how to remove open-jdk completely in ubuntu?

update-alternatives does a great job switching the default java version, just in case you haven't known it:

$ sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/java-7-oracle/jre/bin/java          1071      auto mode
  1            /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java   1061      manual mode
* 2            /usr/lib/jvm/java-7-oracle/bin/java              1065      manual mode
  3            /usr/lib/jvm/java-7-oracle/jre/bin/java          1071      manual mode

Press enter to keep the current choice[*], or type selection number: 

Finally,I got a solution:disconnect from Internet,then do removing,it is removed completely