Oracle Java 8 required by some programs and it is not available on Debian archive due to licensing issues.
To install Java 8 on Debian Stretch with minimum effort, you can use WebUpd8’s Java PPA repositories. Follow the steps below:
$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EEA14886
$ sudo add-apt-repository "deb http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main"
$ sudo apt update
$ sudo apt-get install oracle-java8-installer
If there are other Java/JDK packages on your system, you need to select the default one. Selection can be made with update-alternatives command but WebUpd8 repositories also includes a small package which makes setting Java 8 as default more easily. You just need to install the oracle-java8-set-default package:
$ sudo apt install oracle-java8-set-default
You can also verify the selection with update-alternatives --config java command:
$ 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-8-oracle/jre/bin/java 1081 auto mode
1 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1081 manual mode
* 2 /usr/lib/jvm/java-8-oracle/jre/bin/java 1081 manual mode