How to Read a File in Java

Lewis D.

The Problem

You need to read a file into a Java application. How can you do this?

The Solution

If you read the Sentry answer to How to Write to a File in Java, you’ll know there are many solutions to writing to files in Java. Similarly, there are many ways you can read from a file into a Java application, and each method has unique properties that make it more or less useful for your file-reading circumstances. We’ll take a look at three ways you can read a file in Java.

  • If you need to perform some ad-hoc reading of a whole file at once, use the Files class.
  • If you need to read in a large file line by line, use a BufferedReader.
  • If you need to read a file with contents separated by some delimiter, use a Scanner.

The Files Class

The Files utility class has a very useful readAllLines() method that will read all the lines of a file. The method signature denotes that the lines will be read into a simple List of String elements. If decoding the contents of the file into the standard UTF-8 charset is unsuitable, an overloaded method allows you to provide your own Charset. According to the Java documentation, readAllLines() is meant for simple ad-hoc file reading, so avoid using it for reading in large files.

Here’s an example showing how to read in a file using this method:

import; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import java.util.List; public class ReadFile { public static void main(String[] args) { Path path = Paths.get("example.txt"); try { List<String> lines = Files.readAllLines(path); } catch (IOException ex) { // handle exception... } } }

If you need to read bytes from a file (rather than lines), the readAllBytes() method can be used to read the file into a byte array:

byte[] fileBytes = Files.readAllBytes(path);

The BufferedReader Class

The BufferedReader class is an intuitive and performant approach that you can use to read character-oriented files into your Java applications. Buffering is efficient, so BufferedReader is a good choice for reading larger text files line by line.

Writing the code for a BufferedReader is a little bit more lengthy than the Files method. It is good practice to close resources once we have finished using them, or as in the example below, use the try-with-resources construct where the reader is closed for us:

import; import; import; public class ReadFile { public static void main(String[] args) { try (BufferedReader br = new BufferedReader(new FileReader("example.txt"))) { String line; while ((line = br.readLine()) != null) { // process the line (e.g. add to a List) } } catch (IOException e) { // handle exception... } } }

The Scanner Class

The methods we’ve looked at so far indiscriminately read line by line from a file. The Scanner class provides us with a way to read from files piece by piece. The Scanner works by separating the contents of a file into pieces using a delimiter, so it’s best for reading files with content that is separated by some constant value. This could be a common comma separated file, for example, but the Scanner supports any value for the delimiter.

Let’s take a look at a more unusual delimited file to illustrate this point. You have a file with the following contents that you would like to read into a List:

lion&& tiger&& leopard&& lynx

Using a Scanner, we can read the contents of the file into a List as follows:

import; import; import java.util.ArrayList; import java.util.List; import java.util.Scanner; public class ReadFile { public static void main(String[] args) throws IOException { List<String> animals = new ArrayList<>(); Scanner scanner = new Scanner(new File("example.txt")); scanner.useDelimiter("&& "); while (scanner.hasNext()) { String next =; animals.add(next); } scanner.close(); } }

Further Reading

Loved by over 4 million developers and more than 90,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.

Share on Twitter
Bookmark this page
Ask a questionJoin the discussion

Related Answers

A better experience for your users. An easier life for your developers.

© 2024 • Sentry is a registered Trademark
of Functional Software, Inc.