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 IllegalFormatException("number expected");