Java Programming Transparency No. 1-1 Java Exception Handling Cheng-Chia Chen

Basic Java Syntax Transparency No. 1-2 Contents 1.Traditional approach to program errors/exceptions 2.Java’s exception handling 3.The try statement 4.Exception propagation 5.The catch clause 6.The finally clause

Basic Java Syntax Transparency No. 1-3 Unexpected conditions which may arise during program execution  called exceptions  Examples: System file or i/o errors, disk full or unavailable, socket connection closed. Program file or i/o errors, unexpected EOF, file not found. Programmer errors, array subscript out of bounds, System errors, unable to load class file, problems with JVM.  Traditional approach to exceptions: mechanism: return value indicating occurrence of exceptions + special error_code variable storing exact type or details of errors problems: 1. Exceptions are not forced to be handled and error checking may be ignored entirely. 2. difficult to deal with when there are many types of exceptions that need to be handled.

Basic Java Syntax Transparency No. 1-4 Example: // this call may raise exceptions dbh = dbm_open(dbname, 0, GDBM_WRCREAT, … ); // check/handle all possible exceptions raised from previous call. if (dbh == NULL) { // NULL indicates exceptions logTime(); // error code stored in dbm_errno const char *ret = dbm_strerror(dbm_errno); fprintf(stderr, "ERROR Unable to open database: %s.\n", ret); fflush(stderr); return -1; }

Basic Java Syntax Transparency No. 1-5 Java’s 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 An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught  A program can therefore be separated into a normal execution flow and an exception execution flow

Basic Java Syntax Transparency No Exception Handling  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

Basic Java Syntax Transparency No Exception Handling  If an exception is ignored by the program, the program will terminate and produce an appropriate message  The message includes a call stack trace that indicates on which line the exception occurred The call stack trace also shows the sequence of method calls that lead to the occurrence of the exception. See Zero.java

Basic Java Syntax Transparency No. 1-8 Java’s call stack on executing Zero public class Zero { static int divide(int x,int y) { return x/y; // exception occurs here } public static void main(String[] args) { int x = 10; int y = 0; System.out.println ( divide(x,y)); // and here } // method main } // class Zero  java Zero  java.lang.ArithmeticException: / by zero  at Zero.divide(Zero.java:3)  at Zero.main(Zero.java:11) main(…) divide(…) runtime call stack stack frame for main(…) Exception occurs here

Basic Java Syntax Transparency No. 1-9 Catch an exception where it occurs public class Zero { static int divide(int x,int y) { try { return x/y; // exception occurs here } catch(Exception e) { return 0 ; } } public static void main(String[] args) { int x = 10; int y = 0; System.out.println ( divide(x,y)); } // method main } // class Zero  java Zero  0

Basic Java Syntax Transparency No Catch exceptions by calling methods public class Zero { static int divide(int x,int y) { try { return x/y; // Exception occurs here } catch(UnRelatedException e) { return 0 ; } // Exception not caught by this catch } public static void main(String[] args) { int x = 10; int y = 0; try{ out.println ( divide(x,y)); // Exception is propagated here }catch(Exception e) { // Exception get caught here! out.println( 1 ); } } // method main } // class Zero  java Zero  1

Basic Java Syntax Transparency No The try Statement  Used to detect exceptions that may occurs within its body (a try-block)  A try block is followed by one or more catch clauses, which contain code to process an exception  Each catch clause has an associated exception type  When an exception occurs, processing continues at the first catch clause that matches the exception type  See Product.java

Basic Java Syntax Transparency No import java.io.*; public class Product { public static void main (String[] args) { int x = UserReader.readInt(“Enter a number: "); int y = UserReader.readInt("Enter another number: "); System.out.println ("The product is: " + x*y ); } // method main } // class Adding class UserReader { public static int readInt (String prompt) { BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); int number = 0; boolean valid = false;

Basic Java Syntax Transparency No while (! valid) { System.out.print (prompt); System.out.flush (); try { // statements possibly throwing exceptions put here!! number = Integer.parseInt (stdin.readLine()); valid = true; } catch (NumberFormatException exception) { System.out.println ("Invalid input. Try again."); } catch (IOException exception) { System.out.println ("Input problem. Terminating."); System.exit(0); } return number; } // method readInt } // class UserReader

Basic Java Syntax Transparency No Exception Propagation  If it is not appropriate to handle the exception where it occurs, it can be handled at a higher level of some calling method.  Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the outermost level  A try block that contains a call to a method in which an exception is thrown can be used to catch that exception  See PropagationDemo.java

Basic Java Syntax Transparency No public class Propagation { static public void main (String[] args) { ExceptionScope demo = new ExceptionScope(); System.out.println("program beginning"); demo.level1(); System.out.println("program ending"); } // method main } // class Propagation class ExceptionScope { public void level3 (int a) { int c = 1; System.out.println("level3 beginning"); current = c / a; // divided by zero System.out.println("level3 ending"); } // method level3

Basic Java Syntax Transparency No public void level2() { System.out.println("level2 beginning"); level3 (0); System.out.println("level2 ending"); } // method level2 public void level1() { System.out.println("level1 beginning"); try { level2(); } catch (ArithmeticException p) { System.out.println (p.getMessage()); p.printStackTrace(); } System.out.println("level1 ending"); } // method level1 } // class ExceptionScope

Basic Java Syntax Transparency No main() : … demo.level1() … level1() : … try{ … level2() …} catch( ArithmeticException p) {…} …. level2() : … level3() … level3() : … c = c / 0; // divided by zero … normal flowexception flow Exception propagation

Basic Java Syntax Transparency No Types of Exceptions  An exception is either checked or unchecked  A checked exception can only be thrown within a try block or within a method that is declared (at the throws-clause of the method header) to throw that exception  The compiler will complain if a checked exception is not handled appropriately  An unchecked exception does not require explicit handling, though it could be processed that way In previous example, ArithmeticException is an unchecked exception, hence it need not be declared even it is not catched at method level3() and level2().

Basic Java Syntax Transparency No The throw Statement  A programmer can define an exception by extending the appropriate class  Exceptions are thrown using the throw statement: throw exception-object ;  See ThrowDemo.java  Usually a throw statement is nested inside an if statement that evaluates the condition to see if the exception should be thrown

Basic Java Syntax Transparency No import java.io.IOException; public class ThrowDemo { public static void main (String[] args) throws MyExp { MyExp p = new MyExp ("Alert!"); throw p; // execution never gets to this point } // method main } // class ThrowDemo class MyExp extends IOException { MyExp (String message) { super (message); } // constructor Ooops } // class Ooops

Basic Java Syntax Transparency No The finally Clause  A try statement can have an optional clause designated by the reserved word finally  If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete  Also, if an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete

Basic Java Syntax Transparency No The finally Clause  Exception Handling: try { code that may generate exceptions; } catch (Exception1 e1) { code to handle exception of type Exception1; } catch( …. ){ … } catch (ExceptionN eN) { code to handle exception of type ExceptionN; } finally { code that are guaranteed to be executed before (normally or abnormally) leaving this try-catch statement; }

Basic Java Syntax Transparency No Example finally-clause usage public boolean deposit(int account, int amt ){ int old, balance; old = balance =load(account); try { balance = balance + amt ; // suppose this operation may fail ! } catch (Exception e1) { balance = old ; // recover balance return false ; } finally { store( account, balance ); } return true; }

Basic Java Syntax Transparency No (Part of) the Exception Hierarchy Object  Throwable   Eorror : LinkageError, ThreadDeath, VirtualMachineError, AWTError  Exception: RuntimeException  ArithmeticException  IndexOutOfBoundException  NullPointerException  … IllegalAccessException NoSuchMethodException ClassNotFoundException IOException …  Note: Checked exception must be either catched or declared at the throws- clause of the method header.  All Throwable but Error and RuntimeException are checked Exceptions.

Basic Java Syntax Transparency No What can we get from an Exception java.lang.Throwable:  Constructor Summary Throwable() : Constructs a new Throwable with null as its error message string. Throwable([ [String msg] [, Throwable cause ]]) : Constructs a new Throwable with the specified error message and/or cause.  Method Summary Throwable fillInStackTrace() : Fills in the execution stack trace. String getLocalizedMessage() : Creates a localized description of this Throwable. String getMessage() : Returns the error message string of this throwable object. Throwable getCause(); void printStackTrace(), printStackTrace(PrintStream s), printStackTrace(PrinterWriter s) : Prints this Throwable and its backtrace to the standard error stream or s. StackTraceElement[] getStackTrace() void setStackTraceElement(StackTrace[]) String toString() : Returns a short description of this throwable object. java.lang.Exception: Constructors: Exception(), Exception(String). All methods are inherited from Throwable.

Basic Java Syntax Transparency No StackTraceElement  An element in a stack trace, as returned by Throwable.getStackTrace().  Each element represents a single stack frame. All stack frames except for the one at the top of the stack represent a method invocation. The frame at the top of the stack represents the the execution point at which the stack trace was generated. Typically, this is the point at which the throwable corresponding to the stack trace was created.  Methods String getClassName() String getMethodName() boolean isNativeMethod() String getFileName() int getLineNumber()