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## Defining Exception Classes

- Decide exception should be checked
- Subclass `Exception` or `RuntimeException`
- Provide two constructors

      public class IllegalFormatException extends Exception
      {
         public IllegalFormatException() {}
         public IllegalFormatException(String reason)
         { super(reason); }
      }

- Throw exception when needed:
  `throw new` `IllegalFormat` `Exception("number expected");`

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