Your farmerswife system needs to be upgraded and running on version 6.5.
Java is only needed to be installed on the fw Server host machine, as it is needed by the built-in web server and powers the Web Client and Mobile Web Client (and fw play if licensed).
Download OpenJDK e.g. here: https://adoptopenjdk.net/
Use the Hotspot version.
The installers are called for example:
On Windows: OpenJDK8U-jdk_x64_windows_hotspot_8u222b10.msi
On Mac: OpenJDK8U-jdk_x64_mac_hotspot_8u222b10.pkg
On Linux: OpenJDK8U-jdk_x64_linux_hotspot_8u222b10.tar.gz
Then first UNINSTALL ALL previous Java versions (especially the old deprecated v1.6 and v1.7) on the fw Server host machine; you might need to search for uninstall instructions.
Then Install OpenJDK.
After launching the fw Server application again, check e.g. that the Web Client is properly running.
JRE - Java Runtime Environment
JDK - Java Development Kit