Exceptions

Discuss PA4 Design

Review

• All exceptions inherit from Throwable
• Errors and Exception are direct descendents
• RuntimeException is unchecked.  All others are checked
• try / catch blocks allow you to handle an exception that arises during program execution
• a throws clause can tell the user of the method that we are not handling an exception
• Only exceptions named in the catch block are handled. All others will pass to the JVM default exception handler
• But, we know from polymorphism that a more general catch will catch all of its own and its children.
• But we can use the more specific catch to differentially handle different situations that arise.

New - Creating an exception

• Create an exception the same way that you do any other class, except
• You need to extend Exception or one of its children
• Only do so if you have something that a standard exception cannot handle.
• Use the constructor to pass a more detailed message to the parent exception.

New - Throwing an exception

• You can throw an exception as a result of a problem in a method
• The throw operator takes an exception object as its operand
• The throw operation aborts execution of the current method and sends the exception message to the calling method
• It is up to the calling method to handle or rethrow the exception

Examples

• AccountTest.java, BankAccount.java, NegativeStartingBalance.java
• SalesReport.java, SalesData.txt
• SalesReport2.java
• StackTrace.java

Practice

• worksheet.doc