public class IoWrite
{
    private static Formatter output; // 用于向文件输出文本的工具
    public static void main(String[] args)
    {
        openFile();
        addRecords();
        closeFile();
    }
    // 打开文件 clients.txt
    public static void openFile()
    {
        try
        {
            output = new Formatter("clients.txt"); // 打开文件
        }
        catch (SecurityException securityException)
        {
            System.err.println("写入权限被拒绝。终止程序。");
            System.exit(1); // 终止程序
        }
        catch (FileNotFoundException fileNotFoundException)
        {
            System.err.println("文件打开错误。终止程序。");
            System.exit(1); // 终止程序
        }
    }
    // 向文件添加记录
    public static void addRecords()
    {
        Scanner input = new Scanner(System.in);
        System.out.printf("%s%n%s%n? ",
                "输入账号、名字、姓氏和余额。",
                "输入文件结束指示符以结束输入。");
        while (input.hasNext()) // 循环直到文件结束指示符
        {
            try
            {
                // 将新记录输出到文件；假定输入有效
                output.format("%d %s %s %.2f%n", input.nextInt(),
                        input.next(), input.next(), input.nextDouble());
            }
            catch (FormatterClosedException formatterClosedException)
            {
                System.err.println("写入文件出错。终止程序。");
                break;
            }
            catch (NoSuchElementException elementException)
            {
                System.err.println("无效的输入。请重试。");
                input.nextLine(); // 丢弃输入，以便用户重试
            }
            System.out.print("?");
        }
    }
    // 关闭文件
    public static void closeFile()
    {
        if (output != null)
            output.close();
    }
} // end class CreateTextFile

// 打开文件 clients.txt
public static void openFile()
{
    try
    {
        FileWriter fileWriter = new FileWriter("clients.txt", true);
        output = new Formatter(fileWriter); // 打开文件
    }
    catch (SecurityException securityException)
    {
        System.err.println("写入权限被拒绝。终止程序。");
        System.exit(1); // 终止程序
    }
    catch (FileNotFoundException fileNotFoundException)
    {
        System.err.println("打开文件时出错。终止程序。");
        System.exit(1); // 终止程序
    }
}
注意：在`FileWriter`构造函数中，我将`append`标志设置为true。
  [1]: https://docs.oracle.com/javase/7/docs/api/java/io/FileWriter.html
<details>
<summary>英文:</summary>
You can use a [FileWriter][1] to append to existing file.
Update `openFile` method as,
    // open file clients.txt
    public static void openFile()
    {
        try
        {
            FileWriter fileWriter = new FileWriter(&quot;clients.txt&quot;, true);
            output = new Formatter(fileWriter); // open the file
        }
        catch (SecurityException securityException)
        {
            System.err.println(&quot;Write permission denied. Terminating.&quot;);
            System.exit(1); // terminate the program
        }
        catch (FileNotFoundException fileNotFoundException)
        {
            System.err.println(&quot;Error opening file. Terminating.&quot;);
            System.exit(1); // terminate the program
        }
    }
Note- In `FileWriter` constructor I have kept `append` flag as true
  [1]: https://docs.oracle.com/javase/7/docs/api/java/io/FileWriter.html
</details>