1. 线程的创建与启动 QQ文件传输支持多线程，可以在接收文件的同时进行文件发送。不考虑同一个文件多线程收发的问题，编写ReceiveFileRunnable与SendFileRunnable类（两个类都具有构造方法，带一个String参数，传入收发的文件名），均实现Runnable接口。这两个类的run()方法分别有如下功能： (1)分别打印"I am receiving file:xxx.x" 和"I am sending file:yyy.y"，表示开始收发文件；其中，"xxx.x"和"yyy.y"为收发的文件名； (2)休眠3000ms-6000ms间的随机时间(Thread.sleep..)；分别打印"File:xxx.x has been received."和"File:yyy.y has been sent."，表示收发结束。 编写TestThread类，在main方法中启动5个ReceiveFileRunnable与5个SendFileRunnable线程。当这5个ReceiveFileRunnable与5个SendFileRunnable线程结束，才执行最后一句代码，打印"QQ文件收发任务结束"(使用join)。

代码如下：

public class ReceiveFileRunnable implements Runnable {
    private String fileName;

    public ReceiveFileRunnable(String fileName) {
        this.fileName = fileName;
    }

    @Override
    public void run() {
        System.out.println("I am receiving file: " + fileName);
        try {
            Thread.sleep((long) (3000 + Math.random() * 3000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("File: " + fileName + " has been received.");
    }
}

public class SendFileRunnable implements Runnable {
    private String fileName;

    public SendFileRunnable(String fileName) {
        this.fileName = fileName;
    }

    @Override
    public void run() {
        System.out.println("I am sending file: " + fileName);
        try {
            Thread.sleep((long) (3000 + Math.random() * 3000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("File: " + fileName + " has been sent.");
    }
}

public class TestThread {
    public static void main(String[] args) throws InterruptedException {
        Thread[] threads = new Thread[10];
        for (int i = 0; i < 5; i++) {
            threads[i] = new Thread(new ReceiveFileRunnable("file" + (i + 1) + ".txt"));
            threads[i + 5] = new Thread(new SendFileRunnable("file" + (i + 1) + ".txt"));
        }

        for (Thread thread : threads) {
            thread.start();
        }

        for (Thread thread : threads) {
            thread.join();
        }

        System.out.println("QQ文件收发任务结束");
    }
}

解释：

1. ReceiveFileRunnable类和SendFileRunnable类分别实现了Runnable接口，并在run()方法中完成了题目要求的功能。

2. TestThread类中，我们先创建了10个线程，其中前5个为ReceiveFileRunnable线程，后5个为SendFileRunnable线程。

3. 在启动所有线程之后，我们使用join()方法等待所有线程执行完毕。

4. 最后打印"QQ文件收发任务结束"。