Java FileOutputStream Class – Comprehensive Guide

Java FileOutputStream Class

This tutorial walks through the Java FileOutputStream class, its constructors, key methods, and practical examples.

The FileOutputStream class, part of the java.io package, allows you to write raw byte data to files. It extends the abstract OutputStream class and is essential for binary file operations.

Before diving into FileOutputStream, ensure you understand basic Java file handling concepts.

Creating a FileOutputStream

To create a FileOutputStream, import the class from java.io and instantiate it with either a file path or a File object.

1. Using a file path

// With the append flag
FileOutputStream output = new FileOutputStream(String path, boolean append);

// Without the append flag
FileOutputStream output = new FileOutputStream(String path);

The append boolean determines whether data is added to the end of the file (true) or overwrites existing content (false).

2. Using a File object

FileOutputStream output = new FileOutputStream(File fileObject);

This constructor links the stream to the file represented by fileObject.

Key Methods of FileOutputStream

FileOutputStream implements several inherited methods from OutputStream, each tailored for byte operations.

write()

• write(int b) – writes a single byte.
• write(byte[] b) – writes an entire byte array.
• write(byte[] b, int off, int len) – writes len bytes from b starting at offset off.

Example: Writing Text to a File

import java.io.FileOutputStream;

public class Main {
    public static void main(String[] args) {
        String data = "This is a line of text inside the file.";
        try {
            FileOutputStream output = new FileOutputStream("output.txt");
            byte[] array = data.getBytes();
            output.write(array);
            output.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Running this program creates output.txt containing the sentence shown above. The getBytes() method converts the string into its UTF‑8 byte representation.

flush()

The flush() method forces any buffered bytes to be written to the file immediately. It’s useful when you need to ensure data persistence before closing the stream.

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

public class Main {
    public static void main(String[] args) throws IOException {
        FileOutputStream out = new FileOutputStream("flush.txt");
        String data = "This is demo of flush method";
        out.write(data.getBytes());
        out.flush();
        out.close();
    }
}

close()

After calling close(), the stream is no longer usable. It releases system resources and ensures the file handle is properly freed.

Additional FileOutputStream Methods

For deeper insight, refer to the official Java FileOutputStream documentation.