【Java】ファイルに書き込む方法｜UTF-8の指定・追記と上書きの違い

import java.io.*;
import java.nio.charset.StandardCharsets;

public class Main {
    public static void main(String[] args) {
        try (BufferedWriter bw = new BufferedWriter(
                 new OutputStreamWriter(
                     new FileOutputStream("example.txt"), StandardCharsets.UTF_8))) {
            bw.write("こんにちは");
            bw.newLine();           // 改行
            bw.write("世界");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.*;
import java.util.List;

public class Main {
    public static void main(String[] args) throws IOException {
        Path path = Paths.get("example.txt");

        // 行のリストを書き込む（charset明示）
        Files.write(path, List.of("1行目", "2行目"), StandardCharsets.UTF_8);

        // 文字列を一括で書き込む（Java 11+）
        Files.writeString(path, "こんにちは世界", StandardCharsets.UTF_8);
    }
}

import java.io.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.*;

public class Main {
    public static void main(String[] args) throws IOException {
        // FileWriter は第2引数 true で追記（Java 11+はcharsetも指定可）
        try (BufferedWriter bw = new BufferedWriter(
                 new OutputStreamWriter(
                     new FileOutputStream("log.txt", true), StandardCharsets.UTF_8))) {
            bw.write("追記行");
            bw.newLine();
        }

        // Files は StandardOpenOption.APPEND で追記
        Files.writeString(Paths.get("log.txt"), "追記\n",
            StandardCharsets.UTF_8, StandardOpenOption.CREATE, StandardOpenOption.APPEND);
    }
}

import java.io.*;
import java.nio.charset.StandardCharsets;

public class Main {
    public static void main(String[] args) throws IOException {
        try (PrintWriter pw = new PrintWriter(
                 new OutputStreamWriter(
                     new FileOutputStream("example.txt"), StandardCharsets.UTF_8))) {
            pw.println("名前: 太郎");
            pw.printf("得点: %d点%n", 95);
        }
    }
}