Java Exception Handling

1. Java Exception Handling • Exception = an event that occurs during the execution of a program that disrupts the normal flow of instructions: • Examples: Hard disk crash; Out of bounds array access; Divide by zero, etc • When an exception occurs, the executing method creates an Exception object and hands it to the runtime system--``throwing an exception'' • The runtime system searches the runtime call stack for a method with an appropriate handler, which ``catches the exception''

2. Java's Exception Handling Policy • Java is fussy about (some) exceptions, requiring that the programmer either: • deal with the exceptions when they occur, using try and catch, or • explicitly hand off the exception to the method that calls the method in which the exception occurs, effectively ``passing the buck'' to the calling method. • The exceptions Java is fussy about are called ``checked'' exceptions, because the compiler will check that one of the above options is satisfied

3. Advantages of Java Exception Handling • Separating Error Handling Code from ``Regular'' Code • Propagating Errors Up the Call Stack • Grouping Error Types and Error Differentiation

4. Advantage 1:Separating Error Handling Code from ``Regular'' Code Example: Here is pseudocode for reading a file into memory: readFile { open the file; determine its size; allocate that much memory; read the file into memory; close the file; }

5. Problems with readFile • What happens if the file can't be opened? • What happens if the length of the file can't be determined? • What happens if enough memory can't be allocated? • What happens if the read fails? • What happens if the file can't be closed?

6. Error Handling With Traditional Programming errorCodeType readFile{ initialize errorCode = 0; open the file; if (theFileIsOpen) { determine the length of the file; if (gotTheFileLength) { allocate that much memory; if (gotEnoughMemory) { read the file into memory; if (readFailed) { errorCode = -1; } } else { errorCode = -2; } } else { errorCode = -3; } close the file; if (theFileDidntClose && errorCode == 0) { errorCode = -4; } else { errorCode = errorCode and -4; } } else { errorCode = -5; } return errorCode; }

7. Error Handling With Java readFile{ try { open the file; determine its size; allocate that much memory; read the file into memory; close the file; } catch (fileOpenFailed) { doSomething; } catch (sizeDeterminationFailed) { doSomething; } catch (memoryAllocationFailed) { doSomething; } catch (readFailed) { doSomething; } catch (fileCloseFailed) { doSomething; } } Note that the error handling code and ``regular'' code are separate

8. Advantage 2:Propagating Errors Up the Call Stack readFile method1 { call method2; } method2 { call method3; } method3 { call readFile; } call method3 call method2 call method1 Call Stack Suppose also that method1 is the only method interested in the errors that occur within readFile. How does the error code get propogated up the stack?

9. Propagating Errors Using Traditional Programming method1{ errorCodeType error; error = call method2; if (error) doErrorProcessing; else proceed; } errorCodeType method2{ errorCodeType error; error = call method3; if (error) return error; else proceed; } errorCodeType method3{ errorCodeType error; error = call readFile; if (error) return error; else proceed; } readFile return call method3 return call method2 return call method1 Call Stack

10. Propagating Errors Using Java method1{ try { call method2; } catch (exception) { doErrorProcessing; } } method2 throws exception { call method3; } method3 throws exception { call readFile; } readFile call method3 throw call method2 call method1 Call Stack

11. Advantage 3:Grouping Error Types and Error Differentiation Java exceptions are first class Java objects, and so they are grouped into a class hierarchy. For example: Exception RunTimeException IOException NullPointerException FileNotFoundException ArithmeticException MalformedURLException

12. Example Suppose (remember, this is pseudocode): method1 { try { call method2; } catch(Exception) { doErrorProcessing; } } method2 throws ArithmeticException { ... call method3; ... } method3 throws FileNotFoundException { ... call method4; ... } method4 throws NullPointerException { ... }

13. Throwing Structure method4 call method3 call throw method2 call method1 Call Stack In this case, the exception handler in method1 must handle all exception types

14. Modified Example method1 { try { call method2; } catch(RunTimeException) { doErrorProcessing; } } method2 throws ArithmeticException { ... call method3; ... } method3 throws FileNotFoundException { ... call method4; ... } method4 throws NullPointerException { ... }

15. New Throwing Structure method4 call method3 throw call method2 call method1 Call Stack In this case, method1 will handle only runtime exceptions. Any FileNotFoundException thrown by method3 will have to be handled further down the stack.

16. Another Modified Example method1 { try { call method2; } catch(RunTimeException) { doErrorProcessing; } } method2 throws ArithmeticException { try { ... call method3; ... } catch(IOException) { doMoreErrorProcessing; } } method3 throws FileNotFoundException { ... call method4; ... } method4 throws NullPointerException { ... }

17. New Throwing Structure method4 call method3 throw call method2 call method1 Call Stack

18. FileCopy Example Revisited import java.io.*; public class FileCopy { public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); FileReader r; FileWriter w; System.out.print("Source file name: "); String inFileName = br.readLine(); r = new FileReader(inFileName); System.out.print("Destination file name: "); String outFileName = br.readLine(); w = new FileWriter(outFileName); int c; while ((c = r.read()) != -1) w.write(c); w.flush(); } }

19. If The Input File Does Not Exist In the example, the main method chooses to ``pass the buck,'' but there is nowhere to pass it to. Thus, the program will crash: 4% java FileCopy Source file name: foo Exception in thread "main" java.io.FileNotFoundException: foo (No such file or directory) at java.io.FileInputStream.open(Native Method) at java.io.FileInputStream.<init>(FileInputStream.java:103) at java.io.FileInputStream.<init>(FileInputStream.java:66) at java.io.FileReader.<init>(FileReader.java:39) at FileCopy.main(FileCopy.java:12) 5%

20. Catching the Exception public class FileCopy { public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); FileReader r; FileWriter w; System.out.print("Source file name: "); String inFileName = br.readLine(); try { r = new FileReader(inFileName); } catch(FileNotFoundException ex) { System.out.println(ex.getMessage()); System.out.print("Source file name: "); inFileName = br.readLine(); r = new FileReader(inFileName); } ... } }

21. Catching the Exception (cont'd) This approach will catch the first instance of a FileNotFoundException, but only that instance: 5% java FileCopy Source file name: foo foo (No such file or directory) Source file name: bar Exception in thread "main" java.io.FileNotFoundException: bar (No such file or directory) at java.io.FileInputStream.open(Native Method) at java.io.FileInputStream.<init>(FileInputStream.java:103) at java.io.FileInputStream.<init>(FileInputStream.java:66) at java.io.FileReader.<init>(FileReader.java:39) at FileCopy.main(FileCopy.java:19) 6% Note that you can use the getMessage() method (inherited by the Exception class) in your handler.

22. Generating True Looping Behavior public class FileCopy { public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); FileReader r = null; FileWriter w = null; boolean fileFound; do { fileFound = true; System.out.print("Source file name: "); String inFileName = br.readLine(); try { r = new FileReader(inFileName); } catch(FileNotFoundException ex) { fileFound = false; System.out.println(ex.getMessage()); } } while ( !fileFound ); ... } }

23. New Output 226% java FileCopy Source file name: foo foo (No such file or directory) Source file name: bar bar (No such file or directory) Source file name: foo foo (No such file or directory) Source file name: bar bar (No such file or directory) Source file name: X.java Destination file name: Y.java 227%

24. Another Way public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); FileReader r = getFileReader(br); FileWriter w = getFileWriter(br); int c; while ((c = r.read()) != -1) w.write(c); w.flush(); }

25. Generating Looping Behavior Through Recursion private static FileReader getFileReader(BufferedReader br) { System.out.print("Source file name: "); try { String inFileName = br.readLine(); return new FileReader(inFileName); } catch(IOException ex) { System.out.println(ex.getMessage()); return getFileReader(br); } } Note: No looping code needed. Since the exception is caught, no throws clause is necessary.

26. Generating Looping Behavior Through Recursion (cont'd) private static FileWriter getFileWriter(BufferedReader br) { System.out.print("Destination file name: "); try { String outFileName = br.readLine(); return new FileWriter(outFileName); } catch(IOException ex) { System.out.println(ex.getMessage()); return getFileWriter(br); } } Now the only calls in main that can throw an exception are read, write, and flush

27. A Version of main that Does Not throw public static void main(String[] args) { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); FileReader r = getFileReader(br); FileWriter w = getFileWriter(br); int c; try { while ((c = r.read()) != -1) w.write(c); w.flush(); } catch(IOException ex) { System.out.println(ex.getMessage()); } } Now the program will not crash if/when an I/O error occurs.

28. Checked vs. Unchecked Exceptions • A program that catches all I/O exceptions may still experience a different type of exception (say, a runtime exception) • I/O exceptions are ``checked'' by the compiler • That is, you are required to either catch them or mention them in a throws clause • Unlike I/O exceptions, runtime exceptions are ``unchecked'' • That is, you can choose to ignore them at your peril

29. Some Checked Exceptions • IOException and any of its subclasses • InterruptedException (thrown when a thread is interrupted) • Any exception you invent by subclassing Exception

30. Some Unchecked Exceptions • Subclasses of RuntimeException: • ArithmeticException • IllegalArgumentException • NumberFormatException • IndexOutOfBoundsException • ArrayIndexOutOfBoundsException • StringIndexOutOfBoundsException • NullPointerException • Those that you invent by subclassing RuntimeException

31. Exceptions vs. Errors • Exceptions are processing conditions that a program ought to be able to recover from if it is robust enough • Errors are conditions that are ordinarily not recoverable: • Death of a thread • Linking error • Virtual machine error

32. The Throwable Class • Exceptions and Errors are both subclasses of the Throwable class: Object Throwable Exception Error LinkageError ThreadDeath VirtualMachineError