Throwing Exceptions

File Factorials.java contains a program that calls the factorial method of the MathUtils 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.

1. 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.

2. Returning 1 as the factorial of any negative integer is not correctmathematically, the factorial function is not defined for negative integers. Java class IllegalArgumentException, which extends RuntimeException, defines exceptions of such a situation. Modify your program as follows:

* Modify the header of the factorial method to indicate that factorial can throw an IllegalArgumentException.

* Modify the body of factorial to check the value of the argument and, if it is negative, throw an IllegalArgumentException. Note that what you pass to throw is actually an instance of the IllegalArgumentException class, and that the constructor takes a String parameter. Use this parameter to be specific about what the problem is.

* Compile and run your Factorials program after making these changes. Now when you enter a negative number an exception will be thrown, terminating the program. The program ends because the exception is not caught, so it is thrown by the main method, causing a runtime error.

* Modify the main method in your Factorials class to catch the exception thrown by factorial and print an appropriate message. The loop should still continue until the user chooses to exit. Think carefully where you should put the try and catch.

3. Returning a negative number for values over 16 also is not correct. The problem is arithmetic overflowthe factorial is bigger than can be represented by an int. This can also be thought of as an IllegalArgumentExceptionthis factorial method is only defined for arguments up to 16. Modify your code in factorial to check for an argument over 16 as well as for a negative argument. You should throw an IllegalArgumentException in either case, but pass different messages to the constructor so that the problem is clear.

// ****************************************************************

// Factorials.java

//

// Reads integers from the user and prints the factorial of each.

//

// ****************************************************************

import java.util.Scanner;

public class Factorials

{

public static void main(String[] args)

{

String keepGoing = "y";

Scanner scan = new Scanner(System.in);

while (keepGoing.equals("y") || keepGoing.equals("Y"))

{

System.out.print("Enter an integer: ");

int val = scan.nextInt();

System.out.println("Factorial(" + val + ") = "

+ MathUtils.factorial(val));

System.out.print("Another factorial? (y/n) ");

keepGoing = scan.next();

}

}

}

// Provides static mathematical utility functions.

//

// ****************************************************************

public class MathUtils

{

//-------------------------------------------------------------

// Returns the factorial of the argument given

//-------------------------------------------------------------

public static int factorial(int n)

{

int fac = 1;

for (int i=n; i>0; i--)

fac *= i;

return fac;

}

}