Java ArrayList 寫入文本文件：BufferedWriter 高效實現

標準實現

import java.io.*;
import java.util.ArrayList;

public class ArrayListToFile {
    public static void main(String[] args) throws IOException {
        ArrayList<String> list = new ArrayList<>();
        list.add("first");
        list.add("second");
        list.add("third");

        File file = new File("d:/output.txt");
        try (BufferedWriter bw = new BufferedWriter(new FileWriter(file))) {
            for (String line : list) {
                bw.write(line);
                bw.newLine();  // 跨平台換行
            }
        }
    }
}

進階：泛型工具方法

public static <T> void writeList(List<T> list, File file, Function<T, String> mapper) throws IOException {
    try (BufferedWriter bw = new BufferedWriter(new FileWriter(file))) {
        for (T item : list) {
            bw.write(mapper.apply(item));
            bw.newLine();
        }
    }
}

// 調用
writeList(list, new File("out.txt"), Object::toString);
writeList(userList, new File("users.txt"), u -> u.getName() + "," + u.getEmail());

大文件優化

• 增大緩衝區：new BufferedWriter(new FileWriter(file), 64 * 1024)。
• 異步寫入：Files.writeAsync(path, lines, StandardOpenOption.CREATE) (Java 11+)。
• 分批 flush：每 1000 行 bw.flush()，平衡內存與磁盤 I/O。

編碼指定

// Java 10+
try (BufferedWriter bw = Files.newBufferedWriter(path, StandardCharsets.UTF_8)) { ... }

// Java 8
try (BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file), "UTF-8"))) { ... }

常見錯誤