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Printing to a File

Printing to a File#

Note

Above, you’ll see there’s a video titled CSE 121: Printing to a File Video Walkthrough. The video and the reading both have the same information! You’re not required to go through both the video and the reading, as the video just walks through the reading to help contextualize it!

Printing With PrintStream#

So far our view of file processing has been very one-sided in that we’ve only been taking data from files, and not putting any data into them. Java has a construct though that allows us to print to a File using the same syntax that we use to print to the console, with a construction similar to that of a Scanner. Given a file "output.txt" that we want to print to, we would construct a PrintStream like this:

Note

Just like when creating a Scanner over a File, we do need to include a throws FileNotFoundException on the headers of methods that create a PrintStream.

PrintStream output = new PrintStream(new File("output.txt"));

Notice that, similarly to with Scanners, you pass a Java File construct representing the file you wish to print to into the new PrintStream() call. PrintStreams can actually be made by just passing in the name of the File as a String, but creating the File construct is the preferred method of PrintStream creation in this class.

PrintStream methods#

The PrintStream construct conveniently implements the print and println methods for us to use. Given the PrintStream created above, we would use the methods as such:

  • output.print()

    • Prints the given value

  • output.println()

    • Prints the given value and then terminates the line

As an example of these methods in use, here is a basic program that prints increasing digits from 1 to 10 on new lines to a file named "output.txt":

import java.io.*;

public class ExampleProgram {
    public static void main(String[] args) {
        PrintStream output = new PrintStream(new File("output.txt"));
        
        for (int i = 1; i <= 10; i++) {
            output.println(i);
        }
    }
}

Caution

Notice the import statement at the top of the program. The PrintStream and File constructs are both from the java.io library, and so we must import said library before we can use constructs from it.

The program above would produce the following output in a text file named output.txt in the same directory as the program is located:

1
2
3
4
5
6
7
8
9
10

We now know all that we need to set up a PrintStream and print output to a File! On the next slide we have a code challenge for you to test these new skills. Make sure to read the description on the left side of the screen for information on what to do before starting the challenge.

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