这道题的答案是什么？

okone96

楼上用什么工具了？， 我这里debug仍然是4啊

justforregister

就eclipse啊

okone96

最初由 justforregister 发布
[B]就eclipse啊 [/B]

你用的jdk是哪个版本？
结果仍然是4，问题出现在URL这个类，
URL url1 = new URL(URL_NAMES[0]);
URL url2 = new URL(URL_NAMES[1]);

if(url1.equals(url2)){
     System.out.println("yes";
}else
    System.out.println("no ";

输出yes

我跟进JDKSRC看了一下，因为这两个url除了url.host不一样外其他都一样
输出url1.host()和url2.host()肯定是不相等的，但是i(url1.equals(url2))执行这个语句的时候，url类里比较host（因为在这里其他信息都相等），有个hostsEqual(u1, u2)方法，表面上看不相等，但恰恰相等了，这是因为这两个url的IP地址是一样的，cmd 运行ping url 试一下，说明jdk url 类 host比较时用了IP不是host串

okone96

最初由 justforregister 发布
[B]就eclipse啊 [/B]

太刺激了
楼上偶说的有点误
我把网线拔了，执行了一下，令人吃惊的效果
结果是 和你所说的结果一样
所以肯定：
1 在联网时比较的是new Url(url).equals() 时的对应IP地址
2 不联网时直接比较host串

因为
"http://javapuzzlers.com"
"http://apache2-snort.skybar.dreamhost.com"
这两个url的ip地址是一样，但host不相等的
所以
连网时这2个相等和不连网的情况下执行结果是不相等的

okone96

justforregister
同志的在eclipse的电脑估计是断网的吧

justforregister

最初由 okone96 发布
[B]justforregister
同志的在eclipse的电脑估计是断网的吧 [/B]

待会在 看看

cfbtw

for (String urlName : URL_NAMES)
                            favorites.add(new URL(urlName));


for (String urlName :URL_NAMES){
          URL url = new URL(urlName);
          System.out.println("URL:"  + URL  + " --- > " + url.hashcode()) ;
          favorites.add(url);
}

打印出hashcode就清楚了， URL 类的hashcode 的实现的 bug

cfbtw

和okone说的一样

偶google下，发现原来是javaone 演讲的东东的 ...
http://java.sun.com/javaone/sf/2007/articles/puzzlers.jsp

The second puzzle, aptly titled "More Joys of Sets" has the user create HashMap keys that consist or several URL objects. Again, most of the audience was unable to guess the correct answer.

The important thing the audience learned here is that the URL object's equals() method is, in effect, broken. In this case, two URL objects are equal if they resolve to the same IP address and port, not just if they have equal strings. However, Bloch and Pugh point out an even more severe Achilles' Heel: the equality behavior differs depending on if you're connected to the network, where virtual addresses can resolve to the same host, or if you're not on the net, where the resolve is a blocking operation. So, as far as lessons learned, they recommend:

    * Don't use URL; use URI instead. URI makes no attempt to compare addresses or ports. In addition, don't use URL as a Set element or a Map key.
    * For API designers, the equals() method should not depend on the environment. For example, in this case, equality should not change if a computer is connected to the Internet versus standalone.

braveone

太强了，知道这道题决不简单，没有想到如此复杂。

justforregister

学习了