In various tutorials, it is said, that you should remove existing JRE/JDK before installing new one (e.g. remove OpenJDK before installing the one from Oracle). Is it really necessary? I remember having 2 JREs at one point, being able to choose in my IDE which to run program on.
Don't remember to have any conflicts, what's the chance of having any?
You can install multiple JRE/JDK at a time. You just need to configure which one to use by using update-alternatives
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-7-openjdk-amd64/jre/bin/java 1051 manual mode
Press enter to keep the current choice[*], or type selection number: 1
Similarly, you can configure your JDK with
sudo update-alternatives --config javac
