$ update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jdk-7u21/bin/java" 1
$ update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/jdk-7u21/bin/javac" 1
$ update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/jdk-7u21/bin/javaws" 1
$ chmod a+x /usr/bin/java
$ chmod a+x /usr/bin/javac
$ chmod a+x /usr/bin/javaws
$ chown -R root:root /usr/lib/jvm/jdk-7u21
Make sure you read the below command output clear and choose which Java you want.
$ update-alternatives --config java
There are 2 alternatives which provide `java'.
Selection Alternative
-----------------------------------------------
+ 1 /usr/lib/jvm/java-6-openjdk/jre/bin/java
* 2 /usr/lib/jvm/jdk-7u21/bin/java
Press enter to keep the default[*], or type selection number: (If you want java 6, write 1 and if you java 7 write 2)
* This commands are being good in debian/ubuntu system. I am not sure about Red Hats/CentOs
