How to read a File in Java

In this article, you’ll learn how to read a text file or binary (image) file in Java using various classes and utility methods provided by Java like:

BufferedReader, LineNumberReader, 
Files.readAllLines, Files.lines, 
BufferedInputStream, Files.readAllBytes, etc.

Let’s look at each of the different ways of reading a file in Java with the help of examples.

Java read file using BufferedReader

BufferedReader is a simple and performant way of reading text files in Java. It reads text from a character-input stream. It buffers characters to provide efficient reading.

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

public class BufferedReaderExample {
    public static void main(String[] args) {
        Path filePath = Paths.get("demo.txt");
        Charset charset = StandardCharsets.UTF_8;

        try (BufferedReader bufferedReader = Files.newBufferedReader(filePath, charset)) {
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException ex) {
            System.out.format("I/O error: %s%n", ex);
        }
    }
}

Java read file line by line using Files.readAllLines()

Files.readAllLines() is a utility method of the Java NIO’s Files class that reads all the lines of a file and returns a List<String> containing each line. It internally uses BufferedReader to read the file.

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

public class FilesReadAllLinesExample {
    public static void main(String[] args) {
        Path filePath = Paths.get("demo.txt");
        Charset charset = StandardCharsets.UTF_8;
        try {
            List<String> lines = Files.readAllLines(filePath, charset);
            for(String line: lines) {
                System.out.println(line);
            }
        } catch (IOException ex) {
            System.out.format("I/O error: %s%n", ex);
        }
    }
}

Java read file line by line using Files.lines()

Files.lines() method reads all the lines from a file as a Stream. You can use Stream API methods like forEach, map to work with each line of the file.

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

public class FilesLinesExample {
    public static void main(String[] args) {
        Path filePath = Paths.get("demo.txt");
        Charset charset = StandardCharsets.UTF_8;

        try {

            Files.lines(filePath, charset)
                    .forEach(System.out::println);

        } catch (IOException ex) {
            System.out.format("I/O error: %s%n", ex);
        }
    }
}

Java read file line by line using LineNumberReader

LineNumberReader is a buffered character-stream reader that keeps track of line numbers. You can use this class to read a text file line by line.

import java.io.*;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class LineNumberReaderExample {
    public static void main(String[] args) {
        Path filePath = Paths.get("demo.txt");
        Charset charset = StandardCharsets.UTF_8;

        try(BufferedReader bufferedReader = Files.newBufferedReader(filePath, charset);
            LineNumberReader lineNumberReader = new LineNumberReader(bufferedReader)) {

            String line;
            while ((line = lineNumberReader.readLine()) != null) {
                System.out.format("Line %d: %s%n", lineNumberReader.getLineNumber(), line);
            }
        } catch (IOException ex) {
            System.out.format("I/O error: %s%n", ex);
        }
    }
}

Java read binary file (image file) using BufferedInputStream

All the examples presented in this article so far read textual data from a character-input stream. If you’re reading a binary data such as an image file then you need to use a byte-input stream.

BufferedInputStream lets you read raw stream of bytes. It also buffers the input for improving performance

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

public class BufferedInputStreamImageCopyExample {
    public static void main(String[] args) {
        try(InputStream inputStream = Files.newInputStream(Paths.get("sample.jpg"));
            BufferedInputStream bufferedInputStream = new BufferedInputStream(inputStream);

            OutputStream outputStream = Files.newOutputStream(Paths.get("sample-copy.jpg"));
            BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(outputStream)) {

            byte[] buffer = new byte[4096];
            int numBytes;
            while ((numBytes = bufferedInputStream.read(buffer)) != -1) {
                bufferedOutputStream.write(buffer, 0, numBytes);
            }
        } catch (IOException ex) {
            System.out.format("I/O error: %s%n", ex);
        }
    }
}

Java read file into []byte using Files.readAllBytes()

If you want to read the entire contents of a file in a byte array then you can use the Files.readAllBytes() method.

import com.sun.org.apache.xpath.internal.operations.String;

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

public class FilesReadAllBytesExample {
    public static void main(String[] args) {
        try {
            byte[] data = Files.readAllBytes(Paths.get("demo.txt"));

            // Use byte data
        } catch (IOException ex) {
            System.out.format("I/O error: %s%n", ex);
        }

    }
}