利用Java编写两个线程，其一利用合适的文件流类把文件test.docx复制文件mytest.docx，其二统计文本文件myfile.txt中英文字母a-z出现的次数（不区分大小写），并输出统计结果；

好的，我明白了您的要求。下面是您提出的问题的答案：

问题1：

import java.io.*;

public class CopyFileUsingThread {
    public static void main(String[] args) {
        Thread t1 = new Thread(new CopyFile());
        Thread t2 = new Thread(new CountLetters());
        t1.start();
        t2.start();
    }
}

class CopyFile implements Runnable {
    @Override
    public void run() {
        FileInputStream is = null;
        FileOutputStream os = null;
        try {
            is = new FileInputStream("test.docx");
            os = new FileOutputStream("mytest.docx");
            byte[] buffer = new byte[1024];
            int length;
            while ((length = is.read(buffer)) > 0) {
                os.write(buffer, 0, length);
            }
            System.out.println("文件复制成功");
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (is != null) {
                    is.close();
                }
                if (os != null) {
                    os.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

class CountLetters implements Runnable {
    @Override
    public void run() {
        FileInputStream is = null;
        try {
            is = new FileInputStream("myfile.txt");
            int[] count = new int[26];
            int letterCount = 0;
            int ch;
            while ((ch = is.read()) != -1) {
                if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z')) {
                    letterCount++;
                    if (ch >= 'a' && ch <= 'z') {
                        count[ch - 'a']++;
                    } else {
                        count[ch - 'A']++;
                    }
                }
            }
            for (int i = 0; i < 26; i++) {
                System.out.println((char)('a' + i) + ": " + count[i]);
            }
            System.out.println("字母总数：" + letterCount);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (is != null) {
                    is.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

问题2的代码使用了Java I/O类，它们在Java SE 11中已经被废弃，建议使用更现代的文件I/O类，例如NIO 2。

希望这个回答对您有所帮助，如您需要进一步的解释或者有其他问题，请随时联系我。