PROJECT: Exception Handling

Given a program (Test.java in HW8.zip) that gets two numbers from user input and then carries out a division, revise it to handle exceptional inputs. The given program does those:

Prompt 1

Input 1

Prompt 2

Input 2

Calculation

For the purpose of this project, input 1 should be a double value and input 2 should be a non-zero int value. Upon an invalid input, your program should allow a user to re-enter a piece of data. This would require a loop to validate each input and allow for additional data entry. We need a loop instead of an if-else as you can't assume a second input would be valid.

You will complete this exercise using two approaches:

a. use try-catch.

b. Use defensive coding: check for invalid data ahead of time.

Save each version in a different source code file.

Version A: Try-Catch

Because invalid data must be removed from an input stream object before additional input could happen, you need to read in each number as a string and then parse the data in the string.

Follow this pseudo code for the try-catch version:

Do // loop if wrong type of input

isValid true

Try

Prompt 1

Input 1: read a string and parse double

Catch XxxException

isValid false

display err msg

End Try-Catch

While !isValid

Do // loop if wrong type or zero

isValid true

Try

Prompt 2

Input 2: read a string and parse int

If Zero

isValid false

display err msg

End If

Catch XxxxException

isValid false

display err msg

End Try-Catch

While !isValid

Calculation

For full credit, you must use proper exception classes instead of a generic one. The try-catch version may work like this:

Enter dividend: 4.2.3

Invalid input: multiple points

Enter dividend: 4/5

Invalid input: For input string: "4/5"

Enter dividend: 4.5

Enter divisor: 1.5

Invalid input: (divisor) For input string: "1.5"

Enter divisor: 0

Input error: divisor can't be zero. Please input again.

Enter divisor: 3

4.5 / 3 = 1.5

User input is provided right after each "Enter xxx: ". The message for 0 input is a fixed message. The message for other invalid input is generated with:

System.out.println("Invalid input: " +yourExceptionObj.getMessage());

Or

System.out.println("Invalid input: (divisor) " +yourExceptionObj.getMessage()); // for input 2

Version B: Defensive Coding

You need to use the hasNextXXX methods of Scanner class for the defensive coding version.

public boolean hasNextDouble()

public boolean hasNextInt()

A hasNextXXX method peeks (does NOT read) into user input to decide if the next string to read contains a valid XXX. Use Java API 8 documentation if needed. The following snippet of code illustrates how hasNextXXX may be used:

if (stdIn.hasNextDouble()) { //check before read

dividend = stdIn.nextDouble();

}

else {// no valid input

stdIn.next(); // read string and discard

System.out.println("Invalid dividend");

isValid = false;

}

Follow this pseudo code for defensive coding version:

Do // first num

isValid true

Prompt 1

If has double input

Input 1: nextDouble()

Else

extract input: next()

display err msg

isValid false

End If

While !isValid

Do // second num

isValid true

Prompt 2

If has int input

Input 2: nextInt()

If zero

display err msg

isValid false

End If

Else

extract input: next()

display err msg

isValid false

End If

While !isValid

calculation

This version may work like this:

Enter dividend: 4.2.3

Invalid dividend

Enter dividend: 4/5

Invalid dividend

Enter dividend: 4.5

Enter divisor: 1.5

Invalid divisor

Enter divisor: 0

Input error: divisor can't be zero. Please input again.

Enter divisor: 3

4.5 / 3 = 1.5

Overall comment your program appropriately. Pay attention to standard stuff like coding style, indention, heading, and curly braces.

/**

* Test.java

* CS219. Exception Handling practice

*

* If input is of wrong format, or if 0 is provided for divisor (the 2nd input),

* keep asking until valid data is provided.

*/

import java.util.Scanner;

public class Test {

public static void main(String[] args) {

Scanner stdIn = new Scanner(System.in);

double dividend;

int divisor;

// input

System.out.print("Enter dividend: ");

dividend = stdIn.nextDouble();

System.out.print("Enter divisor: ");

divisor = stdIn.nextInt();

stdIn.close();

// process and result

System.out.println(dividend + " / " + divisor + " = " + dividend / divisor);

} // end main

} // end Test class