Chapter 8Exception HandlingCS 180Sunil PrabhakarDepartment of Computer Science Purdue UniversityClarificationsAuto cast from char to String does not happen. Cast between int and char happens automatically.Class is a data type. It is possible to get the class of any object. Part of Reflection in JavaAdvanced topic 23When things go wrongGood programs should be robust -- I.e., they should be able to handle exceptional situations.What happens if we are trying to input an integer value and the user enters “ten”, or “3.45”?A good program should tell the user to re-enter a valid integer.So far, a situation such as this would result in the termination of our program when we execute Integer.parseInt() on this invalid string.How do we prevent this?4Handling errorsOne idea is to use if -then style tests whenever we expect that an error may arise.This is the style in C -- return values can signal the existence of an error.But this is clumsy, and inelegant. In Java, the exception handling mechanism is used instead.Unexpected (or unusual) cases are handled by a special type of control flow.5ExceptionsAn exception is used to indicate that something unusual (that prevents regular processing) has occurred.When an exception occurs, or is thrown, an Exception object is created, and the normal sequence of flow is terminated. An exception handling mechanism is invoked which is responsible for handling or catching the thrown exception.6Uncaught ExceptionsWhen an exception is thrown, and the program does not specify how to catch it, it causes the program to terminate:import javax.swing.*;public class Test{public static void main(String[] args){String inputStr;int i;inputStr = JOptionPane.showInputDialog(null, “Enter Deposit Amount”);i = Integer.parseInt(inputStr);}}Catching an exception7String inputStr;int i;inputStr = JOptionPane.showInputDialog(null, “Enter an integer”);try{i = Integer.parseInt(inputStr);} catch (Exception e){System.out.println(“Invalid integer”);}Exception control-flow8. . .try{ . . . stmt; . . .} catch (Exception e){ . . .}. . .. . .try{ . . . stmt; . . .} catch (Exception e){ . . .}. . . Exception is thrown when executing this statement.No exception Exception thrown9Exception objectAn exception is thrown by creating an Exception object.The exception object is passed to the catch block as a parameter.It contains details about the actual exception that was thrown.try { . . .} catch (Exception e){ . . .}e is a catch block parameter corresponding to the exception object.10Exception objectThe exception object contains details about the exception.The getMessage() method simply returns a string of text that describes the exception.The printStackTrace() method gives us the order (and line numbers) in which methods had been called when the exception took place. In reverse order of the calls The last method call is listed first, main is last.11The Exception HierarchyExceptionNullPointerExceptionArithmeticExceptionIllegalArgumentExceptionNumberFormatExceptionRunTimeExceptionIOExceptionMany more.See Java APISQLException12Multiple catch BlocksIf more than one type of exception can take place, we may want to handle each one differently.A single try-catch statement can include multiple catch blocks, one for each type of exception.Only the first matching catch block is executed.Matching is based on the class of the exception.Make sure to list classes lower in the hierarchy before listing classes higher up.13Multiple catch BlocksStudent s;try { . . . i = Integer.parseInt(inputStr); . . . s.getName();} catch (NumberFormatException e){ . . . // code to handle NumberFormatExceptions.} catch (NullPointerException e){ . . . // code to handle NullPointerExceptions.} catch (Exception e){ . . . // code to handle all other exceptions.}14Terminating a programIt is possible to terminate a program at any point in its execution (maybe because a very serious error has occurred).This is achieved by calling System.exit(0)This call takes any integer value as a parameter.The program is immediately terminated.15The finally BlockThere are situations where we need to take certain actions regardless of whether an exception is thrown or not.We place statements that must be executed regardless of exceptions, in the finally block.Commonly used to perform cleanup (e.g., closing disconnecting from a database, or closing a network connection)Exception control-flow16. . .try{ . . . stmt; . . .} catch (Exception e){ . . .}finally { . . .}. . .. . .try{ . . . stmt; . . .} catch (Exception e){ . . .}finally { . . .}. . . Exception is thrown when executing this statement.No exception Exception thrownfinally block is always executed.17Salient pointsIf multiple catch blocks are defined they are tested in order -- only the first that matches the thrown exception gets executed.List them from more specific to general.CAUTION: if A is a subclass of B, then an exception of class A is also an exception of class B!Even if there is a return from the try or catch blocks, the finally block is executed before returning!If no matching catch block is found for an exception, the finally block gets executed18Caution: order of catch blocksStudent s;try { . . . i = Integer.parseInt(inputStr); . . . s.getName();} catch (Exception e){ . . . // code to handle general exceptions.} catch (NullPointerException e){ . . . // code to handle NullPointerExceptions.} catch (NumberFormatException e){ . . . // code to handle NumberFormatExceptions.} Will never get executed!Propagating exceptionsIf an exception occurs and there is no matching catch block, then the exception is propagated.control passes to the callerif the caller has no matching catch block, the same happenseventually, if the main method does not handle the exception, the runtime system handles it.19Exception handling20public static void main(String[] args){...a.methodA();...}public void methodA(){try{methodB();} catch (NumberFormatException e){. . .}. . .}public void methodB(){stmt;}NumberFormatException is thrown when executing this statement.Exception is propagatedException is handled.Exception handling21public static void main(String[] args){...a.methodA();...}public void methodA(){try{methodB();} catch (NumberFormatException e){. . .}. . .}public void methodB(){stmt;}NullPointerException is thrown when