老是，我这写的URL出现这个情况？

java.net.UnknownHostException: www.baidu.com

at java.net.AbstractPlainSocketImpl.connect(Unknown Source)

at java.net.PlainSocketImpl.connect(Unknown Source)

at java.net.SocksSocketImpl.connect(Unknown Source)

at java.net.Socket.connect(Unknown Source)

at java.net.Socket.connect(Unknown Source)

at sun.net.NetworkClient.doConnect(Unknown Source)

at sun.net.www.http.HttpClient.openServer(Unknown Source)

at sun.net.www.http.HttpClient.openServer(Unknown Source)

at sun.net.www.http.HttpClient.<init>(Unknown Source)

at sun.net.www.http.HttpClient.New(Unknown Source)

at sun.net.www.http.HttpClient.New(Unknown Source)

at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(Unknown Source)

at sun.net.www.protocol.http.HttpURLConnection.plainConnect(Unknown Source)

at sun.net.www.protocol.http.HttpURLConnection.connect(Unknown Source)

at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)

at java.net.URL.openStream(Unknown Source)

at com.helloworld.HelloWorld.main(HelloWorld.java:33)