File Objects ใน Java - การสร้างและใช้งานไฟล์

Class java.io.File แทน **Path/ไฟล์/ไดเรกทอรี** ใน File System ไม่ใช่เนื้อหาไฟล์ ใช้สำหรับ metadata และ file system operations

Constructors

Constructor	คำอธิบาย
`File(String pathname)`	สร้างจาก path string (absolute/relative)
`File(String parent, String child)`	สร้างจาก parent path + child name
`File(File parent, String child)`	สร้างจาก parent File object + child name
`File(URI uri)`	สร้างจาก file: URI

ตัวอย่างการใช้งาน

import java.io.File;
import java.io.IOException;

public class FileObjectDemo {
    public static void main(String[] args) {
        // 1. สร้าง File objects ด้วย constructors ต่างๆ
        File f1 = new File("test.txt");                           // Relative path
        File f2 = new File("/home/user/documents/test.txt");      // Absolute path
        File f3 = new File("/home/user/documents", "test.txt");   // Parent + child
        File f4 = new File(f3.getParentFile(), "another.txt");    // File parent + child
        
        // 2. Path information
        System.out.println("Name: " + f1.getName());              // test.txt
        System.out.println("Path: " + f1.getPath());              // test.txt
        System.out.println("Absolute Path: " + f1.getAbsolutePath()); // /current/dir/test.txt
        System.out.println("Parent: " + f1.getParent());          // null (relative)
        System.out.println("Canonical: " + f1.getCanonicalPath()); // Resolved absolute
        
        // 3. File existence & properties
        File existing = new File("/etc/passwd"); // Linux system file
        if (existing.exists()) {
            System.out.println("Exists: Yes");
            System.out.println("Is File: " + existing.isFile());
            System.out.println("Is Directory: " + existing.isDirectory());
            System.out.println("Readable: " + existing.canRead());
            System.out.println("Writable: " + existing.canWrite());
            System.out.println("Executable: " + existing.canExecute());
            System.out.println("Size: " + existing.length() + " bytes");
            System.out.println("Last Modified: " + new Date(existing.lastModified()));
        }
        
        // 4. Directory operations
        File dir = new File("myapp/data");
        if (!dir.exists()) {
            boolean created = dir.mkdirs(); // สร้าง parent directories ด้วย
            System.out.println("Directory created: " + created);
        }
        
        // List files
        File[] files = dir.listFiles();
        if (files != null) {
            for (File file : files) {
                System.out.println(file.getName() + " - " + (file.isDirectory() ? "[DIR]" : "[FILE]"));
            }
        }
        
        // Filter listing
        File[] txtFiles = dir.listFiles((d, name) -> name.endsWith(".txt"));
        File[] dirs = dir.listFiles(File::isDirectory);
    }
}

File Operations

Operation	Method	Return
Create file	`createNewFile()`	boolean
Create directory	`mkdir()` / `mkdirs()`	boolean
Delete	`delete()`	boolean
Delete on exit	`deleteOnExit()`	void (JVM shutdown hook)
Rename/Move	`renameTo(File dest)`	boolean
Set permissions	`setReadable/Writeable/Executable()`	boolean
Set last modified	`setLastModified(long time)`	boolean

สร้างไฟล์ชั่วคราว (Temp Files)

// สร้าง temp file ใน default temp directory
File tempFile = File.createTempFile("myapp-", ".tmp");
tempFile.deleteOnExit(); // ลบตอน JVM ปิด

// สร้าง temp directory
File tempDir = Files.createTempDirectory("myapp-").toFile();
tempDir.deleteOnExit();

Path vs File (Java 7+ NIO.2)

แนะนำใช้ java.nio.file.Path และ Files แทน File class

File (Legacy)	Path/Files (Modern)
`new File(path)`	`Paths.get(path)` หรือ `Path.of(path)` (Java 11+)
`file.exists()`	`Files.exists(path)`
`file.length()`	`Files.size(path)`
`file.delete()`	`Files.delete(path)` / `Files.deleteIfExists(path)`
`file.renameTo()`	`Files.move(src, dest, REPLACE_EXISTING)`
`file.listFiles()`	`Files.list(path)` (Stream) / `Files.walk()`
ไม่มี symbolic link support	รองรับ Symbolic Links, Attributes, ACLs
Boolean return (ไม่รู้สาเหตุ)	Throw Exception (รู้สาเหตุ失敗)

NIO.2 Files Example

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

public class NioFileExample {
    public static void main(String[] args) throws IOException {
        Path path = Path.of("data", "config.json");
        
        // Read all lines
        List lines = Files.readAllLines(path);
        
        // Read as String (Java 11+)
        String content = Files.readString(path);
        
        // Write
        Files.writeString(path, "New content", StandardOpenOption.CREATE, StandardOpenOption.WRITE);
        Files.write(path, List.of("Line 1", "Line 2"), StandardOpenOption.APPEND);
        
        // Walk directory (Stream)
        try (Stream paths = Files.walk(Paths.get("data"))) {
            paths.filter(Files::isRegularFile)
                 .filter(p -> p.toString().endsWith(".json"))
                 .forEach(System.out::println);
        }
        
        // Copy/Move with options
        Files.copy(Paths.get("src.txt"), Paths.get("dest.txt"), StandardCopyOption.REPLACE_EXISTING);
        Files.move(Paths.get("old.txt"), Paths.get("new.txt"), StandardCopyOption.ATOMIC_MOVE);
    }
}

สรุป

java.io.File - Legacy, ใช้สำหรับ metadata/path operations แบบง่าย
java.nio.file.Path + Files - **Modern API**, รองรับทุกอย่าง, Exception handling ดีกว่า, Stream support
โค้ดใหม่ควรใช้ NIO.2 ทั้งหมด