Java Notes

Java NIO Package

Complete guide to Java NIO (New I/O) — Path, Files, Channels, Buffers, non-blocking I/O, WatchService, and modern file operations that replace traditional java.io approaches.

What is NIO?

NIO (New I/O) was introduced in Java 1.4 (enhanced in Java 7 as NIO.2) as a modern alternative to traditional java.io. It provides:

java.nio.file.Path — replacement for java.io.File
java.nio.file.Files — utility methods for file operations
Channels & Buffers — for high-performance I/O
Non-blocking I/O — for scalable network applications
WatchService — file system monitoring

Traditional I/O (java.io)	NIO (java.nio):
File	Path
FileInputStream	Files.newInputStream()
FileOutputStream	Files.newOutputStream()
FileReader/Writer	Files.newBufferedReader/Writer()
File.listFiles()	Files.list(), Files.walk()
File.delete()	Files.delete()

Path — Modern File Path Handling

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Lesson example

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Output

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Output:

path1	src/main/java/App.java
Filename	App.java
Parent	src/main/java
Root	null
Name count	4
Base	/home/user
Relative	documents/file.txt
Resolved	/home/user/documents/file.txt
Before	./data/../config/app.properties
After	config/app.properties
From	/home/user/docs
To	/home/user/photos/vacation
Relative	../../photos/vacation

Files — Powerful File Operations

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Lesson example

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Output

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Walking Directory Trees

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Lesson example

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Channels and Buffers

NIO introduces a channel-based I/O model using Buffers:

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Lesson example

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Output

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Output:

✓ Written via FileChannel	48 bytes
✓ Read via FileChannel	Hello from NIO Channels! This is efficient I/O.
Capacity	20
Position	0
Limit	20
Position	12
Position	0
Limit	12
Read int	42
Read double	3.14

WatchService — File System Monitoring

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Lesson example

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NIO vs Traditional I/O Comparison

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Lesson example

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Output

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Common Mistakes

1. Not closing streams from Files methods

java example

// ❌ BAD: Stream is never closed (resource leak!)
Files.walk(Path.of(".")).forEach(System.out::println);

// ✅ GOOD: Use try-with-resources
try (Stream<Path> paths = Files.walk(Path.of("."))) {
    paths.forEach(System.out::println);
}

2. Using readAllBytes on huge files

java example

// ❌ BAD: Loads entire 2GB file into memory!
byte[] data = Files.readAllBytes(Path.of("huge_file.dat"));

// ✅ GOOD: Stream line by line
try (Stream<String> lines = Files.lines(Path.of("huge_file.txt"))) {
    lines.filter(l -> l.contains("ERROR")).forEach(System.out::println);
}

3. Forgetting to flip ByteBuffer

Example

ByteBuffer buf = ByteBuffer.allocate(100); buf.put("Hello".getBytes()); // ❌ WRONG: Position is at end, nothing to read! byte[] data = new byte[5]; buf.get(data); // BufferUnderflowException // ✅ CORRECT: Flip first buf.flip(); // position=0, limit=5 byte[] data = new byte[5]; buf.get(data); // Works!

Interview Questions

Q1: What is the difference between java.io.File and java.nio.file.Path?

Answer: File is a legacy class (Java 1.0) representing a file path. Path (Java 7) is its modern replacement offering: better error handling (exceptions with reasons vs booleans), symbolic link support, file system independence, and integration with the Files utility class. New code should always use Path.

Q2: What are Channels and Buffers in NIO?

Answer: Channels are bidirectional connections to I/O sources (files, sockets). Buffers are containers that hold data being transferred. Data flows: Channel → Buffer (read) or Buffer → Channel (write). This is more efficient than stream-based I/O because channels can use OS-level optimizations like DMA transfers.

Q3: What is the difference between blocking and non-blocking I/O?

Answer: Blocking I/O: the thread waits (blocks) until the operation completes. Non-blocking I/O (NIO): the thread can continue doing other work — it checks if data is available without waiting. This allows one thread to manage many connections (key for scalable servers).

Q4: What is WatchService used for?

Answer: WatchService monitors file system changes (create, modify, delete events) in a directory. Use cases: hot-reloading config files, auto-compiling changed source files, sync tools (like Dropbox), log file monitoring.

Q5: What are the advantages of Files.copy() over manual stream copying?

Answer: Files.copy() uses OS-level operations (like sendfile on Linux) that can transfer data without copying through user space. It handles atomic operations, preserves file attributes, supports options like REPLACE_EXISTING and COPY_ATTRIBUTES, and provides clear exceptions on failure.

Q6: Explain ByteBuffer's flip() method.

Answer: flip() prepares a buffer for reading after writing. It sets limit = position (marks the end of written data) and position = 0 (starts reading from the beginning). Without flip, you'd try to read from the current position (end of data) and get nothing.

Continue the lesson path

Study next in this module

4 connected lessons

01File HandlingDeserialization in JavaComplete guide to Deserialization in Java — ObjectInputStream, reading objects from files, type safety, version compatibility, security risks, and best practices for safe deserialization.02File HandlingSerialization in JavaComplete guide to Serialization in Java — converting objects to byte streams, Serializable interface, ObjectOutputStream, transient keyword, serialVersionUID, and custom serialization.03File HandlingBuffered Streams in JavaComplete guide to Buffered Streams in Java — BufferedInputStream, BufferedOutputStream, BufferedReader, BufferedWriter, performance benefits, line-by-line reading, and real-world file processing.04File HandlingCharacter Streams in JavaComplete guide to Character Streams in Java — FileReader, FileWriter, handling Unicode text, character encoding, reading/writing text files with proper internationalization support.

PreviousDeserialization in Java NextLambda Expression in Java 8

Java Notes

Java NIO Package

Complete guide to Java NIO (New I/O) — Path, Files, Channels, Buffers, non-blocking I/O, WatchService, and modern file operations that replace traditional java.io approaches.

What is NIO?

NIO (New I/O) was introduced in Java 1.4 (enhanced in Java 7 as NIO.2) as a modern alternative to traditional java.io. It provides:

java.nio.file.Path — replacement for java.io.File
java.nio.file.Files — utility methods for file operations
Channels & Buffers — for high-performance I/O
Non-blocking I/O — for scalable network applications
WatchService — file system monitoring

Traditional I/O (java.io)	NIO (java.nio):
File	Path
FileInputStream	Files.newInputStream()
FileOutputStream	Files.newOutputStream()
FileReader/Writer	Files.newBufferedReader/Writer()
File.listFiles()	Files.list(), Files.walk()
File.delete()	Files.delete()

Path — Modern File Path Handling

Try it in WoHoCode

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Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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Output:

path1	src/main/java/App.java
Filename	App.java
Parent	src/main/java
Root	null
Name count	4
Base	/home/user
Relative	documents/file.txt
Resolved	/home/user/documents/file.txt
Before	./data/../config/app.properties
After	config/app.properties
From	/home/user/docs
To	/home/user/photos/vacation
Relative	../../photos/vacation

Files — Powerful File Operations

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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Walking Directory Trees

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Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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Channels and Buffers

NIO introduces a channel-based I/O model using Buffers:

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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Output:

✓ Written via FileChannel	48 bytes
✓ Read via FileChannel	Hello from NIO Channels! This is efficient I/O.
Capacity	20
Position	0
Limit	20
Position	12
Position	0
Limit	12
Read int	42
Read double	3.14

WatchService — File System Monitoring

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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NIO vs Traditional I/O Comparison

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

Loading editor...

Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

Run this example to see its output.

Common Mistakes

1. Not closing streams from Files methods

java example

// ❌ BAD: Stream is never closed (resource leak!)
Files.walk(Path.of(".")).forEach(System.out::println);

// ✅ GOOD: Use try-with-resources
try (Stream<Path> paths = Files.walk(Path.of("."))) {
    paths.forEach(System.out::println);
}

2. Using readAllBytes on huge files

java example

// ❌ BAD: Loads entire 2GB file into memory!
byte[] data = Files.readAllBytes(Path.of("huge_file.dat"));

// ✅ GOOD: Stream line by line
try (Stream<String> lines = Files.lines(Path.of("huge_file.txt"))) {
    lines.filter(l -> l.contains("ERROR")).forEach(System.out::println);
}