For a list of installed versions you can use the following command:
update-java-alternatives -l
If you have many versions installed, you can select which one will be the default via the following command:
update-alternatives --config java
You can install version 8 specifically (recommended for Java Web applications) install via command:
sudo apt install openjdk-8-jdk
You may install any specific version with the following command (replace X with your version):
sudo apt install openjdk-X-jdk
To install the latest one use the command:
sudo apt install default-jdk
To learn more about current state of Java check these articles: