How to Read a File (Text)

Reading files from disk is an important and common task that is required in many programs. We'll take a look at one way to read text files using Java.

Using the BufferedReader Class

The java.io package contains many input and output (I/O) classes. One useful class for reading text files is the java.io.BufferedReader class. It is particularly useful because of its BufferedReader.readLine() method, which allows for reading a line of text from a file. Another class that will be used is the java.io.FileReader class, which will read a specified file. The BufferedReader will be used as a wrapper to allow the reading of file a line at a time.

The following is an example of a program that reads the contents of a text file line-by-line and reads it into a String:

import java.io.*;

/**
 * This program demonstrates how to read a text file.
 * @author coobird
 */
public class TextReadingApp
{
  public static void main(String[] args)
  {
    // Will be used later to store the contents of a file.
    String contents= "";
    
    try
    {
      // A BufferedReader is instantiated with a newly instantiated
      // FileReader as its underlying reader. 
      BufferedReader br = new BufferedReader(new FileReader("textfile.txt"));
      
      // A while-loop will continue to execute until the end of file
      // is reached, at which point br.ready() returns false.
      while (br.ready())
      {
        // Concatenates the current line to another string.
        // The newline character is added, as the readLine()
        // method returns a String without the newline.
        contents += br.readLine() + "\n";
      }
    }
    catch (FileNotFoundException e)
    {
      // Thrown by FileReader(String)
      e.printStackTrace(System.out);
    }
    catch (IOException e)
    {
      // Thrown by br.ready()
      e.printStackTrace(System.out);
    }
    
    // Displays the contents of the file.
    System.out.println(contents);
  }
}

In the above program runs as follows:

1. A BufferedReader wraps around a FileReader to read a text file called "textfile.txt".
2. Once the file is opened and is ready for reading, the while loop runs as long as there is more lines to read from the text file.
3. The BufferedReader.ready() method, which returns true when BufferedReader has more lines to read, is used as the loop condition.
4. BufferedReader.readLine() method return a line from the text file as a String without the newline and carriage feed.
5. The returned String is concatenated to the String variable contents which is used to store the text being read from the file. (Hence there's a "\n" that's concatenated to the string.)
6. At the end of the program, it displays the contents of the text file to the console.