首先查询有多少种JDK已经被安装了：
sudo update-alternatives --list java
其次 配置你想默认的JDK：
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-6-openjdk/jre/bin/java 1061 auto mode
1 /usr/lib/jvm/java-6-openjdk/jre/bin/java 1061 manual mode
2 /usr/lib/jvm/java-6-sun/jre/bin/java 63 manual mode
Press enter to keep the current choice[*], or type selection number: 2
此外有些需要编译的 还需要设定 javac 和 javadoc。