chapter15

chapter15 - Chapter 15 Exception Handling 15.1 Throwing...

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Unformatted text preview: Chapter 15 Exception Handling 15.1 Throwing Exceptions What is a good program? A program that is reliable? Not just giving correct answer on correct input Should protect against possible errors (invalid password, not a picture file, etc) Good programs Need a robust program, reliable in the eyes of a novice user It should not ("cannot") crash easily - what if Word always crashed when you hit the control key? Good Programs How do we ensure a robust program? Option 1 return a special value to determine if the method succeeded or failed Ex. Make withdraw return true or false What's wrong with this? Calling method may not check answer Calling method may not know what to do Good Programs Option 2 - Use exceptions Exception an error condition that can occur during the normal course of a program execution Exception handling is another form of control structure (like ifs and switch statements) When an error is encountered, the normal flow of the program is stopped and the exception is handled Exceptions We say an exception is thrown when it occurs When the exception-handling code is executed, the error is caught Examples: Dividing by zero Accessing a null object Exceptions What happens when exceptions occur? An Exception object is thrown What happens when an Exception is thrown? normal execution stops and exception handling begins What does the Exception object know? the name of the problem the location where it occurred and more... Exceptions Why use exceptions? consistency (everyone else does it) Java API classes use exceptions. Other programming languages do too! flexibility programmer can decide how to fix problems. simplicity Easy to pinpoint problems. Types of Exceptions Null Pointer Exception FoodIterator fi = afc.getAllFood(); while ( !hasFood() && fi.hasNext() { Food f = fi.getNextFood(); if (f.getLocation().equals(myLoc)) myFood=f; } Types of Exceptions Arithmetic Exception: Divide by zero public double divide( int data1, int data2 ) { return data1 / data2; } Handling Exceptions Do nothing program crashes if the exception occurs! Propagate (throw) it tell the method's caller about it and let them decide what to do Resolve (try-catch) it in our method fix it or tell the user about it and let them decide what to do Handling Exceptions So far, we have let the system handle exceptions int score = in.nextInt(); If the user enters "abc123" Exception in thread "main" java.util.InputMismatchException at java.util.Scanner.throwFor(Scanner.java:819) at java.util.Scanner.next(Scanner.java:1431) at java.util.Scanner.nextInt(Scanner.java:2040) at java.util.Scanner.nextInt(Scanner.java:2000) at Test.main(Test.java:5) Handling Exceptions Says in English: System has caught an error described as a InputMismatchException Thrown because a String cannot be converted to an integer When system handles, we often get a program crash Instead of the system, we can handle to improve robustness Handling Exceptions Better solution: Instead of letting the program crash, use Exception Handling to improve program's robustness Exceptions cannot be ignored Exceptions are handled by Exception Handlers directly Throwing Exceptions If there is an error in the value of a parameter, we can throw exceptions to make the user accountable Decide what type of exception to throw Test for condition, and throw the exception if condition is violated 1. 2. Example Throw an exception object to signal an exceptional condition Example: What do we do if the amount to withdraw is greater than the balance? IllegalArgumentException: illegal parameter value Problem public class BankAccount { public void withdraw(double amount) { if (amount > balance) { ????????? } balance = balance - amount; } . . . } public class BankAccount { public void withdraw(double amount) { if (amount > balance) { IllegalArgumentException exception = new IllegalArgumentException("Amount exceeds balance"); throw exception; } balance = balance - amount; } . . . } Solution #1 public class BankAccount { public void withdraw(double amount) { if (amount > balance) { throw new IllegalArgumentException("Amount exceeds balance"); } balance = balance - amount; } . . . } Solution #2 Throwing Exceptions: Syntax throw exceptionObject; Checked/Unchecked Exceptions Checked Exception checked at compile time Complier ensures that you are handling a possible problem These errors are due to external circumstances that the programmer cannot prevent Majority occur when dealing with input and output For example, IOException Checked/Unchecked Exceptions Unchecked Exception = Runtime Exceptions Extend the class RuntimeException or Error They are the programmer's fault Examples of runtime exceptions: NumberFormatException IllegalArgumentException NullPointerException Optional to deal with these Example of error: OutOfMemoryError Can't do anything about these catastrophic problems, so don't deal with it Checked/Unchecked Exceptions Unchecked Exceptions result from deficiencies in your code, so should check on own Null object reference Sending a negative value to Math.sqrt() Checked Exceptions are not the fault of the coder Problems with the file format, user input, etc. Checked/Unchecked Exceptions Categories aren't perfect: Scanner.nextInt throws unchecked InputMismatchException Programmer cannot prevent users from entering incorrect input This choice makes the class easy to use for beginning programmers Checked/Unchecked Exceptions Deal with checked exceptions principally when programming with files and streams For example, use a Scanner to read a file String filename = . . .; FileReader reader = new FileReader(filename); Scanner in = new Scanner(reader); But, FileReader constructor can throw a FileNotFoundException Handling Checked Exceptions 1. 2. Handle the exception Tell compiler that you want method to be terminated when the exception occurs Use throws specifier so method can throw a checked exception public void read(String filename) throws FileNotFoundException { FileReader reader = new FileReader(filename); Scanner in = new Scanner(reader); . . . } Handling Checked Exceptions throws tells the compiler to "pass the buck" to the method that called this method Can propagate multiple exceptions: public void read(String filename) throws IOException, ClassNotFoundException Can also group using hierarchy If method can throw an IOException and FileNotFoundException, only use IOException Handling Checked Exceptions Why propagate when we could handle the error ourselves? We may not know how to Let user of my code decide Better to declare exception than to handle it incompetently Catching Exceptions At some point, an exception should be dealt with If not, program terminates with error message Professional code requires more sophistication cannot just allow errors to kill program What would happen if all of my.wisc.edu turned off if you entered wrong password? Catching Exceptions To deal with this problem, install exception handlers in your code to deal with possible exceptions Handlers are try/catch statements Try-Catch Put statement(s) that could cause an error in the try block Error handling code goes in catch block Only is executed if there was an error Can have multiple catch blocks, one for each possible type of exception try-catch Syntax try { <try block> } catch ( <ExceptionClass> <name> ) { <catch block> } catch ( <ExceptionClass> <name> ) { <catch block> }... try { String filename = . . .; FileReader reader = new FileReader(filename); Scanner in = new Scanner(reader); String input = in.next(); int value = Integer.parseInt(input); . . . } catch (IOException exception) { exception.printStackTrace(); } catch (NumberFormatException exception) { System.out.println("Input was not a number"); } Example 3 types of error can be thrown FileNotFoundException is thrown by FileReader constructor caught by IOException clause NoSuchElementException is thrown by Scanner.next not caught, thrown to caller NumberFormatException is thrown by Integer.parseInt() caught by second clause Catching Exceptions If there are no errors, the catch block is skipped If an exception is thrown, the try block stops executing immediately and jumps to the catch block Catching Exceptions Exception Information Why do we have Exception objects? We can get information on where the error happened and what exactly happened Exception objects have 2 methods defined: getMessage() (what happened?) printStackTrace() (where did it happen?) getMessage Returns the data that cause the error Example: ... }catch(NumberFormatException e){ System.out.println(e.getMessage()); } printStackTrace Prints out a trace of methods that caused the error starting at the root of the error Where did the exception occur? What method called this code to execute..etc. This is what the system does when an exception is thrown Example: ... }catch(NumberFormatException e){ e.printStackTrace(); } Catching ALL Exceptions public double average( String data ) { try { int sum = 0; for ( int i=0; i < data.length(); i++ ) sum += Integer.parseInt(data.charAt(i)); return sum/data.length(); } catch ( Exception e ) // catch ALL exceptions { System.out.println( e ); return 0; } } Catching ALL Exceptions Why is this probably a bad idea? Probably want to handle each type of exception differently Don't want to catch things like NullPointerExeption Catching Exceptions IMPORTANT! Order of catch blocks matters } catch (Exception e){ System.out.println(e.getMessage()); } catch (NumberFormatException e){ System.out.println("'" + str + "'not valid input, Please use digits only"); } You should go specific to generic NumberFormatException is a Exception (inheritance) specific type of the class Demo p.s.v. main( String args ) { try { String filename = args[0], input; scanner = new Scanner( new File(filename) ); input = scanner.nextLine(); int value = Integer.parseInt( input ); double reciprocal = 1.0 / value; System.out.println( "rec=" + rec ); } catch ( Exception e ) { S.o.pln( "Found Exception: " + e ); } S.o.pln( "Done" ); } String filename=args[0], input; try { scanner = new Scanner(new File(filename)); input = scanner.nextLine(); int value = Integer.parseInt( input ); double reciprocal = 1.0 / value; System.out.println( "rec=" + rec ); } catch ( NumberFormatException e ) { S.o.pln( input + " is not an integer"); } catch ( ArithmeticException e ) { S.o.pln( "Can't divide by zero" ); } catch ( FileNotFoundException e ) { S.o.pln( filename + " Not Found" ); } finally { S.o.pln( "Done" ); } Catching Exceptions What if an exception is thrown and there is no catch block to match? The system handles it (terminates program and prints the stack trace) The finally clause Recall that whenever an exception occurs, we immediately go to the catch block What if there is some code we want to execute regardless of whether there was an exception? finally block is used The finally clause try{ distance = Double.parseDouble(str); if (distance < 0){ throw new Exception("Negative distance is not valid"); } return distance; } catch (NumberFormatException e){ System.out.println("'" + str + "'not valid input, Please use digits only"); } catch (Exception e){ System.out.println(e.getMessage()); } finally { System.out.println("Done"); } The finally clause finally is executed NO MATTER WHAT Even if there is a break or return in the try block Good for "cleanup" of a method Throwable Exception Inheritance Hierarchy Exception RuntimeException NullPointerException ArithmeticException NegativeArrayIndexException IndexOutOfBoundsException ArrayIndexOutOfBoundsException StringIndexOutOfBoundsException IllegalArgumentException NumberFormatException IOException FileNotFoundException EOFException try-catch Control Flow code before try try block no exceptions occur code after try try-catch Control Flow code before try try block exception occurs catch block code after try try-catch Control Flow code before try try block no exceptions occur exception occurs catch block code after try try-catch Control Flow code before try try block no exceptions occurred finally block (if it exists) code after try try-catch Control Flow code before try try block exception occurs catch block finally block (if it exists) code after try try-catch Control Flow code before try exception occurs true matches first catch block? try block no exceptions occurred 1st catch block 2nd catch block true false matches next catch block? false finally block (if it exists) code after try Propagating Exceptions When a method may throw an exception, either directly or indirectly, we call the method an exception thrower. Every exception thrower must be one of two types: a catcher, or a propagator. Propagating Exceptions An exception catcher is an exception thrower that includes a matching catch block for the thrown exception. An exception propagator does not contain a matching catch block. A method may be a catcher of one exception and a propagator of another. Example The following figure shows a sequence of method calls among the exception throwers. Method D throws an instance of Exception. The green arrows indicate the direction of calls. The red arrows show the reversing of call sequence, looking for a matching catcher. Method B is the catcher. The call sequence is traced by using a stack. Example Propagating Exceptions If a method is an exception propagator, we need to modify its header to declare the type of exceptions the method propagates. We use the reserved word throws for this declaration. public void C( ) throws Exception { ... } public void D( ) throws Exception { ... } Propagating Exceptions Without the required throws Exception clause, the program will not compile. However, for exceptions of the type called runtime exceptions, the throws clause is optional. Propagating vs. Catching Do not catch an exception that is thrown as a result of violating a condition set by the client programmer. Instead, propagate the exception back to the client programmer's code and let him or her handle it. Exceptions in a Constructor We can thrown an exception in the constructor if a condition is violated For example, we may throw an IllegalArgumentException if a parameter value is invalid ...
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