Java 建立一个多线程程序，分别用两个线程读取x1,x2两个文件数据内容，数字部分乘以2倍，间隔两秒中往X3文件中复制写入一次数据

在Java中创建一个多线程程序，你可以使用`Thread`类或者更推荐的`ExecutorService`和`Callable`/`Future`来实现。这里是一个简单的示例，我们将使用`java.util.concurrent`包下的工具：

import java.io.*;
import java.util.concurrent.*;

public class MultiThreadedFileProcessor {
    private static final String FILE_X1 = "x1.txt";
    private static final String FILE_X2 = "x2.txt";
    private static final String FILE_X3 = "x3.txt";

    public static void main(String[] args) throws IOException, InterruptedException {
        ExecutorService executor = Executors.newSingleThreadExecutor();
        Future<String> future1 = executor.submit(new FileReaderAndWriter(FILE_X1, FILE_X3, 2000));
        Future<String> future2 = executor.submit(new FileReaderAndWriter(FILE_X2, FILE_X3, 2000));

        // 等待所有任务完成并关闭线程池
        try {
            String result1 = future1.get(); // 获取第一个文件处理结果
            String result2 = future2.get(); // 获取第二个文件处理结果
            System.out.println("Results: " + result1 + ", " + result2);
        } catch (ExecutionException e) {
            e.printStackTrace();
        } finally {
            executor.shutdown();
        }
    }

    // 文件读取并乘以2后写入新文件的线程类
    private static class FileReaderAndWriter implements Callable<String> {
        private String inputFile;
        private String outputFile;
        private long interval;

        public FileReaderAndWriter(String inputFile, String outputFile, long interval) {
            this.inputFile = inputFile;
            this.outputFile = outputFile;
            this.interval = interval;
        }

        @Override
        public String call() {
            try (BufferedReader reader = new BufferedReader(new FileReader(inputFile));
                 BufferedWriter writer = new BufferedWriter(new FileWriter(outputFile))) {

                String line;
                while ((line = reader.readLine()) != null) {
                    int number = Integer.parseInt(line); // 假设都是整数
                    int doubledNumber = number * 2;
                    writer.write(doubledNumber + "\n"); // 写入新的数值
                    Thread.sleep(interval); // 暂停两秒
                }
                return "Write to file " + outputFile + " completed.";
            } catch (IOException | InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

在这个例子中，我们创建了一个单线程的`ExecutorService`，然后提交了两个任务（`future1`和`future2`），每个任务代表读取一个文件并每两秒写入到`FILE_X3`。注意，实际应用中需要处理可能出现的异常，并在适当的时候关闭输入输出流。