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1.11 Handling Exceptions
Example 1-11 shows a
program that uses the Integer.parseInt( ) method
to convert a string specified on the command line to a number. The
program then computes and prints the factorial of that number, using
the Factorial4.factorial( ) method defined in
Example 1-10. That much is simple; it takes only two
lines of code. The rest of the example is concerned with exception
handling, or, in other words, taking care of all of the things that
can go wrong. You use the try/catch statement in
Java for exception handling. The try clause
encloses a block of code from which exceptions may be thrown. It is
followed by any number of catch clauses; the code
in each catch clause takes care of a particular
type of exception.In Example 1-11, there are three possible user-input
errors that can prevent the program from executing normally.
Therefore, the two main lines of program code are wrapped in a
try clause followed by three
catch clauses. Each clause notifies the user about
a particular error by printing an appropriate message. This example
is fairly straightforward. You may want to consult Chapter 2 of Java in a Nutshell, as
it explains exceptions in more detail.
Example 1-11. FactComputer.java
package je3.basics;
/**
* This program computes and displays the
factorial of a number specified
* on the command line. It handles possible user input
errors with try/catch.
**/
public class FactComputer {
public static void main(String[ ] args) {
// Try to compute a factorial.
// If something goes wrong, handle it in the catch clause below.
try {
int x = Integer.parseInt(args[0]);
System.out.println(x + "! = " + Factorial4.factorial(x));
}
// The user forgot to specify an argument.
// Thrown if args[0] is undefined.
catch (ArrayIndexOutOfBoundsException e) {
System.out.println("You must specify an argument");
System.out.println("Usage: java FactComputer <number>");
}
// The argument is not a number. Thrown by Integer.parseInt( ).
catch (NumberFormatException e) {
System.out.println("The argument you specify must be an integer");
}
// The argument is < 0. Thrown by Factorial4.factorial( )
catch (IllegalArgumentException e) {
// Display the message sent by the factorial( ) method:
System.out.println("Bad argument: " + e.getMessage( ));
}
}
}
