Files ユーティリティ：読み取り、書き込み、コピー、移動

Files クラス

import java.nio.file.*;
import java.io.IOException;

Path file = Path.of("example.txt");
// All Files methods throw IOException — handle or propagate

ファイルの読み取り

Path path = Path.of("notes.txt");

// Read as a single String (Java 11+)
String content = Files.readString(path);
System.out.println(content);

// Read as List of lines
List<String> lines = Files.readAllLines(path);
lines.forEach(System.out::println);

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

ファイルの書き込み

Path out = Path.of("output.txt");

// Write a String (overwrites by default)
Files.writeString(out, "Hello, NIO.2!");

// Write with StandardOpenOption.APPEND
Files.writeString(out, "\nAppended line", StandardOpenOption.APPEND);

// Write lines
List<String> lines = List.of("line1", "line2", "line3");
Files.write(out, lines);

ファイルのプロパティの確認

Path path = Path.of("data.txt");

System.out.println(Files.exists(path));       // true/false
System.out.println(Files.isRegularFile(path)); // true if not dir
System.out.println(Files.isDirectory(path));   // true if dir
System.out.println(Files.isReadable(path));    // readable?
System.out.println(Files.size(path));          // bytes
System.out.println(Files.getLastModifiedTime(path));

ファイルのコピー

Path src = Path.of("original.txt");
Path dst = Path.of("backup.txt");

// Throws if destination exists (default)
Files.copy(src, dst);

// Overwrite if exists:
Files.copy(src, dst, StandardCopyOption.REPLACE_EXISTING);

// Copy preserving file timestamps:
Files.copy(src, dst, StandardCopyOption.COPY_ATTRIBUTES);

ファイルの移動と名前変更

Path from = Path.of("temp.txt");
Path to   = Path.of("archive/report_2024.txt");

Files.move(from, to, StandardCopyOption.REPLACE_EXISTING);

// Rename in same directory:
Path renamed = from.resolveSibling("new_name.txt");
Files.move(from, renamed, StandardCopyOption.REPLACE_EXISTING);

ファイルの削除

Path file = Path.of("temp.txt");

// Throws NoSuchFileException if file doesn't exist:
Files.delete(file);

// Returns false if file didn't exist (no exception):
boolean deleted = Files.deleteIfExists(file);
System.out.println("Deleted: " + deleted);

ディレクトリの作成

Path dir = Path.of("output/reports/2024");

// Creates all missing intermediate directories:
Files.createDirectories(dir);

// Creates a single directory (fails if parent missing):
Files.createDirectory(Path.of("output/reports/2025"));

一時ファイルと一時ディレクトリ

// Temp file in default temp directory
Path tmp = Files.createTempFile("prefix_", ".txt");
Files.writeString(tmp, "temp data");
System.out.println(tmp); // something like /tmp/prefix_12345.txt

// Temp directory
Path tmpDir = Files.createTempDirectory("work_");
System.out.println(tmpDir);

Files.lines による行のストリーミング

try (var stream = Files.lines(Path.of("large.log"))) {
    long errorCount = stream
        .filter(l -> l.contains("ERROR"))
        .count();
    System.out.println("Errors: " + errorCount);
} // stream (and underlying reader) closed automatically

クラスパスリソースの読み取り

try (var in = getClass().getResourceAsStream("/config.json")) {
    Path tmp = Files.createTempFile("config", ".json");
    Files.copy(in, tmp, StandardCopyOption.REPLACE_EXISTING);
    String json = Files.readString(tmp);
    System.out.println(json);
}

クイックチェック

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