Switch between multiple java versions

Apt-get won't overwrite the existing java versions.

To switch between installed java versions, use the update-java-alternatives command.

List all java versions:

update-java-alternatives --list

Set java version as default (needs root permissions):

sudo update-java-alternatives --set /path/to/java/version

...where /path/to/java/version is one of those listed by the previous command (e.g. /usr/lib/jvm/java-7-openjdk-amd64).

---

Additional information:

update-java-alternatives is a convenience tool that uses Debian's alternatives system (update-alternatives) to set a bunch of links to the specified java version (e.g. java, javac, ...).

Use

sudo update-alternatives --config java

which lists all installed versions with current active one marked and provides dialog to switch:

There are 3 choices for the alternative java (providing /usr/bin/java).

Selection    Path...
------------------------------------------------------------
  0            /usr/lib/jvm/java-9-oracle/bin/java...
* 1            /usr/lib/jvm/java-7-oracle/jre/bin/java...
  2            /usr/lib/jvm/java-8-oracle/jre/bin/java...
  3            /usr/lib/jvm/java-9-oracle/bin/java...

Press <enter> to keep...[*], or type selection number: 

Use

export JAVA_HOME="$(jrunscript -e 'java.lang.System.out.println(java.lang.System.getProperty("java.home"));')"

to set $JAVA_HOME from current active version

Based on the answer from @muet, I found this to work seamlessly:

Add this to ~/.bashrc:

export JAVA_HOME="$(jrunscript -e 'java.lang.System.out.println(java.lang.System.getProperty("java.home"));')"

Add to aliases:

alias useJava8='yes | sudo apt-get install oracle-java8-set-default && source ~/.bashrc'
alias useJava7='yes | sudo apt-get install oracle-java7-set-default && source ~/.bashrc'

Then you can switch within the same shell using only: useJava7or useJava8