Exceptions

Exceptions = use to signal/indicate problems (not necessarily errors)

Some Causes: Code or Data Errors use array index out of bounds. Standard Method Exceptions some methods defined to "throw" exceptions Throwing your own Exceptions define your own method and have it throw an exception e.g. readFile() throws IOException Java Errors run time errors (usually can't catch)

 

 

The Class Hierarcy

Object Throwable Exception                     Error can catch               you can't catch

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Handling Exceptions

using try, catch, finally

 

• try
  • encloses code that can lead to 1 or more exceptions
  • such code must always be inside a try statement (or the method it is inside of must be declared to throw these exceptions)
• catch
  • follows a try block and contains code to handle the exception. has parameter exception thrown.
  • can have multiple catch blocks.....each one handling a different Exceptions
  • If you have catches for Exceptions in the same heirarchy chain, put in order of lowest subclass first...otherwise, will never catch the lower subclass Exceptions.
    • E.g.
    • try { }
    • catch (Arithmetic Exception e)
    • { ....}
    • catch (Exception e)
    • {....}
      
• finally
  • follows any catch block and is always exceuted no mater what
  • e.g. use to close streams
  • optional.

   

Re-throwing an exception

 

• you can rethrow an exception ....in a sense returns it from the method you are in
• e.g.
  • try{ ....}
  • catch(IOExceoption e)
  • { throws e; }

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Creating your Own Exception Class and Thowing It

 

1. Create a subclass of Exception and you should have a default constructor and a constructor with 1 string argument
2. Create instance of this new class....lets call it P.
3. Then trow P..... throw P; as appropriate in your code