23. Exception Handling, Exercises of Logic

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23. Exception Handling

Java

Fall 2009 Instructor: Dr. Masoud Yaghini

Exception Handling

Outline^
^ Exception-Handling Overview^
^ Example: No Exception Handling^
^ Example: Exception Handling^
^ When to Use Exception Handling
^ When to Use Exception Handling^
^ References

Exception Handling

Introduction
^ Exception^ –^ an indication of a problem that occurs during aprogram’s execution
^ Exception handling^ –^ resolving exceptions that may occur so program can–^ resolving exceptions that may occur so program can^ continue or terminate well
^ Exception handling enables programmers tocreate programs that are more robust

Exception Handling

Examples
^ ArrayIndexOutOfBoundsExceptionArrayIndexOutOfBoundsExceptionArrayIndexOutOfBoundsExceptionArrayIndexOutOfBoundsException^ –^ an attempt is made to access an element past theend of an array
^ NullPointerExceptionNullPointerExceptionNullPointerExceptionNullPointerException^ –^ when a

nullnull^ reference is used where an object isnullnull

-^ when a

nullnull^ reference is used where an object isnullnull expected

^ InputMismatchExceptionInputMismatchExceptionInputMismatchExceptionInputMismatchException^ –^ occurs when

ScannerScannerScannerScanner

method^

nextIntnextIntnextIntnextInt

receives a

string that does not represent a valid integer
ArithmeticExceptionArithmeticExceptionArithmeticExceptionArithmeticException – can arise from a number of different problems inarithmetic

Exception Handling

Performance Tip
^ If the potential problems occur infrequently,intermixing program and error-handling logiccan degrade a program’s performance,
^ Because the program must perform (potentially
^ Because the program must perform (potentially^ frequent) tests to determine whether the taskexecuted correctly and the next task can beperformed.

Example: No Exception Handling

Exception Handling

DivideByZeroNoExceptionHandling.java
^ Output 1: Please enter an integer numerator: 100Please enter an integer denominator: 7Result: 100 / 7 = 14

Exception Handling

DivideByZeroNoExceptionHandling.java
^ Output 2: Please enter an integer numerator: 100Please enter an integer denominator: 0 Exception in thread "main" java.lang.ArithmeticException: / by zero^ at DivideByZeroNoExceptionHandling.quotient(DivideByZeroNoExceptionHandling.java:11)^ at^ DivideByZeroNoExceptionHandling.main

(DivideByZeroNoExceptionHandling.java:

23 )

at^ DivideByZeroNoExceptionHandling.main

(DivideByZeroNoExceptionHandling.java:

23 )

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)at java.lang.reflect.Method.invoke(Method.java:597)at com.intellij.rt.execution.application.AppMain.main(AppMain.java:90)

Example: Exception Handling

Exception Handling

Example: Exception Handling
^ With exception handling, the program catchesand handles the exception
^ Next example allows user to try again if invalidinput is entered (zero for denominator, or non-^ integer input)integer input)
^ Example:^ –^ DivideByZeroWithExceptionHandling.java

Exception Handling

DivideByZeroWithExceptionHandling.java
^ Output 2:^ Please enter an integer numerator: 100Please enter an integer denominator: 0Exception: java.lang.ArithmeticException: / by zero^ Zero is an invalid denominator. Please try again.Zero is an invalid denominator. Please try again.^ Please enter an integer numerator: 100Please enter an integer denominator: 7Result: 100 / 7 = 14

Exception Handling

DivideByZeroWithExceptionHandling.java
^ Output 3:^ Please enter an integer numerator: 100Please enter an integer denominator: helloException: java.util.InputMismatchException^ You must enter integers. Please try again.You must enter integers. Please try again.^ Please enter an integer numerator: 100Please enter an integer denominator: 7Result: 100 / 7 = 14

Exception Handling

Catching Exceptions
^ catchcatchcatchcatch

block – catches (i.e., receives) and handles an exception,contains: – Begins with keyword

catchcatchcatchcatch

-^ Exception parameter in parentheses – exception^ parameter identifies the exception type and enablesparameter identifies the exception type and enables^ catchcatchcatchcatch

block to interact with caught exception object

-^ Block of code in curly braces that executes whenexception of proper type occurs

Exception Handling

Catching Exceptions
^ Matching

catchcatchcatchcatch

block

-^ the type of the exception parameter matches thethrown exception type exactly
Uncaught exception –^ an exception that occurs for which there are no^ matching

catchcatchcatchcatch^ blocks matching

catchcatchcatchcatch^ blocks