CS112 2012S 06 Exceptions and ArrayLists 06 0 Errors Errors can occur in program Invalid input bad data Unexpected situation Logic error in code Like to handle these errors gracefully not just halt the program Running a web server don t want one piece of bad data to bring the whole thing down 06 1 Error Checking We could check for any concievable error ArrayList Integer A Initialize A A set i new Integer x y 06 2 Error Checking We could check for any concievable error if i 0 i A size if y 0 A set i new Integer x y else Handle division by zero case Handle outside bounds of the array case Problems with this method 06 3 Exceptions We can let the system catch all the errors for us A set i new Integer x y Throws an exception if i 0 i A size y 0 Program ends Problems with this method 06 4 Exceptions We can let the system catch the errors for us We can catch the errors ourselves try A set i new Integer x y catch Exception e do some work to clean up after the exception 06 5 Try Catch Block 1 CS112 2012S 06 Exceptions and ArrayLists try Any Java Code catch Exception e Any Java Code If an exception is rasied inside the try block Stop immediately and execute the code in the catch block Coninue after the catch block as normal If no exception is raised inside the try block Ignore the code in the catch block 06 6 Exceptions int x int y try x 3 y 0 x x y System out println Can t get here catch Exception e System out println Exception caught System out println Done with try block 06 7 Exceptions int x int y try x 3 y 5 x x y System out println We will get here catch Exception e System out println We won t get here System out println Done with try block 06 8 Exceptions ArrayList Integer A new ArrayList Integer for int i 0 i 10 i A add new Integer i try for int i 0 i 10 i System out println i for int i 10 i 0 i System out println i catch Exception e System out println Error System out println Done with try block 06 9 Exceptions ArrayList Integer A new ArrayList Integer for int i 0 i 10 i A add new Integer i try for int i 0 i 10 i System out println A get i for int i 10 i 0 i System out println A get i catch Exception e System out println Error System out println Done with try block 2 CS112 2012S 06 Exceptions and ArrayLists 06 10 Exceptions Variables declared within a try block are not visible outside Actually variables declated within any block are not visible outside the block try ArrayList Integer A new ArrayList Integer for int i 0 i 10 i A add new Integer i catch Exception e System out println Error A set 3 new Integer 5 ERROR 06 11 Exceptions What went wrong try A set new Integer x y catch Exception e System out println e getMessage We ll do more with this after Inheritance 06 12 Uncaught Exceptions int divide int x int y int result x y System out println result return result void foo int x 6 x divide x x divide x x divide x x divide x 2 1 0 3 06 13 Uncaught Exceptions int divide int x int y int result x y System out println result return result void foo try int x 6 x divide x 3 x divide x 2 x divide x 0 x divide x 2 catch ArithmeticException e System out println Error 06 14 Uncaught Exceptions int divide int x int y int result x y System out println result return result void divideBySelf ArrayList Integer A for int i 0 i A size i void foo ArrayList int A new ArrayList int A add new Integer 4 A add new Integer 0 A add new Integer 3 try divideBySelf A 3 CS112 2012S 06 Exceptions and ArrayLists int res divide A get i A get i A set res catch ArithmeticException e System out println Excp caught 06 15 Uncaught Exceptions class Silly public int x int badFunc x int y x 0 x void foo x badFunc x void bar x foo x void start try x 0 bar catch Exception e System out println x 06 16 Different Kinds of Exceptions There are many different kinds of exceptions Array Out of bounds Arithmetic divide by zero I O file doesn t exist etc We ll come back to different kinds of exceptions after we ve covered inheritance 06 17 Errors Ahead You can warn other programmers using you code that your code might throw an exception They will need to either deal with the exception themselves or throw it on to who called them We ve already seen this class File examples in class Examples 06 18 ArrayList Fun Write a function that reverse an ArrayList of strings void reverse ArrayList string list 06 19 ArrayList Fun void reverse ArrayList string list String tmp for int i 0 i list size 2 i tmp list get i list set i list get list size 1 i list set list size 1 i tmp 4 CS112 2012S 06 Exceptions and ArrayLists 5 06 20 In Class Assignment Go to lecture note website Get ListFun java Fill in body of reverse2 so that it returns a reversed copy of the list passed in without changing the list passed in
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