Install Oracle JDK 8 and switch between JDKs in Ubuntu 18.04

1. Check installed Open JDK

Note that by default Ubuntu 18.04 includes Open JDK 10 or 11. Check your current Java version first, or list the JDK package installed.

yao@DESKTOP:~$ sudo apt list --installed | grep jdk

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

openjdk-11-jdk/bionic-updates,bionic-security,now 10.0.2+13-1ubuntu0.18.04.4 amd64 [installed]
openjdk-11-jdk-headless/bionic-updates,bionic-security,now 10.0.2+13-1ubuntu0.18.04.4 amd64 [installed,automatic]
openjdk-11-jre/bionic-updates,bionic-security,now 10.0.2+13-1ubuntu0.18.04.4 amd64 [installed,automatic]
openjdk-11-jre-headless/bionic-updates,bionic-security,now 10.0.2+13-1ubuntu0.18.04.4 amd64 [installed,automatic]

2. Install Oracle JDK 8 (or 9)

Follow Install Oracle Java 8 / 9 in Ubuntu 16.04, Linux Mint 18. It also works in Ubuntu 18.04.

Add PPA

sudo add-apt-repository ppa:webupd8team/java

Install Oracle JDK 8 (or 9)

sudo apt update

sudo apt install oracle-java8-installer

sudo apt install oracle-java9-installer

Now the javac -verison should be switched to the new Oracle JDK 8 (or 9), but better check before we go on.

yao@DESKTOP:~$ javac -version
javac 1.8.0_201

Set Java environment variables

yao@DESKTOP:~$ sudo apt install oracle-java8-set-default
Reading package lists... Done
Building dependency tree
Reading state information... Done
oracle-java8-set-default is already the newest version (8u201-1~webupd8~1).
oracle-java8-set-default set to manually installed.
0 upgraded, 0 newly installed, 0 to remove and 182 not upgraded.

3. Switch between JDKs

yao@DESKTOP:~$ 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-11-openjdk-amd64/bin/java   1101      auto mode
  1            /usr/lib/jvm/java-11-openjdk-amd64/bin/java   1101      manual mode
* 2            /usr/lib/jvm/java-8-oracle/jre/bin/java       1081      manual mode

Press <enter> to keep the current choice[*], or type selection number:2