M OBILE C OMPUTING Syntax Errors Violate language rules program - PowerPoint PPT Presentation

Excep&ons EECS1012 M OBILE C OMPUTING § Syntax Errors Violate language rules à program won't compile Source: programmer Defense: modern IDEs expose these 😁 § Run&me Errors Make an invalid opera?on à program will crash Source: programmer, end-user, environment Defense: use a defensive and/or excep?on approach § Logic Errors Violate requirement à program will run but with a bug Source: programmer, analyst P ROF . Y. L ESPÉRANCE Defense: tes?ng (unit + integra?on) with coverage Dept. of Electrical Engineering & Computer Science 1 2 1. Error source leads to an incorrect opera&on User types in a string s. It is expected (but not a precondi?on) that s is of the form n/x, where n is a month number. Find the 2. Incorrect opera&ons may be valid or invalid three-leLer month name whose number is n, e.g. 3 à MAR 3. An invalid opera&on throws an excep&on 4. The excep&on causes a crash unless caught 1. Defensive Approach An&cipate all invalid opera&ons and guard against them Sources Programmer, End User, or Logic Handler 2. Excep&on Approach Error Environment Assume all is well and handle invalidity as an excep&on yes yes Error Valid Incorrect Runtime Exception Caught? Operation? Operations no Error no 3 4 EECS1022/Lespérance 1

Excep&ons String result = null; 
 String names = "JanFebMarAprMayJunJulAugSepOctNovDec"; String result = null; 
 int slash = s.indexOf("/"); String names = "JanFebMarAprMayJunJulAugSepOctNovDec"; if (slash != -1) int slash = s.indexOf("/"); { String left = s.substring(0, slash); String left = s.substring(0, slash); int monthNumber = Integer.parseInt(left); if (left.matches("\\d{1,2}")) { int start = 3 * (monthNumber - 1); int monthNumber = Integer.parseInt(left); result = names.substring(start, start + 3); if (monthNumber >= 1 && monthNumber <= 12) { int start = 3 * (monthNumber - 1); result = names.substring(start, start+3); } } For a fine-grained return: use else to return a custom code } 5 6 String result; 
 Object try 
 { 
 String names = "JanFebMarAprMayJunJulAugSepOctNovDec"; Th rowable int slash = s.indexOf("/"); String left = s.substring(0, slash); int monthNumber = Integer.parseInt(left); int start = 3 * (monthNumber - 1); Error Exception result = names.substring(start, start + 3); 
 } 
 catch (StringIndexOutOfBoundsException e) VirtualMachineError RuntimeException { result = null; For a fine-grained return: AssertionError IOException } - custom codes, catch (NumberFormatException e) AWTError PrinterException - e.getMessage() { result = null; - Log.getStackTraceString(e) ... ... } return result; 7 8 EECS1022/Lespérance 2

Excep&ons String result; 
 try 
 { 
 String names = "JanFebMarAprMayJunJulAugSepOctNovDec"; int slash = s.indexOf("/"); String left = s.substring(0, slash); int monthNumber = Integer.parseInt(left); int start = 3 * (monthNumber - 1); result = names.substring(start, start + 3); 
 } 
 catch (Exception e) { result = null; } return result; See EH0, EH1, EH2, and EH3.java 9 EECS1022/Lespérance 3