File I/O
This section will describe how to read from a text file and how to write to a text file. In both
cases, you will need to include the line:
import java.io.*;
You will also need to add:
throws IOException
to the end of the main method. Thus, the main method will look like this:
public static void main(String[] args) throws IOException {
}
This means that interacting with files can cause some unexpected problems (like if the file isn’t
there), but that we’re not going to handle them. Later in the course we will discuss how to
handle these problems.
Reading from a File
First, we need to open a connection to the file. We do this by creating a Scanner (like we do to
get user input). This time, however, we’ll connect the Scanner to a file:
Scanner s = new Scanner(new File(filename));
Here, filename is a string that is the name of the input file. This file should be stored in the
same directory as the class that’s using it. Alternatively, you can specify the absolute path of the
file (e.g., “C:\Documents and Settings\...”).
To read a single line in the file, do this:
String line = s.nextLine();
The first line of the file will be stored in the line variable. If we call nextLine() again, it
will read the second line, and so on.
To see if you’ve reached the end of the input file, use the hasNext() command. This will
return false if we’re at the end of the file, and true otherwise.
When you’re done reading from a file, you need to close it:
s.close();
