A problem?... Exceptions. A problem?... A problem?... Suggested Reading: Bruce Eckel, Thinking in Java (Fourth Edition) Error Handling with Exceptions

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1 A problem?... Exceptions Suggested Reading: Bruce Eckel, Thinking in Java (Fourth Edition) Error Handling with Exceptions 2 A problem?... A problem?

A problem?... A problem?... 5 6 A problem?

2 A problem?... A problem? A problem?... An exception is a special event that might happen during the execution of a program and prevents its normal continuation. This might happen, but...not in my code!!! 7 8

A simplified view on Exceptions Class Throwable Constructor Description (from docs API ) Throwable() constructs a new throwable with null as its detail message Throwable(String

description of this throwable String getmessage() returns the detail message string of this throwable void printstacktrace() prints this throwable and its backtrace to the standard

3 A simplified view on Exceptions Class Throwable Constructor Description (from docs API ) Throwable() constructs a new throwable with null as its detail message Throwable(String message) constructs a new throwable with the specified detail message 9 10 Class Throwable Method Description (from docs API ) String getlocalizedmessage() creates a localized description of this throwable String getmessage() returns the detail message string of this throwable void printstacktrace() prints this throwable and its backtrace to the standard error stream void printstacktrace(printstream s) prints this throwable and its backtrace to the specified print stream String tostring() returns a short description of this throwable 11 12

4 13 14 About throws 15 16

5 Catching exceptions Catching exceptions try {... } catch(exception1 e) { //zero or more catch sections... } catch(exception2 e) {... } finally { //zero or one finally section... } Catching exceptions Throwing exceptions throw new Exception(); exception IOException is never thrown!!! 19 20

6 X X FullStringOfIntegersException has already been caught!!! IndexOutOfRangeException

7 X X IndexOutOfRangeException is never thrown FullStringOfIntegersException is never thrown

8 E is thrown in method M Step 1 Does E belong to a trycatch-finally block? NO M finishes throwing E YES Step 2 Finds the surrounding try-catchfinally block; the current try section is finished YES Iterate through each existing catch in order to find a parameter whose type is E or a supertype of E Does the current try-catchfinally block belong to another try-catch-finally block? NO M finishes throwing E Have a corresponding catch section been found? NO Execute the instructions from the finally section (if it is a finally section) YES Step 3 Execute the instructions belonging to the corresponding catch section Execute the instructions from the finally section (if it is a finally section) Continue the execution of M with the first instruction after the trycatch-finally block Exception1 is lost!!! 31 32

9 MyException is lost!!! Exception Classification Checked versus Unchecked Checked versus Unchecked Exceptions 35 36

10 37 38 Exceptions and Constructors Exceptions and Constructors call to super must be first statement in constructor unreported exception MyException1 in default constructor A subclass constructor cannot catch exceptions raised by its superclass constructor!!! 39 40

and overriding overridden method does not

11 Exceptions and Constructors Exceptions and Constructors The subclass constructor can throw additional exceptions!!! Exceptions and overriding Exceptions and overriding overridden method does not throw NewException A subclass may choose not to throw any exception!!! 43 44

12 Exceptions and overriding Exceptions and overriding overridden method does not throw MyException1 overridden method does not throw NewException Exceptions and overriding Exceptions and overriding Does not throw any exception!!! 47 48

13 57 Use exceptions only for exceptional conditions? Effective Java Companion slides of Joshua Bloch, Effective Java (Second Edition) 50! 51 52

14 58 59 try {... Avoid unnecessary use of checked exceptions } catch(myexception e) { throw new RuntimeException( MyMessage ); Use checked exceptions for recoverable conditions and runtime exceptions for programming errors //this will not happen!!! } try {... } catch(myexception e) { System.exit(1); } MyException is better to be an unchecked exception!!! Don t ignore exceptions Include failure-capture information in detail message try {... } catch(myexception e) { } 55 56

CSC207H: Software Design. Exceptions. CSC207 Winter 2018

CSC207H: Software Design. Exceptions. CSC207 Winter 2018 Exceptions CSC207 Winter 2018 1 What are exceptions? Exceptions represent exceptional conditions: unusual, strange, disturbing. These conditions deserve exceptional treatment: not the usual go-tothe-next-step,