Binary File I/O in Java (FileInputStream & FileOutputStream)

Binary File I/O in Java is used to read and write raw bytes, making it suitable for non-text files such as images, videos, audio files, executables, PDFs, and ZIP archives. Unlike text data, binary data must be handled using byte streams; using character streams (FileReader, BufferedReader) corrupts binary content.

What Are Binary Files?

Binary files store data exactly as bytes (0–255). These files are not human-readable and cannot be processed using character-based classes.

Examples of Binary Files

• Images → .jpg, .png, .gif
• Videos → .mp4, .mkv
• Audio → .mp3, .wav
• Documents → .pdf, .docx
• Compressed files → .zip, .rar
• Executables → .exe
• Serialized Java objects → .ser

Because these files contain raw bytes, Java requires byte-based streams for safe reading and writing.

Binary I/O Streams in Java

Java provides two important classes from java.io package:

• FileInputStream — Reads bytes from a file
• FileOutputStream — Writes bytes to a file

Both operate strictly on byte data, making them ideal for binary files.

1. Reading Binary Files using FileInputStream

How FileInputStream Works

• Reads data byte-by-byte
• Returns integer values from 0 to 255
• Returns 1 when end of file is reached
• Does not perform character conversion → safe for all binary formats

Example: Read a binary file byte by byte

import java.io.FileInputStream;
import java.io.IOException;

public class ReadBinaryFile {
    public static void main(String[] args) {
        try (FileInputStream fis = new FileInputStream("image.jpg")) {

            int data;
            while ((data = fis.read()) != -1) {
                System.out.print(data + " ");
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2. Writing Binary Files using FileOutputStream

How FileOutputStream Works

• Writes data byte-by-byte
• Can overwrite or append to a file
• No encoding conversion

Example: Write bytes to a binary file

import java.io.FileOutputStream;
import java.io.IOException;

public class WriteBinaryFile {
    public static void main(String[] args) {
        try (FileOutputStream fos = new FileOutputStream("output.bin")) {

            byte[] data = {10, 20, 30, 40, 50};
            fos.write(data);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3. Copying a Binary File (Most Realistic Use Case)

Copying files like images, videos, and PDFs is the most common binary I/O operation.

Example: Fast binary file copying using buffer

import java.io.*;

public class CopyBinaryFile {
    public static void main(String[] args) {
        try (FileInputStream fis = new FileInputStream("source.jpg");
             FileOutputStream fos = new FileOutputStream("copy.jpg")) {

            byte[] buffer = new byte[4096];
            int bytesRead;

            while ((bytesRead = fis.read(buffer)) != -1) {
                fos.write(buffer, 0, bytesRead);
            }

            System.out.println("File copied successfully!");

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Why Buffering Is Used?

• Faster reading and writing
• Reduces number of I/O calls
• Prevents performance issues with large files

Key Methods of FileInputStream & FileOutputStream

Why Use Byte Streams for Binary Files?

Common Mistakes to Avoid

• Using FileReader to read binary files → Causes corruption because of character conversion
• Printing raw bytes expecting readable text → Bytes are not printable characters
• Avoid reading byte-by-byte for large files → Always use a buffer for high performance

Points to Remember

• Binary files contain raw bytes
• Use FileInputStream for reading
• Use FileOutputStream for writing
• Always use buffer (byte[]) for performance
• Character streams are not suitable for binary data
• Works for images, videos, PDFs, ZIPs, audio, etc.