10Exceptions

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10. Exceptions : 

10. Exceptions Checked vs. Unchecked

Arian 5 : 

Arian 5

June 4, 1996 : 

June 4, 1996 The inertial guidance computer had told the steering computer that the rocket had gone way off course, when in fact it was not off course at all.

They borrowed the computer from A4! : 

They borrowed the computer from A4! guidance computer converted a 64-bit floating point number into a 16-bit signed integer number. After 36 seconds N was larger than 32768, which is the largest possible 16-bit signed integer! steering computer, was fed bad numbers!

Java to the rescue : 

Java to the rescue T- 39 seconds the rocket exploded, destroying the rocket and cargo valued at half a billion dollars They needed to throw an arithmetic number EXCEPTION….

Exception Class : 

Exception Class Resides in Java.lang Subclasses of Exception are checked exceptions Try-catch statements are required! Defer error handling until later. try { doTheGoodThing()}catch(Exception e){}

Why not guard the input? : 

Why not guard the input? public double invert f(double x) { if (x==0) System.out.println(“danger x is zero!”); return 0; return 1/x; }

Create our own Exception : 

Create our own Exception class CreditCardException extends Exception { public static check (int i) throws CreditCardException { if (i <= 0) throw new CreditCardException(); } }

Checking the Card : 

Checking the Card public void checkCard(int ccn) { try { CreditCardException.check(ccn); } catch(CreditCardException e) { e.printStackTrace(); } }

Rule -try-catch : 

Rule -try-catch You must throw a checked exception OR try to catch it! public class Wow { public void printCheck(int ccn) throws CreditCardException { System.out.println(“checking ccn:”+ccn); CreditCardException.check(ccn); }}

Catching Multiple Exceptions : 

Catching Multiple Exceptions try { ..... } catch (Exception1 e) {// the subclass ..... } catch (Exception2 e) {// parent class .... }

Catch Rule : 

Catch Rule Have as many catches as you like, but you must at least one! You can always catch all exception with: try { } catch (Exception e) { e.printStackTrace(); Log.store(e); }

Example Exception : 

Example Exception class IOException extends Exception{ } class FileIOException extends IOException { } class UserIOException extends IOException { }

Finally! : 

Finally! Finally is optional in the try-catch block. try { ..... } catch (Exception e) { } finally { // this is optional }

Finally Rule1 : 

Finally Rule1 Finally is ALWAYS executed unless the JVM fails for some out-side cause. Finally is good for house keeping close a file close database connections, etc.

IsDigits : 

IsDigits public static int getNumberOfDigits(String s) { char ca[] = s.toCharArray(); int nod = 0; for (int i=0; i < ca.length; i++) { char c = ca[i]; if (Character.isDigit(c)) nod++; } return nod; }

Make a cce : 

Make a cce public class CreditCardException extends Exception { }

CheckCC : 

CheckCC public static void checkCreditCard(String cc) throws CreditCardException { if (getNumberOfDigits(cc) != 16) throw new CreditCardException(); }

Make app : 

Make app public static void main(String args[]) { try { IsDigitExample.checkCreditCard( "123467wefewfewewrwe890123456"); System.out.println("MO MONEY"); }catch(CreditCardException cce) { System.out.println("DEADBEAT!"); } }