NIO.2 File API in Java

Master java.nio.file API, Path interface, Files class, directory traversal, file creation, reading, writing, file properties, and NIO.2 features for the OCP 21 exam.

Table of Contents

• 1. NIO.2 Overview
• 2. Path Interface
• 3. Files Class
• 4. Directory Operations
• 5. File Operations
• 6. Exam Key Points

1. NIO.2 Overview

NIO.2 (New I/O 2) was introduced in Java 7, providing improved file I/O operations through the java.nio.file package.

1.1 Key Classes

• Path - Represents a file or directory path
• Paths - Factory class for creating Path objects
• Files - Utility class for file operations
• FileSystem - File system abstraction

2. Path Interface

2.1 Creating Path Objects

Example:

import java.nio.file.Path;
import java.nio.file.Paths;

// Creating Path objects
Path path1 = Paths.get("file.txt");
Path path2 = Paths.get("/home/user/file.txt");
Path path3 = Paths.get("dir", "subdir", "file.txt");
Path path4 = Path.of("file.txt");  // Java 11+

// Path operations
Path absolute = path1.toAbsolutePath();
Path normalized = path1.normalize();
Path resolved = path1.resolve("other.txt");
Path parent = path1.getParent();
String fileName = path1.getFileName().toString();
int nameCount = path1.getNameCount();

3. Files Class

The Files class provides static methods for file operations.

3.1 Reading Files

Example:

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;

Path path = Paths.get("file.txt");

// Read all lines
List<String> lines = Files.readAllLines(path);

// Read all bytes
byte[] bytes = Files.readAllBytes(path);

// Read as stream
Files.lines(path).forEach(System.out::println);

// Read with charset
List<String> linesUTF8 = Files.readAllLines(path, StandardCharsets.UTF_8);

3.2 Writing Files

Example:

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.Arrays;

Path path = Paths.get("output.txt");

// Write lines
List<String> lines = Arrays.asList("Line 1", "Line 2");
Files.write(path, lines);

// Write bytes
byte[] data = "Hello".getBytes();
Files.write(path, data);

// Append to file
Files.write(path, "New line".getBytes(), StandardOpenOption.APPEND);

// Write with options
Files.write(path, lines, StandardOpenOption.CREATE, 
    StandardOpenOption.TRUNCATE_EXISTING);

4. Directory Operations

4.1 Creating and Traversing Directories

Example:

import java.nio.file.*;
import java.util.stream.Stream;

// Create directory
Path dir = Paths.get("mydir");
Files.createDirectory(dir);
Files.createDirectories(Paths.get("dir1/dir2/dir3"));  // Creates parent dirs

// List directory contents
try (Stream<Path> stream = Files.list(dir)) {
    stream.forEach(System.out::println);
}

// Walk directory tree
try (Stream<Path> stream = Files.walk(dir)) {
    stream.forEach(System.out::println);
}

// Find files
try (Stream<Path> stream = Files.find(dir, Integer.MAX_VALUE,
        (path, attrs) -> path.toString().endsWith(".txt"))) {
    stream.forEach(System.out::println);
}

5. File Operations

5.1 File Properties and Operations

Example:

import java.nio.file.*;
import java.nio.file.attribute.BasicFileAttributes;

Path path = Paths.get("file.txt");

// Check file properties
boolean exists = Files.exists(path);
boolean isDirectory = Files.isDirectory(path);
boolean isRegularFile = Files.isRegularFile(path);
boolean isReadable = Files.isReadable(path);
boolean isWritable = Files.isWritable(path);
long size = Files.size(path);

// Get file attributes
BasicFileAttributes attrs = Files.readAttributes(path, BasicFileAttributes.class);
FileTime creationTime = attrs.creationTime();
FileTime lastModified = attrs.lastModifiedTime();

// Copy file
Files.copy(source, target, StandardCopyOption.REPLACE_EXISTING);

// Move file
Files.move(source, target, StandardCopyOption.REPLACE_EXISTING);

// Delete file
Files.delete(path);
Files.deleteIfExists(path);  // No exception if doesn't exist

6. Exam Key Points

Critical Concepts for OCP 21 Exam:

• Path: Represents file or directory path
• Paths.get(): Create Path object
• Path.of(): Create Path (Java 11+)
• Files.readAllLines(): Read all lines from file
• Files.readAllBytes(): Read all bytes from file
• Files.lines(): Read file as Stream
• Files.write(): Write to file
• Files.createDirectory(): Create single directory
• Files.createDirectories(): Create directory hierarchy
• Files.list(): List directory contents
• Files.walk(): Walk directory tree
• Files.find(): Find files matching predicate
• Files.copy(): Copy file or directory
• Files.move(): Move/rename file or directory
• Files.delete(): Delete file or directory
• StandardOpenOption: Options for file operations