Exceptions Chapter 10. Instructor: Scott Kristjanson CMPT 125/125 SFU Burnaby, Fall 2013

Transcription

1 Exceptions Chapter 10 Instructor: Scott Kristjanson CMPT 125/125 SFU Burnaby, Fall 2013

2 2 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) Wk09.3 Slide 2

3 3 Exceptions An exception is an object that describes an unusual or erroneous situation Exceptions are thrown by a program, and may be caught and handled by another part of the program A program can be separated into a normal execution flow and an exception execution flow An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught Wk09.3 Slide 3

4 Exception Handling 4 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 Wk09.3 Slide 4

5 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; Error occurred in Class Zero Error occurred in method main Error in file Zero.java Error at line 11 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 is Div by Zero Wk09.3 Slide 5

6 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 Wk09.3 Slide 6

7 try-catch Example 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 // 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(); Wk09.3 Slide 7

8 The try-catch Statement 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); Wk09.3 Slide 8

9 9 The finally Clause A try statement can have an optional clause following the catch clauses, designated by the reserved word finally The statements in the finally clause always are executed If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete If an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete Wk09.3 Slide 9

10 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 Wk09.3 Slide 10

11 Exception Propagation Example 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."); Wk09.3 Slide 11

12 ExceptionScope call will indirectly call an Exception 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()); // // 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."); System.out.println( \nthe call stack trace:"); int result = numerator/denominator; problem.printstacktrace(); System.out.println("Level 3 ending."); System.out.println(); System.out.println("Level 1 ending."); This will cause a divide by zero exception! Wk09.3 Slide 12

13 Exception Handling Flow of Control 13 static public void main() System.out.println("Program beginning."); demo.level1(); System.out.println("Program ending."); public void level1() System.out.println("Level 1 beginning."); try level2(); catch (ArithmeticException problem) System.out.println( \nthe exception message is: " + problem.getmessage()); public void level3() int numerator = 10, denominator = 0; System.out.println("Level 3 beginning."); int result = numerator/denominator; System.out.println("Level 3 ending."); Program beginning. Level 1 beginning. Level 2 beginning. Level 3 beginning. The exception message is: / by zero System.out.println( \nthe call stack trace:"); problem.printstacktrace(); System.out.println(); System.out.println("Level 1 ending."); public void level2() System.out.println("Level 2 beginning."); level3(); System.out.println("Level 2 ending."); 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. Wk09.3 Slide 13

14 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 Wk09.3 Slide 14

15 15 The Exception Class Hierarchy Part of the error and exception class hierarchy in the Java API: Wk09.3 Slide 15

16 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 Wk09.3 Slide 16

17 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 Wk09.3 Slide 17

18 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 Wk09.3 Slide 18

19 Throwing an Exception Example 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 Wk09.3 Slide 19

20 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); Wk09.3 Slide 20

21 Throwing an Exception bypasses normal Flow of Control 21 Normal Flow of Control A calls B B opens file and calls C C returns to B B closes file and returns to A Flow of Control with Exceptions A calls B B opens files and calls C C Throws Exception X A Catches Exception X File remains open! (A bug!) Method A Method A Catch X call B B returns to A call B Open File Method B Close File Open File Method B Close File call C C returns to B call C Method C Method C Throw X Wk09.3 Slide 21

22 22 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, but nobody handled them! 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 Wk09.3 Slide 22

23 23 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 Wk09.3 Slide 23

24 24 Standard I/O There are three standard I/O streams: We use System.out when we execute println statements System.out and System.err typically represent a particular window on the monitor screen System.in typically represents keyboard input, which we've used with Scanner objects Wk09.3 Slide 24

25 25 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 Wk09.3 Slide 25

26 26 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 print and println methods objects, which have Wk09.3 Slide 26

27 27 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 Wk09.3 Slide 27

28 Writing Text Files 28 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 ); Contents of file test.dat 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); Wk09.3 Slide 28

29 29 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. Wk09.3 Slide 29

30 30 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 stream System.out Standard output stream System.err Standard error stream for error messages Wk09.3 Slide 30

31 31 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 Wk09.3 Slide 31