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 java.io.IOException;
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 java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

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 java.io.File;
import java.io.IOException;
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 = scanner.next();
            animals.add(next);
        }
        scanner.close();
    }
}

Further Reading

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