message.
Explain it without notes
How do you decide whether a custom exception should extend Exception or RuntimeException?
What is exception chaining, and why is it important?
What constructors should a custom exception provide, and why?
When is a custom exception worth creating, and when should you use a standard one?
What is exception translation, and where do you do it?
Expected output
error: balance 200 is less than requested 450
you need 250 more
is RuntimeException? true
balance: 200