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Exceptions

Exceptions. Download as Power Point file for saving or printing . Overview . An exception is an unusual circumstance, such as an error condition, that must be handled in a non-standard way.

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Exceptions

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  1. Exceptions Download as Power Point file for saving or printing. Modern Programming Languages - Exceptions

  2. Overview • An exception is an unusual circumstance, such as an error condition, that must be handled in a non-standard way. • Java and many other languages (including C++), exceptions serve as a standard mechanism for handling execution error conditions. • Provides for handling unusual cases at the appropriate level(s). • For example, an airplane could have a stuck wing flap detected in a very low-level method monitoring a wing flap's position. • No ability to communicate information directly to the pilot. • A wing flap exception can be reported upward to the higher levels that call the low-level method where the exception is detected. • The higher level layers, with broader status information, could automatically adjust other control surfaces and report the wing flap exception to the pilot. Modern Programming Languages - Exceptions

  3. A Little Demo public class Test { public static void main(String[] args) { System.out.println(5/0); }} javac Test.javajava TestException in thread "main" java.lang.ArithmeticException: / by zero at Test.main(Test.java:3) Modern Programming Languages - Exceptions

  4. Exception Example public class Test { public static void main(String[] args) { try { System.out.println( 5 / 0); } catch (ArithmeticException a) { System.out.println("You're dividing by zero!"); } }} This will catch and handle any ArithmeticException. Other exceptions will still get the language system’s default behavior. javac Test.javajava TestYou're dividing by zero! Modern Programming Languages - Exceptions

  5. Exception Mechanism The exception mechanism consists of three parts: • definition of the exception class • throwing the exception when exceptional condition is detected • catching or handling the exception (try and catch). Modern Programming Languages - Exceptions

  6. Exercise 1A - List line numbers executed and output • System.out.print("1"); • try { • System.out.print("2"); • if (true) throw new Exception(); • System.out.print("3"); • } • catch (Exception e) { • System.out.print("4"); • } • finally { • System.out.print("5"); • } • System.out.println("6"); Modern Programming Languages - Exceptions

  7. Exercise 1B - List line numbers executed and output • public class Exercise1 { • static void f1() { • System.out.print("1"); • try { • System.out.print("2"); • f2(); • System.out.print("3"); • } • catch (Exception e) { System.out.print("4"); } • finally { System.out.print("5"); } • System.out.println("6"); • } • static void f2 () throws Exception { • if (true) throw new Exception(); • } • public static void main(String s[]) { f1(); } • }

  8. Exercise 1C - List line numbers executed and output • public class Exercise1 { • static void f1() throws Exception { • System.out.print("1"); • try { • System.out.print("2"); • f2(); • System.out.print("3"); • } • catch (Exception e) { System.out.print("4"); throw e; } • finally { System.out.print("5"); } • System.out.println("6"); • } • static void f2 () throws Exception { • if (true) throw new Exception(); • } • public static void main(String s[]) throws Exception { • f1(); • } • } 1245Exception thread "main" java.lang.Exception at Exercise1.f2(Exercise1.java:15) at Exercise1.f1(Exercise1.java:6) at Exercise1.main(Exercise1.java:19)

  9. Define • Generally, an exception class inherits from the parent class Exception. • The exception class definition minimally defines a class constructor but may also provide methods for analysis of the conditions causing the exception. • The following is a typical class definition for counterExceptionwith a constructor that copies a string message complaintpresumably detailing the cause of the exception. • An example of the constructor's use illustrates a new counterException created with the complaint that count failed. Modern Programming Languages - Exceptions

  10. Define and Use Example Define class counterException extends Exception {   String complaint; public counterException(String complaint){      this.complaint = complaint;   }  public String toString(){  return "counter Exception " + complaint; } } Use new counterException("count failed."); Modern Programming Languages - Exceptions

  11. Throw • When an error condition is detected, it is necessary to handle it immediately where detected or throw the exception to the calling method. • Exception handling is a mechanism for transferring control from where an error occurred to where it can be handled most appropriately. • After the exception is thrown, the throwing method terminates and execution control is immediately passed backward along the chain of callers from where the exception was thrown. • Any method along the calling chain can: • handle the exception and continue with execution, • handle the exception and again throw the exception to the calling method to handle • or do nothing, terminate immediately and let the calling method deal with the exception. • The down method below is an example of throwing an exception, in this case when the counter n becomes negative. Modern Programming Languages - Exceptions

  12. Throw Example public int down() throws counterException {   if (n <= 0) throw new counterException( n + " count Down failed.");   return --n; } • The down method is an example of throwing an exception, in this case when the counter n becomes negative. • If exception is thrown, execution does not reach return --n; Modern Programming Languages - Exceptions

  13. Catch • Handling an exception consists of trying a method that throws an exception and catching a thrown exception. • Catching an exception prevents the exception from being passed to a higher level calling method. • If an exception is not caught, the method terminates immediately and control is passed to the higher level calling method so that it might catch the exception. • Consider the following example which causes a counterException to be thrown by method down when the counter becomes negative. • The try and catch go together to define the method that throws the exception, down, and the exception to be caught, counterException. • Only if an exception is thrown will it be caught and the catch statement(s) executed. • Any statements following the catch would be executed as normal. Modern Programming Languages - Exceptions

  14. Catch Example Catch try {  aCounter.down( ); } catch (counterException ce) { System.out.println("" + ce); } Throw public int down() throws counterException {   if (n <= 0) throw new counterException( n + " count Down failed.");   return --n; } Modern Programming Languages - Exceptions

  15. Throwing exceptions to caller • When an exception is not handled in a method it must be thrown to the higher level calling method. • That method must state that it throws an exception as throws counterException. • The down method listed earlier does not handle or catch the counterException thrown so the exception is thrown back to the calling method, action. • The direct effect of down throwing an exception is to terminate its execution, passing execution to the catch, so that the remaining statements of down (i.e. return --n) are not executed. public int down() throws counterException {   if (n <= 0) throw new counterException( n + " count Down failed.");   return --n; }

  16. Finally • try – Invokes a method that throws an exception. • catch – Point of execution of exception throw. • finally - Always executed at end of a try block. Modern Programming Languages - Exceptions

  17. class counterException extends Exception {  // Define •   String complaint; •   public counterException(String c)  { •     this.complaint = c;  •   } •   public String toString( ) { •     return "counter Exception “ + complaint;  •   } • } • class counter { •   int n = 0; •   public int zero() { return n=0; } •   public int up() { return ++n; } •   public int down() throws counterException { // Throw • if (n <= 0) • throw new counterException •          (n + " count Down failed."); •     return --n; •   } • } Exception Class Example Modern Programming Languages - Exceptions

  18. public class Example { • public static void main( String args[] ) { • counter aCounter = new counter( ); • aCounter.zero( ); • aCounter.up(); • try {  aCounter.down( ); } • catch(counterException ce) {  // Catch •         System.out.println("" + ce);  •   } • try {  aCounter.down( ); } • catch(counterException ce) {  // Catch •         System.out.println("" + ce);  •   } • finally { • System.out.println(“Finally”); •   } • } • } Exception Class Use Example

  19. Exercise 2 • class counterException extends Exception { •   String complaint; •   public counterException(String c){ this.complaint = c; } •   public String toString( ) { return "counter Exception “ + complaint; } • } • class counter { •   int n = 0; •   public int zero() { return n=0; } •   public int up() { return ++n; } •   public int down() throws counterException { • if (n <= 0) throw new counterException (n + " count Down failed."); •     return --n; •   } • } • public class Example { • public static void main( String args[] ) { • counter aCounter = new counter( ); • aCounter.zero( ); • aCounter.up(); • try {  aCounter.down( ); } • catch(counterException ce) { System.out.println("" + ce); } • try {  aCounter.down( ); } • catch(counterException ce) { System.out.println("" + ce); } • finally { System.out.println(“Finally”); } • } • } • List the sequence of line numbers executed and output. • The catch is defined in main method but is executed by the throw. Is the catch visible to the throw? • What does the throw and catch resemble?

  20. Exercise 2 Continued • class counterException extends Exception { •   String complaint; •   public counterException(String c){ this.complaint = c; } •   public String toString( ) { return "counter Exception “ + complaint; } • } • class counter { •   int n = 0; •   public int zero() { return n=0; } •   public int up() { return ++n; } •   public int down() throws counterException { • if (n <= 0) throw new counterException (n + " count Down failed."); •     return --n; •   } • } • public class Example { • public static void main( String args[] ) throws Exception{ • counter aCounter = new counter( ); • aCounter.zero( ); • aCounter.up(); • aCounter.down( );  • aCounter.down( ); • System.out.println(“Completed”);  • } • } • List the sequence of line numbers executed and output. • What occurs at Line 21?

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