File I/O - Java - page 531

PITFALL: (continued)

in Display 10.1 with the following:

try
{
PrintWriter outputStream =
new PrintWriter( new FileOutputStream("stuff.txt"));
}

This replacement looks innocent enough, but it makes the variable outputStream a
local variable for the try block, which would mean that you could not use output-
Stream outside of the try block. If you make this change and try to compile the
changed program, you will get an error message saying that outputStream when used
outside the try block is an undefined identifier.

■

PITFALL: Overwriting an Output File

When you connect a stream to a text file for writing to the text file, as illustrated by
what follows, you always produce an empty file:

outputStream =
new PrintWriter( new FileOutputStream("stuff.txt"));

If there is no file named stuff.txt , this will create an empty file named stuff.txt . If
a file named stuff.txt already exists, then this will eliminate that file and create a
new, empty file named stuff.txt . So if there is a file named stuff.txt before this file
opening, then all the data in that file will be lost. The section “The File Class” tells
you how to test to see whether a file already exists so that you can avoid accidentally
overwriting a file. The following subsection shows you how to add data to a text file
without losing the data that is already in the file. ■

Appending to a Text File
When you open a text file for writing in the way we did it in Display 10.1 and a file
with the given name already exists, the old contents are lost. However, sometimes you
instead want to add the program output to the end of the file. This is called append-
ing to a file . If you wanted to append program output to the file stuff.txt , you
would connect the file to the stream outputStream in the following manner:

appending

outputStream =
new PrintWriter( new FileOutputStream("stuff.txt", true ));