CS112-2012S-06 Exceptions and ArrayLists 106-0: 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 down06-1: Error Checking• We could check for any concievable errorArrayList<Integer> A;// Initialize AA.set(i, new Integer(x/y));06-2: Error Checking• We could check for any concievable errorif (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 usA.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 ourselvestry{A.set(i, new Integer(x,y));}catch (Exception e){// do some work to clean up after the exception}06-5: Try-Catch BlockCS112-2012S-06 Exceptions and ArrayLists 2try{// 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 block06-6: Exceptionsint 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: Exceptionsint 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: ExceptionsArrayList<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: ExceptionsArrayList<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!");CS112-2012S-06 Exceptions and ArrayLists 306-10: Exceptions• Variables declared within a try block are not visible outside• Actually, variables declated within any block are not visible outside the blocktry{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 Inheritance06-12: Uncaught Exceptionsint divide(int x, int y){int result = x / y;System.out.println(result);return result;}void foo(){int x = 6;x = divide(x, 2);x = divide(x, 1);x = divide(x, 0);x = divide(x, 3);}06-13: Uncaught Exceptionsint 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 Exceptionsint divide(int x, int y) void foo(){ {int result = x / y; ArrayList<int> A = new ArrayList<int>();System.out.println(result); A.add(new Integer(4));return result; A.add(new Integer(0));} A.add(new Integer(3));void divideBySelf(ArrayList<Integer> A) try{ {for (int i = 0; i < A.size(); i++ divideBySelf(A);CS112-2012S-06 Exceptions and ArrayLists 4{ }int res = divide(A.get(i), A.get(i));A.set(res); catch (ArithmeticException e)} {} System.out.println("Excp. caught!");}}06-15: Uncaught Exceptionsclass Silly {public int x;int badFunc() void bar(){ {x++; x++;int y = x / 0; foo();x++; x++;} }void foo() void start(){ {x++; trybadFunc(); {x++; 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 inheritance06-17: 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 stringsvoid reverse(ArrayList<string> list){}06-19: ArrayList Funvoid 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);}}CS112-2012S-06 Exceptions and ArrayLists 506-20: 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
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