Projeto de Software / Programação 3 Tratamento de Exceções. Baldoino Fonseca/Márcio Ribeiro

Transcription

1 Projeto de Software / Programação 3 Tratamento de Exceções Baldoino Fonseca/Márcio Ribeiro baldoino@ic.ufal.br

2 What can go wrong?! result = n1 / n2; In the following slides: 1) Analyze the code; 2) read the documentation; 3) and tell me what can go wrong 2

3 System.out.println("Enter the first number: "); int n1 = Integer.parseInt(input.next());

4 Reader reader = new FileReader("shopping-cart.json");

5 Exceptions!

6 NumberFormatException Message Exception type Exception in thread "main" java.lang.numberformatexception: For input string: "e at java.lang.numberformatexception. forinputstring(numberformatexception.java:48) at java.lang.integer.parseint(integer.java:449) at java.lang.integer.parseint(integer.java:499) at Calculadora.main(Calculadora.java:21) Exception trace Where it happened 6

7 FileNotFoundException java.io.filenotfoundexception: shopping-cart.json (No such file or directory) at java.io.fileinputstream.open(native Method) at java.io.fileinputstream.<init>(fileinputstream.java:120) at java.io.fileinputstream.<init>(fileinputstream.java:79) at java.io.filereader.<init>(filereader.java:41) at Calculadora.main(Calculadora.java:14) 7

8 What is an exception?! Shorthandforthe phrase exceptionalevent An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions. From 8

9 Why they are important? Exceptions have information about the error that happened Makes our programs (more) fault tolerant 9

10 Again: an exception is an event that happened when executing the program that interrupted the normal flow execution When this event occurs an Exception object is created This object encapsulates the exceptioninformation and provides methods to manipulate this information 10

11 There are two responsibilities Someone creates the exception new Exception(); and someone can handle it! catch (Exception e) { 11

12 Exception handling Process of responding to the occurrence of exceptions often changing the normal flow of program execution Examples Wrong input? Ask again for a right one! Locked file? Network problems? GPS waiting for location error? Wait a second and try again! End of the world? Shows at least a decent message explaining what s going on From 12

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14 How to handle exceptions In Java?!

15 We already know where the exceptions come from! System.out.println("Enter the first number: "); int n1 = Integer.parseInt(input.next()); Reader reader = new FileReader("shopping-cart.json"); Where should we handle them?! 15

16 Inside the method where it can be raised public void loadinput() { Scanner input = new Scanner(System.in); boolean correctinput = false; while (!correctinput) { try { System.out.println("Enter the first number: "); n1 = Integer.parseInt(input.next()); System.out.println("Enter the second number: "); n2 = Integer.parseInt(input.next()); correctinput = true; catch (NumberFormatException e) { System.out.println("Wrong input. " + "You typed a value that is not an integer. " + e.getmessage()); 16

17 Externally public void loadinput() throws NumberFormatException, FileNotFoundException { Scanner input = new Scanner(System.in); System.out.println("Enter the first number: "); this.n1 = Integer.parseInt(input.next()); System.out.println("Enter the second number: "); this.n2 = Integer.parseInt(input.next()); this.jsonfilereader = new FileReader("shopping-cart.json"); 17

18 The method which calls loadinput handles! public void process() { boolean ready = false; while (!ready) { try { loadinput(); ready = true; catch (NumberFormatException e) { System.out.println("Wrong input. " + "You typed a value that is not an integer. " + e.getmessage()); catch (FileNotFoundException e) { System.out.println("File not available yet. " + "Wait few seconds until it is created..."); //process... 18

19 How does it work? From

20 Stack trace getmessage() String that contains the exceptiondescription printstacktrace() main() method1() method2() java.lang.exception: Exception thrown in method2 method2() method1() main() at ShoppingCart.method2(ShoppingCart.java:9) at ShoppingCart.method1(ShoppingCart.java:10) at ShoppingCart.main(ShoppingCart.java:41) Stack 20

21 This code works fine public void process() { boolean ready = false; while (!ready) { try { loadinput(); ready = true; catch (FileNotFoundException e) { System.out.println("File not available yet. " + "Wait few seconds until it is created..."); //process... Why? 21

22 Three kinds of exceptions Checked exceptions Subject to catchblocks Examples: FileNotFoundException, SocketException Error Not subject to catchblocks External to the application: usually cannot recover Examples: OutOfMemoryError, StackOverflowError Runtime exceptions Not subject to catchblocks Usually: programming bugs, improper use of an API Examples: ArrayIndexOutOfBoundsException, 22 NullPointerException

23 So, Exceptions are classes...

24 Object Unchecked Throwable Error Exception SortedSet SortedSet RuntimeException

25 Exceptions are classes... And they use inheritance... So, can I define my own exception?

26 public class CannotSaveShoppingCartException extends Exception { public CannotSaveShoppingCartException () { super("cannot save shopping cart."); Checked public CannotSaveShoppingCartException (String message) { super(message); public class CannotSaveShoppingCartException extends RuntimeException { public CannotSaveShoppingCartException () { super("cannot save shopping cart."); Unchecked public CannotSaveShoppingCartException (String message) { super(message);

27 Raising exceptions public class FilePersistence implements PersistenceSession public void save(shoppingcart shoppingcart) throws CannotSaveShoppingCartException { try { //do something that can raise IOException catch (IOException e) { throw new CannotSaveShoppingCartException( "File 'shopping-carts.ufal' cannot be created or was not found"); Need to update the PersistenceSession interface! 27

28 Be aware of the flow!

29 Can you see any problem with out? try { // out.close(); catch (FileNotFoundException e) { System.err.println("FileNotFoundException: "+e.getmessage()); throw new RuntimeException(e); catch (IOException e) { System.err.println("Caught IOException: " + e.getmessage()); 29

30 If an exception occurs, out is never closed...

31 What about this solution? try { // out.close(); catch (FileNotFoundException e) { out.close(); System.err.println("FileNotFoundException: "+e.getmessage()); throw new RuntimeException(e); catch (IOException e) { System.err.println("Caught IOException: " + e.getmessage()); 31

32 What is going on? Cloned code

33 Finally block The finally block is always executed whether an exception is raised or not try { //do something that can raise IOException catch (IOException e) { //handles the IOException finally { //always executed 33

34 Important The finally block is a key tool for preventing resource leaks. When closing a file or otherwise recovering resources, place the code in a finally block to ensure that resource is always recovered. 34 From

35 Finally block in action! Closing the PrintWriter finally { if (out!= null) { System.out.println("Closing PrintWriter"); out.close(); else { System.out.println("PrintWriter not open"); 35

36 Exceptions in Java SE 7

37 Raising exceptions within the finally block try { URL url = new URL(" reader = new BufferedReader(new InputStreamReader(url.openStream())); String line = reader.readline(); Date date = new SimpleDateFormat("MM/DD/YY").parse(line); catch (MalformedURLException exception) { // handle passing in the wrong type of URL. catch (IOException exception) { // handle I/O problems. catch (ParseException exception) { // handle date parse problems. finally { if (reader!= null) { try { reader.close(); catch (IOException ex) { ex.printstacktrace(); Quite a bit of code to close! 37

38 Try with resources + Exception handling sharing try (BufferedReader reader = new BufferedReader( new InputStreamReader(new URL( " { String line = reader.readline(); SimpleDateFormat format = new SimpleDateFormat("MM/DD/YY"); Date date = format.parse(line); catch (ParseException IOException exception) { // handle I/O problems. catch (MalformedURLException exception) { // handle passing in the wrong type of URL. Reader will be closed automatically BufferedReader implements AutoCloseable 38

39 Use exceptions only for exceptional conditions!

40 When not to handle exceptions?

41 Avoid using exceptions for something that will happen int[] intarr =... try { int i = 0; while (true) { System.out.println(intArr[i++]); catch(arrayindexoutofboundsexception e) { 41