File Processing - Building Java Programs: A Back to Basics Approach - page 378

Unfortunately, when you try to compile a program that constructs a Scanner in
this manner, you'll run into a snag. Say you write a main method that begins by
opening a Scanner as follows:

// flawed method--does not compile
public static void main(String[] args) {
Scanner input = new Scanner(new File("hamlet.txt"));
...

}

This program does not compile. It produces a message like the following:

CountWords.java:8:
unreported exception java.io.FileNotFoundException;
must be caught or declared to be thrown
Scanner input = new Scanner(new File(hamlet.txt));
^

1 error

The issue involves exceptions, which were described in Chapter 3. Remember that
exceptions are errors that prevent a program from continuing normal execution. In
this case the compiler is worried that it might not be able to find a file called
hamlet.txt . What is it supposed to do if that happens? It won't have a file to read
from, so it won't have any way to continue executing the rest of the code.
If the program is unable to locate the specified input file, it will generate an error
by throwing what is known as a FileNotFoundException . This particular exception
is known as a checked exception.

Checked Exception
An exception that must be caught or specifically declared in the header of
the method that might generate it.

Because FileNotFoundException is a checked exception, you can't just ignore
it. Java provides a construct known as the try/catch statement for handling such
errors (described in Appendix D), but it allows you to avoid handling this error as
long as you clearly indicate the fact that you aren't handling it. All you have to do is
include a clause in the header for the main method to clearly state the fact

that your main method might generate this exception.

throws

throws Clause
A declaration that a method will not attempt to handle a particular type of
exception.