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Reading text files in Java

In Reading text files in Java tutorial we show how to read text files in Java. We use build-in tools including FileReader, InputStreamReader, and Scanner. In addition, we use API Google Guava library.

Google Guava is set of common libraries for Java; the set includes IO API, too.

The following examples use this text file.

src/resources/thermopylae.txt
The Battle of Thermopylae was fought between an alliance of Greek city-states, 
led by King Leonidas of Sparta, and the Persian Empire of Xerxes I over the 
course of three days, during the second Persian invasion of Greece. 

The file is located in the src/resources/ directory.

Java read text classes

We can use the following Java classes to read text files in Java.

Java read text file with FileReader

FileReader is a class used for reading character files. It reads text from character files using a default buffer size. Decoding from bytes to characters uses either a specified charset or the platform's default charset.

Note: In the past, FileReader relied on the default platform's encoding. Since Java 11, the issue was corrected. It is possible now to explicitly specify the encoding.

com/zetcode/FileReaderEx.java
package com.zetcode;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.nio.charset.StandardCharsets;

public class FileReaderEx {

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

        var fileName = "src/resources/thermopylae.txt";

        try (BufferedReader br = new BufferedReader(
                new FileReader(fileName, StandardCharsets.UTF_8))) {

            var sb = new StringBuilder();

            String line;
            while ((line = br.readLine()) != null) {

                sb.append(line);
                sb.append(System.lineSeparator());
            }

            System.out.println(sb);
        }
    }
}

The code example reads text from the thermopylae.txt file.

var fileName = "src/resources/thermopylae.txt";

In the fileName variable, we store the path to the file.

try (BufferedReader br = new BufferedReader(
    new FileReader(fileName, StandardCharsets.UTF_8))) {

The FileReader takes the file name as the first parameter. The second parameter is the charset used. The FileReader is passed to the BufferedReader, which buffers read operations for better performance. This is a try-with-resources statement which ensures that the resource (the buffered reader) is closed at the end of the statement.

var sb = new StringBuilder();

String line;
while ((line = br.readLine()) != null) {

    sb.append(line);
    sb.append(System.lineSeparator());
}

System.out.println(sb);

Printing lines to the console consumes additional resources. Therefore, we use the StringBuilder to build the output string and print it in one operation. This is an optional optimization. The System.lineSeparator() returns the system-dependent line separator string.

Java read text file with Files.readAllLines

The Files.readAllLines() method reads all lines from a file. This method ensures that the file is closed when all bytes have been read or an exception is thrown. The bytes from the file are decoded into characters using the specified charset.

Note that this method reads the whole file into the memory; therefore, it may not be suitable for very large files.

com/zetcode/ReadAllLinesEx.java
package com.zetcode;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;

public class ReadAllLinesEx {

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

        var fileName = "src/resources/thermopylae.txt";

        List<String> lines = Files.readAllLines(Paths.get(fileName),
                StandardCharsets.UTF_8);

        for (String line : lines) {

            System.out.println(line);
        }
    }
}

The contents of the thermopylae.txt file are read and printed to the console using the Files.readAllLines() method.

Reading text file with Java 8 streaming API

Another option to read text files is to use the Java 8 streaming API. The Files.lines() reads all lines from a file as a stream. The bytes from the file are decoded into characters using the StandardCharsets.UTF-8 charset.

com/zetcode/FilesLinesEx.java
package com.zetcode;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class FilesLinesEx {

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

        var fileName = "src/resources/thermopylae.txt";

        Files.lines(Paths.get(fileName)).forEachOrdered(System.out::println);
    }
}

The contents of the thermopylae.txt file are read and printed to the console using the Files.lines() method.

Java read text file with Scanner

A Scanner is simple text scanner which can parse primitive types and strings using regular expressions.

com/zetcode/ScannerEx.java
package com.zetcode;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ScannerEx {

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

        var fileName = "src/resources/thermopylae.txt";

        try (var scanner = new Scanner(new File(fileName))) {

            while (scanner.hasNext()) {

                String line = scanner.nextLine();
                System.out.println(line);
            }
        }
    }
}

The example reads a text file using a Scanner.

while (scanner.hasNext()) {

    String line = scanner.nextLine();
    System.out.println(line);
}

The file is read line by line with the nextLine() method.

Java read text file with InputStreamReader

InputStreamReader is a bridge from byte streams to character streams. It reads bytes and decodes them into characters using a specified charset.

com/zetcode/InputStreamReaderEx.java
package com.zetcode;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;

public class InputStreamReaderEx {

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

        var fileName = "src/resources/thermopylae.txt";

        try (var br = new BufferedReader(new InputStreamReader(
                new FileInputStream(fileName), StandardCharsets.UTF_8))) {

            String line;

            while ((line = br.readLine()) != null) {

                System.out.println(line);
            }
        }
    }
}

The example reads a text file using an InputStreamReader.

try (var br = new BufferedReader(new InputStreamReader(
        new FileInputStream(fileName), StandardCharsets.UTF_8))) {

The InputStreamReader is created from a FileInputStream, which creates an input stream by opening a connection to an actual file. The InputStreamReader is then passed to a BufferedReader for better efficiency.

Java 7 introduced a more convenient API to work with an InputStreamReader. A new buffered InputStreamReader can be created with Files.newBufferedReader.

com/zetcode/InputStreamReaderEx2.java
package com.zetcode;

import java.io.BufferedReader;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;

public class InputStreamReaderEx2 {

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

        var fileName = "src/resources/thermopylae.txt";
        var filePath = Paths.get(fileName);

        try (BufferedReader br = Files.newBufferedReader(
            filePath, StandardCharsets.UTF_8)) {

            String line;

            while ((line = br.readLine()) != null) {

                System.out.println(line);
            }
        }
    }
}

The example reads the thermopylae.txt file with the Files.newBufferedReader() method.

Java read text file with Files.readAllBytes

The Files.readAllBytes() method reads all the bytes from a file. It ensures that the file is closed when all bytes have been read.

com/zetcode/ReadAllBytesEx.java
package com.zetcode;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class ReadAllBytesEx {

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

        var fileName = "src/resources/thermopylae.txt";
        var filePath = Paths.get(fileName);

        byte[] data = Files.readAllBytes(filePath);
        var content = new String(data);

        System.out.println(content);
    }
}

The example reads all bytes from a file and passes them to the String constructor.

Java read text with Files.readString

Java 11 introduces a convenient method that allows to read the whole file into a string in one shot.

The Files.readString reads all content from a file into a string, decoding from bytes to characters using the specified or the default (StandardCharsets.UTF_8) charset. It ensures that the file is closed when all content have been read.

com/zetcode/ReadFileAsStringEx.java
package com.zetcode;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class ReadFileAsStringEx {

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

        var fileName = "src/resources/thermopylae.txt";
        var filePath = Paths.get(fileName);

        var content = Files.readString(filePath);

        System.out.println(content);
    }
}

The example reads the contents of the thermopylae.txt file into a string a prints it to the terminal.

Java read text file with FileChannel

FileChannel is a channel for reading, writing, mapping, and manipulating a file. The advantages of file channels include reading and writing at a specific position of a file, loading a section of a file, or locking a section of a file.

com/zetcode/FileChannelEx.java
package com.zetcode;

import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class FileChannelEx {

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

        var fileName = "src/resources/thermopylae.txt";

        try (RandomAccessFile myFile = new RandomAccessFile(fileName, "rw");
             FileChannel inChannel = myFile.getChannel()) {

            ByteBuffer buf = ByteBuffer.allocate(48);

            int bytesRead = inChannel.read(buf);

            while (bytesRead != -1) {

                buf.flip();

                while (buf.hasRemaining()) {

                    System.out.print((char) buf.get());
                }

                buf.clear();
                bytesRead = inChannel.read(buf);
            }
        }
    }
}

The example reads the text file with FileChannel.

try (RandomAccessFile myFile = new RandomAccessFile(fileName, "rw");
        FileChannel inChannel = myFile.getChannel()) {

A FileChannle is created from a RandomAccessFile.

ByteBuffer buf = ByteBuffer.allocate(48);

int bytesRead = inChannel.read(buf);

We allocate a buffer and read initial data.

while (bytesRead != -1) {

    buf.flip();

    while (buf.hasRemaining()) {

        System.out.print((char) buf.get());
    }

    buf.clear();
    bytesRead = inChannel.read(buf);
}

We read the data into the buffer and write it to the terminal. We use flip() to change buffer from reading to writing.

Reading text file with Google Guava

Google Guava is a Java helper library which has IO tools, too. The following two Guava methods would consume a lot of system resources if the file to be read is very large.

pom.xml
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
            http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.zetcode</groupId>
    <artifactId>readtextguavaex</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>12</maven.compiler.source>
        <maven.compiler.target>12</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>28.0-jre</version>
        </dependency>
    </dependencies>

</project>

This is the Maven POM file.

com/zetcode/ReadTextGuavaEx.java
package com.zetcode;

import com.google.common.base.Charsets;
import com.google.common.io.Files;

import java.io.File;
import java.io.IOException;
import java.util.List;

public class ReadTextGuavaEx {

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

        var fileName = "src/main/resources/thermopylae.txt";

        List<String> lines = Files.readLines(new File(fileName),
                Charsets.UTF_8);

        var sb = new StringBuilder();

        for (String line: lines) {

            sb.append(line);
            sb.append(System.lineSeparator());
        }

        System.out.println(sb);
    }
}

In the example, we read all of the lines from a file with the Files.readLines() method. The method returns a list of strings. A default charset is specified as the second parameter.

In the second example, we use Files.asCharSource().

com/zetcode/ReadTextGuavaEx2.java
package com.zetcode;

import com.google.common.base.Charsets;
import com.google.common.io.Files;

import java.io.File;
import java.io.IOException;

public class ReadTextGuavaEx2 {

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

        var fileName = "src/main/resources/thermopylae.txt";

        var charSource = Files.asCharSource(new File(fileName), 
            Charsets.UTF_8).read();

        System.out.println(charSource);
    }
}

The Files.asCharSource() for reading character data from the given file using the given character set. Its read() method reads the contents of this source as a string.

In this article, we have read text files in various ways in Java.

You might also be interested in the following related tutorials: Java list directory contents, Java FileOutputStream tutorial, Copying file in Java, Java tutorial, Joining strings with Java 8's StringJoiner, Reading a web page in Java, or Introduction to Google Guava.

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