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Lab 7 Exceptions

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;;CSE 1302 J;Fall 2014;Lab 7;Exception handling;Exceptions Aren?t Always Errors;Use the phrase ?I won the lottery for 2 million!!!?;Placing Exception Handlers;Use the 3 phrases: 10 20 30 40 50;1 fish, 2 fish;We have 3 dogs and 2 cats.;Throwing Exceptions;Run the program with: a negative number;a value between 0-16;a value > 16;For each part of the lab, turn in the source and output. Staple all three labs together in the order given.;Exceptions Aren't Always Errors;File CountLetters.java contains a program that reads a word from the user and prints the number of occurrences of each letter in the word. Save it to your directory and study it, then compile and run it to see how it works. In reading the code, note that the word is converted to all upper case first, then each letter is translated to a number in the range 0..25 (by subtracting 'A') for use as an index. No test is done to ensure that the characters are in fact letters.;1. Run CountLetters and enter a phrase, that is, more than one word with spaces or other punctuation in between. It should throw an ArrayIndexOutOfBoundsException, because a non-letter will generate an index that is not between 0 and 25. It might be desirable to allow non-letter characters, but not count them. Of course, you could explicitly test the value of the character to see if it is between 'A' and 'Z'. However, an alternative is to go ahead and use the translated character as an index, and catch an ArrayIndexOutOfBoundsException if it occurs. Since you don't want to do anything when a non-letter occurs, the handler will be empty. Modify this method to do this as follows;? Put the body of the first for loop in a try. NOTE: The body of a for loop is the code in braces under the for heading line.;? Add a catch that catches the exception, but don't do anything with it.;Compile and run your program.;2. Now modify the body of the catch so that it prints a useful message (e.g., "Not a letter") followed by the exception. Compile and run the program. Although it's useful to print the exception for debugging, when you're trying to smoothly handle a condition that you don't consider erroneous you often don't want to. In your print statement, replace the exception with the character that created the out of bounds index. Run the program again, much nicer!;// ****************************************************************;// CountLetters.java;//;// Reads a phrase from the standard input and prints the number of;// occurrences of each letter in that phrase.;//;// ****************************************************************;import java.util.Scanner;public class CountLetters;public static void main(String[] args);int[] counts = new int[26];Scanner scan = new Scanner(System.in);//get word from user;System.out.print("Enter a phrase: ");String word = scan.nextLine();//convert to all upper case;word = word.toUpperCase();//count frequency of each letter in string;for (int i=0, i < word.length(), i++);counts[word.charAt(i)-'A']++;//print frequencies;System.out.println();for (int i=0, i < counts.length, i++);if (counts [i] != 0);System.out.println((char)(i +'A') + ": " + counts[i]);Placing Exception Handlers;File ParseInts.java contains a program that does the following;? Prompts for and reads in a line of input;? Uses a second Scanner to take the input line one token at a time and parses an integer from each token as it is extracted.;? Sums the integers.;? Prints the sum.;Save ParseInts to your directory and compile and run it. If you give it the input;10 20 30 40;it should print;The sum of the integers on the line is 100.;Try some other inputs as well. Now try a line that contains both integers and other values, e.g.;We have 2 dogs and 1 cat.;You should get a NumberFormatException when it tries to call Integer.parseInt on "We", which is not an integer. One way around this is to put the loop that reads inside a try and catch the NumberFormatException but not do anything with it. This way if it's not an integer it doesn't cause an error, it goes to the exception handler, which does nothing. Do this as follows;? Modify the program to add a try statement that encompasses the entire while loop. The try and opening { should go before the while, and the catch after the loop body. Catch a NumberFormatException and have an empty body for the catch.;? Compile and run the program and enter a line with mixed integers and other values. You should find that it stops summing at the first non-integer, so the line above will produce a sum of 0, and the line "1 fish 2 fish" will produce a sum of 1. This is because the entire loop is inside the try, so when an exception is thrown the loop is terminated. To make it continue, move the try and catch inside the loop. Now when an exception is thrown, the next statement is the next iteration of the loop, so the entire line is processed. The dogs-and-cats input should now give a sum of 3, as should the fish input.;// ****************************************************************;// ParseInts.java;//;// Reads a line of text and prints the sum of the integers in the line.;//;// ****************************************************************;import java.util.Scanner;public class ParseInts;public static void main(String[] args);int val, sum=0;Scanner scan = new Scanner(System.in);String line;System.out.println("Enter a line of text");Scanner scanLine = new Scanner(scan.nextLine());while (scanLine.hasNext());val = Integer.parseInt(scanLine.next());sum += val;System.out.println("The sum of the integers on this line is " + sum);Throwing Exceptions;File Factorials.java contains a program that calls the factorial method of the MathUtils HYPERLINK "student code/MathUtils.java" HYPERLINK "student code/MathUtils.java" HYPERLINK "student code/MathUtils.java".java class to compute the factorials of integers entered by the user. Save these files to your directory and study the code in both, then compile and run Factorials to see how it works. Try several positive integers, then try a negative number. You should find that it works for small positive integers (values < 17), but that it returns a large negative value for larger integers and that it always returns 1 for negative integers.;1. Returning 1 as the factorial of any negative integer is not correct?mathematically, the factorial function is not defined for negative integers. To correct this, you could modify your factorial method to check if the argument is negative, but then what? The method must return a value, and even if it prints an error message, whatever value is returned could be misconstrued. Instead it should throw an exception indicating that something went wrong so it could not complete its calculation. You co

 

Paper#65006 | Written in 18-Jul-2015

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