Java File I/O — Introduction

Almost every real-world Java application needs to read from or write to files — configuration files, logs, CSV data exports, JSON API responses, or user uploads. Java provides two main packages for file I/O: the classic java.io (since Java 1.0) and the modern java.nio.file (since Java 7).

In this chapter, you’ll learn the landscape of Java I/O, understand the key abstractions, and know when to use which approach.

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java.io vs java.nio

Feature	java.io (classic)	java.nio.file (modern)
Introduced	Java 1.0 (1996)	Java 7 (2011)
API style	Stream-based (InputStream, OutputStream)	Path-based (Path, Files)
Blocking?	Yes (blocking I/O)	Supports non-blocking (NIO channels)
File operations	Basic (read, write, delete)	Rich (walk, find, copy, move, watch)
Recommendation	Use for legacy code	Use for all new code

Rule of thumb: Use java.nio.file.Files and java.nio.file.Path for all new file operations. Fall back to java.io only when a library requires it.

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Byte Streams vs Character Streams

Java distinguishes between binary data (bytes) and text data (characters):

	Byte Streams	Character Streams
Base classes	InputStream / OutputStream	Reader / Writer
Unit	byte (8 bits)	char (16 bits, Unicode)
Use for	Images, PDFs, binary files	Text files, CSV, JSON, XML
Examples	FileInputStream, BufferedInputStream	FileReader, BufferedReader

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Always Close Your Resources

File handles are limited system resources. Always close them after use. The modern way is try-with-resources:

// ✅ Good — automatically closes the reader
try (BufferedReader reader = new BufferedReader(new FileReader("data.txt"))) {
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
}
// reader is automatically closed here, even if an exception occurs

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What’s Next?

In the next chapter, you’ll learn how to read files using both classic BufferedReader and modern Files.readString().