How does java obtain url domain names and automatically concatenate URLs?

For example, a webpage is called

Http://xx.com/a2/

Parse the content of this webpage, which contains many

<A href = "/b2/"> b2 </a>
<A href = "2.html"> 2.html </a>
<A href = "../3.html"> 3.html </a>

If the calculation is performed one by one, it seems to be tolerable. java is used to determine that, for example, "/" at the beginning indicates that the root directory should be added, and "../" indicates that the root directory should be jumped up to a certain level.

What if.../appears. Jump up to Level 2

.../Jump up to 3 levels

There are some unexpected ones, such ./

There is no need to make judgments one by one. java already has built-in functions to solve the url splicing problem.

The code is as follows:	Copy code
JoinUrl ("http://xx.com/a2/", "/b2 /");

Everything is done.

The code is as follows:	Copy code
Public static String joinUrl (String curl, String file ){ URL url = null; String q = ""; Try { Url = new URL (curl), file ); Q = url. toExternalForm (); } Catch (MalformedURLException e ){     } Url = null; If (q. indexOf ("#")! =-1) q = q. replaceAll ("^ (. + ?) #.*? $ "," $1 "); Return q;  }

 
Used to quickly obtain domain names

The code is as follows:	Copy code
Public static String getDomain (String curl ){ URL url = null; String q = ""; Try { Url = new URL (curl ); Q = url. getHost (); } Catch (MalformedURLException e ){     } Url = null; Return q;  }