Code examples used for this class were from “Java How to Program” Ch. 13

Recommended Study Action:

Review the Self review exercises of Ch 13

Read all exercises of Ch. 13 and formulate ideas as to how these problems would be completed.

Try to implement 13.20, 13.21

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What are exceptions?

Exceptions are Objects that are created that describe an error condition that occurred in a program

Exceptions are Objects

The Exception Hierarchy

Recoverable Exceptions ( Class Exception )

IOException

RuntimeException

Non-Recoverable Exceptions ( Class Error )

OutOfMemoryError

ThreadDeath

AWTError

Checked Exceptions ( Class IOException )

Usually a resource issue.

Compiler checks to verify they are handled.

Unchecked Exceptions ( class RuntimeException )

Usually due to poor coding practices.

Compiler does not force handling.

OutOfBoundsException

ArithmeticException

Convention - Exceptions should end with the word 'Exception'

Exceptions handle synchronous errors not asynchronous events

How do we handle exceptions?

Try Block

Catch Block

Catch or Declare

A method must catch or declare a checked exception

To declare an exception use 'throws ...'

finally

Used to clean up allocated resources and close files

Stack Unwinding

when an un-caught exception is thrown from a method,

the parent method is checked for a catch,

then its parent,

and so on... until a catch is found.

If no catch is found the thread terminates.

Exception Stack trace

Stack traces help determine where/why the exception was thrown?

Chained Exceptions

One exception type occurs

A different exception is thrown to better represent the error to the calling method

pre-condition

a condition the is assumed to be true before a method runs

post-condition

a condition that is supposed to be true after a method runs

assert

used for checking pre and post conditions.

Asserts are not processed by default!

asserts throw AssertionError

Performance?

Use tests if possible not exceptions!

The try block creates overhead that can slow a program down significantly!

How to create new Exceptions:

public class MyException extends Exception {

public MyException() {}

public MyException( String detail ) {

super( detail );

}

}

You can declare non checked exceptions, but DONT!

Exceptions for constructors

When a constructor throws an exception the object is not created!

Multiple Catch blocks

More specific catches should be first!

You can not catch the same item twice!

If you can't really handle the exception throw it again!