File I/O Setup

If we want our program to read from a text file or write to a text file, we need to add two import statements:

import java.io.*;
import java.util.*;

We will also need to add the clause throws IOException to the end of the main method (or, when we start writing multiple methods, to the end of whatever method wants to work with a text file). The main method should instead look like this:

public static void main(String[] args) throws IOException
{

}

(We can similarly put throws IOException at the end of a different method.) The throws IOException clause 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 Text Files

If we want to read information from a text file into a program, we first 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();

Here’s an example that opens the file “in.txt” and prints every line from the file to the screen:

Scanner s = new Scanner(new File("in.txt"));
while (s.hasNext()) 
{
    String line = s.nextLine();
    System.out.println(line);
}
s.close();

If in.txt looked like this:

Hi
Testing
example

Then our program would print to the terminal:

Hi
Testing
example

You will often read files that have a bunch of information on each line – say, a bunch of names separated by spaces. To process each name, read each line (as shown above), and then use a StringTokenizer to break apart each line.

Writing to Text Files

If we want our program to print information to a text file, we first need to open a connection to that file.:

PrintWriter pw = new PrintWriter((filename);

Here, filename is the name of the text file we want to print to. If this file doesn’t exist, it will be created in the same directory as your code. If it does exist, everything in the file will be overwritten.

Now we can write to the file. Here’s how to write a single line of text (and advance to the next line):

pw.println(stuff);

Here, stuff is the String, int, char, or double that you want to write to the file. This function works exactly like System.out.println. To print a line without advancing to the next line, do:

pw.print(stuff);

When you’re done writing to the file, close your connection:

pw.close();

Closing the file is especially important after you’ve written information to it. When you write to a file, the information doesn’t get immediately written. Instead, it gets stored in a temporary buffer. When the buffer gets full, the entire buffer is written to the file at once. When you close the file, it writes whatever is in the buffer to the file. If you forget to close it, then whatever information was still in the buffer will not get written to the file.

Example

These arrays will hold people’s names and ages, respectively:

String[] names;
int[] ages;

Suppose those arrays have been initialized and filled with data. Suppose also that they are the same length, and that names[i] is a person’s name and ages[i] is the same person’s age. We want to print:

name: age

For each person to the file info.txt. Here’s how:

//open a connection to the file
PrintWriter pw = new PrintWriter("info.txt");

//print name: age for each person
for (int i = 0; i < names.length; i++) 
{
    pw.println(names[i] + ": " + ages[i]);
}

//close the file
pw.close();

Note: we could also use the printf command to print formatted values to an output file, just like we use System.out.printf to print formatted information to the console.