exceptions chapter 10 instructor: scott kristjanson cmpt 125/125 sfu burnaby, fall 2013

Exceptions Chapter 10

Instructor: Scott Kristjanson

CMPT 125/125

SFU Burnaby, Fall 2013

Wk07.3 Slide 2Slides based on Java Foundations 3rd Edition, Lewis/DePasquale/Chase

2

Scott Kristjanson – CMPT 125/126 – SFU

Scope

Exceptions: The purpose of exceptions Exception messages The call stack trace The try-catch statement Exception propagation The exception class hierarchy I/O exceptions (and writing text files)


3


Exceptions

An exception is an object that describes an unusual or erroneous situationExceptions are thrown by a program, and may be caught and handled by another part of the programA program can be separated into a normal execution flow and an exception execution flowAn error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught


4


Exception Handling

The Java API has a predefined set of exceptions and errors that can occur during execution

A program can deal with an exception in one of three ways:

• ignore it

• handle it where it occurs

• handle it an another place in the program

The manner in which an exception is processed is an important design consideration


5


Uncaught Exceptions

If an exception is ignored by the program, the program will terminate abnormally and produce an appropriate message

The message includes a call stack trace that• indicates the line on which the exception occurred• shows the method call trail that lead to the attempted execution of the offending line

public class Zero

{ //-------------------------------------------------------

// Deliberately divides by zero to produce an exception.

//-------------------------------------------------------

public static void main(String[] args)

{

int numerator = 10;

int denominator = 0;

System.out.println("Before the attempt to divide by zero.");

System.out.println(numerator / denominator);

System.out.println("This text will not be printed.");

}

} Before the attempt to divide by zero.Exception in thread "main" ava.lang.ArithmeticException: / by zero

at Zero.main(Zero.java:11)

Error in file Zero.java

Error is Div by Zero

Error at line 11

Error occurred in method main

Error occurred in Class Zero


6


The try-catch Statement

To handle an exception in a program, the line that throws the exception is executed within a try block

A try block is followed by one or more catch clauses

Each catch clause has an associated exception type and is called an exception handler

When an exception occurs, processing continues at the first catch clause that matches the exception type


7


//********************************************************************

// ProductCodes.java Java Foundations

//

// Demonstrates the use of a try-catch block.

//********************************************************************

import java.util.Scanner;

public class ProductCodes

{

//-----------------------------------------------------------------

// Counts the number of product codes that are entered with a

// zone of R and and district greater than 2000.

//-----------------------------------------------------------------

public static void main(String[] args)

{

String code;

char zone;

int district, valid = 0, banned = 0;

Scanner scan = new Scanner(System.in);

System.out.print("Enter product code (STOP to quit): ");

code = scan.nextLine();

try-catch Example


8


while (!code.equals("STOP"))

{

try

{

zone = code.charAt(9);

district = Integer.parseInt(code.substring(3, 7));

valid++;

if (zone == 'R' && district > 2000)

banned++;

}

catch (StringIndexOutOfBoundsException exception)

{

System.out.println("Improper code length: " + code);

}

catch (NumberFormatException exception)

{

System.out.println("District is not numeric: " + code);

}

System.out.print("Enter product code (STOP to quit): ");

code = scan.nextLine();

}

System.out.println("# of valid codes entered: " + valid);

System.out.println("# of banned codes entered: " + banned);

}

}

The try-catch Statement


9


The finally Clause

A try statement can have an optional clause following the catch clauses, designated by the reserved word finallyThe statements in the finally clause always are executedIf no exception is generated, the statements in the finally clause are executed after the statements in the try block completeIf an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete


10


Exception Propagation

An exception can be handled at a higher level if it is not appropriate to handle it where it occurs

Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the level of the main method

A try block that contains a call to a method in which an exception is thrown can be used to catch that exception


11


//********************************************************************

// Propagation.java Java Foundations

// Demonstrates exception propagation.

//********************************************************************

public class Propagation

{

//-----------------------------------------------------------------

// Invokes the level1 method to begin the exception demonstration.

//-----------------------------------------------------------------

static public void main(String[] args)

{

ExceptionScope demo = new ExceptionScope(); // Causes an Exception!

System.out.println("Program beginning.");

demo.level1();

System.out.println("Program ending.");

}

}

Exception Propagation Example


12


public class ExceptionScope { //--------------------------------------------

// Catches and handles the exception

// that is thrown in level3.

//--------------------------------------------

public void level1() {

System.out.println("Level 1 beginning.");

try {

level2();

}

catch (ArithmeticException problem) {

System.out.println(

“\nThe exception message is: " +

problem.getMessage());

System.out.println(“\nThe call stack trace:");

problem.printStackTrace(); System.out.println();

}

System.out.println("Level 1 ending.");

}

//-------------------------------------------------

// Serves as an intermediate level. The exception

// propagates through this method back to level1.

//-------------------------------------------------

public void level2() { System.out.println("Level 2 beginning.");

level3();

System.out.println("Level 2 ending.");

}

//------------------------------------------------

// Performs a calculation to produce an exception.

// It is not caught and handled at this level.

//------------------------------------------------

public void level3() {

int numerator = 10, denominator = 0;

System.out.println("Level 3 beginning.");

int result = numerator/denominator; System.out.println("Level 3 ending.");

}

}

ExceptionScope call will indirectly call an Exception

This will cause a divide by zero exception!


13


Exception Handling Flow of Control

static public void main() { System.out.println("Program beginning."); demo.level1(); System.out.println("Program ending.");

Program beginning.Level 1 beginning.Level 2 beginning.Level 3 beginning.

The exception message is: / by zero

The call stack trace:java.lang.ArithmeticException: / by zero at ExceptionScope.level3(ExceptionScope.java:47) at ExceptionScope.level2(ExceptionScope.java:34) at ExceptionScope.level1(ExceptionScope.java:14) at Propagation.main(Propagation.java:17)

Level 1 ending.Program ending.

public void level2() { System.out.println("Level 2 beginning."); level3(); System.out.println("Level 2 ending.");

public void level3() { int numerator = 10, denominator = 0; System.out.println("Level 3 beginning."); int result = numerator/denominator; System.out.println("Level 3 ending.");}public void level1() {

System.out.println("Level 1 beginning."); try { level2(); } } catch (ArithmeticException problem) { System.out.println( “\nThe exception message is: " + problem.getMessage()); System.out.println( “\nThe call stack trace:"); problem.printStackTrace(); System.out.println(); } System.out.println("Level 1 ending.");

static public void main() { System.out.println("Program beginning."); demo.level1(); System.out.println("Program ending.");

public void level2() { System.out.println("Level 2 beginning."); level3(); System.out.println("Level 2 ending.");

public void level3() { int numerator = 10, denominator = 0; System.out.println("Level 3 beginning."); int result = numerator/denominator; System.out.println("Level 3 ending.");}public void level1() {

System.out.println("Level 1 beginning."); try { level2(); } } catch (ArithmeticException problem) { System.out.println( “\nThe exception message is: " + problem.getMessage()); System.out.println( “\nThe call stack trace:"); problem.printStackTrace(); System.out.println(); } System.out.println("Level 1 ending.");


14


The Exception Class Hierarchy

Classes that define exceptions are related by inheritance, forming an exception class hierarchy

All error and exception classes are descendents of the Throwable class

A programmer can define an exception by extending the Exception class or one of its descendants

The parent class used depends on how the new exception will be used


15


The Exception Class Hierarchy

Part of the error and exception class hierarchy in the Java API:


16


Checked Exceptions

An exception is either checked or unchecked

A checked exception either must be caught by a method, or must be listed in the throws clause of any method that may throw or propagate it

A throws clause is appended to the method header

The compiler will issue an error if a checked exception is not caught or asserted in a throws clause


17


Unchecked Exceptions

An unchecked exception does not require explicit handling, though it could be processed that way

The only unchecked exceptions in Java are objects of type RuntimeException or any of its descendants

Errors are similar to RuntimeException and its descendants in that:

• Errors should not be caught

• Errors do not require a throws clause

Use checked exceptions for recoverable conditions and runtime exceptions for programming errors


18


The throw Statement

Exceptions are thrown using the throw statement

Usually a throw statement is executed inside an if statement that evaluates a condition to see if the exception should be thrown


19


//********************************************************************

// CreatingExceptions.java Java Foundations

//

// Demonstrates the ability to define an exception via inheritance.

//********************************************************************

import java.util.Scanner;

public class CreatingExceptions

{

//-----------------------------------------------------------------

// Creates an exception object and possibly throws it.

//-----------------------------------------------------------------

public static void main(String[] args) throws OutOfRangeException

{

final int MIN = 25, MAX = 40;

Scanner scan = new Scanner(System.in);

OutOfRangeException problem =

new OutOfRangeException("Input value is out of range.");

System.out.print("Enter an integer value between " + MIN +

" and " + MAX + ", inclusive: ");

int value = scan.nextInt();

// Determine if the exception should be thrown

if (value < MIN || value > MAX)

throw problem;

System.out.println("End of main method."); // may never reach

}

}

Throwing an Exception Example


20


//********************************************************************

// OutOfRangeException.java Java Foundations

//

// Represents an exceptional condition in which a value is out of

// some particular range.

//********************************************************************

public class OutOfRangeException extends Exception

{

//-----------------------------------------------------------------

// Sets up the exception object with a particular message.

//-----------------------------------------------------------------

OutOfRangeException(String message)

{

super(message);

}

}

Throwing an Exception Example


21


A final word on Java Exceptions

You will see people use Exceptions in Java all the time, so you need to understand it but…

• It is no longer considered a best practice• Ten years ago, Java Exception Handling was frequently used• It resulted in a LOT of bad code and undetected errors• Programmers overused the concept without proper design• Programmers Threw lots of exceptions, that nobody handled!• The Handler code often did nothing with the exception except hide it!

Generally:• Avoid designing with Exceptions for most programs• Use robust design and error checking instead• Exceptions should be the exception, not the rule• Applicable in custom designed run-time environments


22


I/O Exceptions

Let's examine issues related to exceptions and I/O

A stream is a sequence of bytes that flow from a source to a destination

In a program, we read information from an input stream and write information to an output stream

A program can manage multiple streams simultaneously


23


Standard I/O

There are three standard I/O streams:

We use System.out when we execute println statementsSystem.out and System.err typically represent a particular window on the monitor screenSystem.in typically represents keyboard input, which we've used with Scanner objects


24


The IOException Class

Operations performed by some I/O classes may throw an IOException

• A file might not exist

• Even if the file exists, a program may not be able to find it

• The file might not contain the kind of data we expect

An IOException is a checked exception


25


Writing Text Files

In Chapter 4 we explored the use of the Scanner class to read input from a text file

Let's now examine other classes that let us write data to a text file

The FileWriter class represents a text output file, but with minimal support for manipulating data

Therefore, we also rely on PrintWriter objects, which have print and println methods


26


Writing Text Files

We build the class that represents the output file by combining these classes appropriately

Output streams should be closed explicitly

Let's look at a program that writes a test data file with random 2-digit numbers


27


import java.util.Random;

import java.io.*;

public class TestData

{

public static void main(String[] args) throws IOException

{ final int MAX = 10; int value;

Random rand = new Random();

String file = "test.dat";

FileWriter fw = new FileWriter (file);

BufferedWriter bw = new BufferedWriter(fw );

PrintWriter outFile = new PrintWriter (bw );

for (int line=1; line <= MAX; line++)

{

for (int num=1; num <= MAX; num++)

{

value = rand.nextInt(90) + 10;

outFile.print(value + " "); }

outFile.println(); }

outFile.close();

System.out.println("Output file has been created: " + file);

} }

Writing Text Files

46 11 43 54 54 39 79 66 44 51 22 38 62 47 66 83 42 72 94 64 64 32 95 63 13 83 89 21 85 66 32 79 49 54 61 81 91 93 68 55 14 89 29 55 89 58 80 25 18 57 44 55 87 93 31 58 56 85 91 12 25 26 60 17 96 36 36 82 30 45 52 58 72 85 82 72 10 86 98 16 54 76 29 94 12 84 86 81 41 83 90 35 46 97 23 73 10 39 88 24

Contents of file test.dat


28


Key Things to take away:

Exceptions:• Exceptions are objects that represent unusual or invalid processing• The messages printed when an exception is thrown provide a method call

stack trace• Each catch clause handles a particular kind of exception that may be

thrown within the try block• The finally clause is executed whether the try block is exited normally

or because of a thrown exception• If an exception is not caught and handled where it occurs, it is propagated

to the calling method• A Programmer must carefully consider how and where exceptions should

be handled, if at all. Avoid using them unless there is no alternative.• A new exception is defined by deriving a new class form the Exception

Class• The throws clause on a method header must be included for checked

exceptions that are not caught and handled in the method.


29


Key Things to take away:

Files:

• A Stream is a sequential sequence of bytes• A Stream can be used as a source of input or as a destination for output• The Java class library contains many classes for defining I/O streams with

various characteristics.• Output file streams should be explicitly closed, or they may not correctly

retain the data written to them.• Three public reference variables in the System class represent the

standard Input/Output streams:System.in – Standard input streamSystem.out – Standard output streamSystem.err – Standard error stream for error messages


30


References:

1. J. Lewis, P. DePasquale, and J. Chase., Java Foundations: Introduction to Program Design & Data Structures. Addison-Wesley, Boston, Massachusetts, 3rd edition, 2014, ISBN 978-0-13-337046-1

exceptions chapter 10 instructor: scott kristjanson cmpt 125/125 sfu burnaby, fall 2013

Documents