CS1054: Lecture 22

1. CS1054: Lecture 22 - Exception Handling

2. Java Exception • Describes an exceptional condition that has occurred in a piece of code. • An object representing that exception is created and thrown in the method that caused the exception. • At some point the exception has to be caught and processed • They report to the calling method some error condition.

3. Keywords • try • catch • throw • throws • finally

4. Default Handler • Exc1 and Exc2

5. Using try and catch • Default handler helps, still you may want to handle exceptions yourself. • This also prevents program for terminating automatically. • For this we use try and catch block. • Exc3

6. Import points – try and catch • Once exception is thrown program control transfers from try to catch block. • Execution never returns from catch to try block • After catch, program continues with the next line in the program following entire try/catch block.

7. Multiple catch blocks • More than one type of exceptions can be thrown by a single piece of code. • To handle this, you can specify more than one catch clauses. • Each catch statement is inspected, and the first one whose type matches the exception is executed.

8. Important • When using Exceptions: • Subclasses should be written always before superclass.

9. Throw • Allows your program to explicitly throw an exception.

10. Throws • If a method is capable of causing an exception that it doesn’t handle, it must specify this behavior. • A throws clause lists different types of exceptions that a method might throw.

11. finally try { } finally { }

12. Catch vs. Finally • A try statement requires either finally or catch. • Statements in catch block get executed only if an exception is thrown. • Statements in finally block always get executed.

13. Exception – Handling blocks try { // block of code to monitor of errors } catch (ExceptionType1 exOb) { // exception handler for Exception Type 1 } catch (ExceptionType1 exOb) { // exception handler for Exception Type 2 } finally { // block of code to be executed before try block ends }