题目地址：https://www.icourse163.org/learn/PKU-1001941004?tid=1450230441#/learn/hw?id=1222245416

下面的程序可以下载多个网页文件(download方法已写好），请将它改成多线程进行下载（评分占7分），如果可能， 显示计算全部下载完成程序所用的时间（提示：new Date().getTime()可以得到当前时间的毫秒数，评分占3分）。另外，请注意一下，

系统中http:传到平台后，它自动改成了https:，所以请改回http:。  另外，有一些链接访问不了，所以要注意加try...catch。

import java.net.URL;
import java.io.*;
 
class Downloader 
{
    public static void main(String[] args)
        throws Exception
    {
        final URL[] urls = {
            new URL("https://www.pku.edu.cn"),
            new URL("https://www.baidu.com"),
            new URL("https://www.sina.com.cn"),
            new URL("https://www.dstang.com")
        };
        final String[] files = {
            "pku.htm", 
            "baidu.htm",
            "sina.htm", 
            "study.htm",
        };
 
        for(int idx=0; idx<urls.length; idx++){
            try{
                System.out.println( urls[idx] );
                download( urls[idx], files[idx]);
            }catch(Exception ex){
                ex.printStackTrace();
            }
        }
    }
    static void download( URL url, String file)
        throws IOException
    {
        try(InputStream input = url.openStream();
            OutputStream output = new FileOutputStream(file))
        {
            byte[] data = new byte[1024];
            int length;
            while((length=input.read(data))!=-1){
                output.write(data,0,length);
            }
        }
    }
}

解答如下：

package week8;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.util.Date;

public class task1 {

	public static void main(String[] args) throws Exception
	{
		// TODO Auto-generated method stub
		final URL[] urls = {
	            new URL("http://www.pku.edu.cn"),
	            new URL("http://www.baidu.com"),
	            new URL("http://www.sina.com.cn"),
	            new URL("http://www.dstang.com")
	        };
        final String[] files = {
            "pku.htm", 
            "baidu.htm",
            "sina.htm", 
            "study.htm",
        };
        
        Thread [] threadPool = new Thread[urls.length];
        DownloadWeb [] downloadWeb = new DownloadWeb[urls.length];
        
        for(int i=0; i<urls.length; i++)
        {            
            System.out.println( urls[i] );  
            downloadWeb[i] = new DownloadWeb(urls[i],files[i]);
            threadPool[i] = new Thread(downloadWeb[i]);
            threadPool[i].run();
        }
	}

}

class DownloadWeb implements Runnable
{
	URL url;
	String file;
	
	DownloadWeb(URL url, String file)
	{
		this.url = url;
		this.file = file;
	}
	
	public void run()
	{
		long temp = new Date().getTime();
		try
		{
			download(this.url, this.file);
		}
		catch(Exception ex)
		{
			ex.printStackTrace();
			System.out.printf("下载%s失败\n", file); 
		}
		
		System.out.printf("下载%s花了%dl毫秒\n", file, new Date().getTime() - temp); //long的占位符居然是dl，郁闷死
	}
	
	private void download( URL url, String file) throws IOException
    {
        try(InputStream input = url.openStream();
            OutputStream output = new FileOutputStream(file))
        {
            byte[] data = new byte[1024];
            int length;
            
            while((length=input.read(data))!=-1)
            {
                output.write(data,0,length);
            }
        }
    }
}