A sample print out is: is is -11 key entered was: w

Transcription

1 Lab 9 Lesson 9-2: Exercise 1, 2 and 3: Note: when you run this you may need to maximize the window. The modified buttonhandler is: private static class ButtonListener implements ActionListener public void actionperformed(actionevent event) char code; int answer; int one; int two; code = letter.gettext().charat(0); switch (code) //'Q' code moved for Lesson 9-2 Exercise 1 // lowercase case letters added for Exersise 2 // default added for Exercise 3 case 'Q' : case 'q' : outframe.dispose(); System.exit(0); case 'A' : case 'a' : one = Integer.parseInt(first.getText()); answer = (one + two); System.out.println(one + " + " + two case 'S' : case 's' : one = Integer.parseInt(first.getText()); answer = (one - two); System.out.println(one + " - " + two default : System.out.println("key entered was: " + code + "\nplease enter a valid key - A, a, S, s, Q, q"); // moved for Lesson 9-2 Exercise 1 first.settext(""); second.settext(""); letter.settext(""); A sample print out is: is is -11 key entered was: w

2 please enter a valid key - A, a, S, s, Q, q is 25 Exercise 4 and 5: A sample of the program covering exercises 4 and 5 is: // Program Shell1 counts punctuation marks in a file. import java.io.*; public class Shell1 public static BufferedReader infile; public static void main(string[] args) throws IOException, StringIndexOutOfBoundsException String line; infile = new BufferedReader(new FileReader("Shell1.dat")); char symbol; int periodct = 0; int commact = 0; int questionct = 0; int colonct = 0; int semicolonct = 0; int blankct = 0; line = infile.readline(); while (line!= null) // Loop till end of data for (int count = 0; count < line.length(); count++) // Loop till end of line // Note - use of short cut operator ++ which increments the variable by 1 // see page 457 of the main text symbol = line.charat(count); switch (symbol) case '.' : periodct++; case ',' : commact++; case '?' : questionct++; case ':' : colonct++; case ';' : semicolonct++; case ' ' : blankct++; line = infile.readline(); System.out.println( "count of '.' is " + periodct); System.out.println( "count of ',' is " + commact); System.out.println( "count of '?' is " + questionct); System.out.println( "count of ':' is " + colonct);

3 System.out.println( "count of ';' is " + semicolonct); System.out.println( "count of ' ' is " + blankct); The output produced is: count of '.' is 6 count of ',' is 3 count of '?' is 3 count of ':' is 4 count of ';' is 2 count of ' ' is 11 Lesson 9-3: Exercise 1 The sample output is: The first sum is The second sum is Exercise 2 NOTE the second loop only works because there is at least one positive number to read before the negative number is read. The output is the same as Exercise 1 above. The loops are now: counter = 1; do // Ten values read and summed counter++; while (counter <= 10); System.out.println("The first sum is " + sum); do // Values are read and summed until a negative is read while (value >= 0); System.out.println("The second sum is " + sum); Exercise 3 The same NOTE as exercise 2 above applies. The second loops are not examples of good programming practice. The while loop in exercise 1 is the safest solution. The output is the same as Exercise 1 above. The loops are now: for (counter = 1; counter <=10; counter++) // Ten values read and summed

4 System.out.println("The first sum is " + sum); for (counter = 1; value >= 0; counter++) // Values are read and summed until a negative is read ; System.out.println("The second sum is " + sum); Exercise 4 File looping.d2 has eleven numbers. The first ten are positive numbers and the last number is a negative number. The program fails attempting to read past the last number. The safest correction is to replace the second loop with the loop from the initial program in exercise 1. This then gives the correct results: The first sum is The second sum is 0 Lesson 9-4: Exercise 1 NOTE: additional error trapped to handle hitting the Enter button without any action entered. The new button handler is private static class ButtonListener implements ActionListener public void actionperformed(actionevent event) char code; int answer; int one; int two; //check added to ensure trap no input as an error if (letter.gettext().length() > 0) code = letter.gettext().charat(0); else code = ' '; try switch (code) //'Q' code moved for Lesson 9-2 Exercise 1 // lowercase case letters added for Exersise 2 // default added for Exercise 3 case 'Q' : case 'q' : outframe.dispose(); System.exit(0); case 'A' : case 'a' : one = Integer.parseInt(first.getText());

5 answer = (one + two); System.out.println(one + " + " + two case 'S' : case 's' : one = Integer.parseInt(first.getText()); answer = (one - two); System.out.println(one + " - " + two default : throw new MyException(code); catch (MyException myexc )System.out.println(myExc.getMessage()); // moved for Lesson 9-2 Exercise 1 first.settext(""); second.settext(""); letter.settext(""); The Class MyException is: public class MyException extends Exception public MyException(char code) // constructor with String super("key entered was: " + code + "\nplease enter a valid key - A, a, S, s, Q, q"); A sample test run showing the error is: Exercise 2 Exercise is is -22 key entered was: w please enter a valid key - A, a, S, s, Q, q is -11 All the classes that use file IO already use IOException, however they could trap the errors in the programs and field them directly see Exercise 3 for an example. An example code fragment changes Loops.java to prompt the user for the filename, catch an invalid filename, and reprompt the user until the correct name has been input: public static BufferedReader keyinput; public static BufferedReader indata; public static void main(string[] args) throws IOException int value; boolean notfound; String filename;

6 // used to input the file name keyinput = new BufferedReader(new InputStreamReader(System.in)); do notfound = false; System.out.println("please enter filename to read"); filename = keyinput.readline(); try indata = new BufferedReader(new FileReader(fileName)); catch (IOException ex) System.out.println("File not found"); notfound = true; while (notfound); // end of do Lesson 9-5: Exercise 1, 2, 3 and 4: The program (with corrections) is: // Program Bugs demonstrates various looping structures. import java.io.*; public class Bugs public static BufferedReader indata; public static void main(string[] args) throws IOException int filefound; try // corrected for exercise 2 indata = new BufferedReader(new FileReader("Bugs.dat")); filefound = 1; catch (IOException exception) filefound = 2; int value; switch (filefound) case 2 : System.out.println("Bugs.dat not found"); // added to correct error in exercise 1 case 1 : // do loop int counter = 1; int do counter++; // added to correct bug in exercise 3 while (counter <= 10); System.out.println(sum); // for loop

7 for (counter = 1; counter <= 10; counter++) // counter++; to correct the error in exercise 4 System.out.println(sum);