Make project compile against jdk8

Not able to compile the project against JDK8 on my WSL. There are multiple issues it seems:
1. It's referring to a JDK9 flag during compilation.
2. It's using a lambda syntax that fails to compile on JDK8.

Start by reproducing the project compilation with JDK 8 and reading the compiler errors to locate the JDK 9 flag and lambda syntax mentioned in the issue. Then inspect the relevant build configuration and source; done means the project compiles successfully against JDK 8.

Written by the indexing model from the issue text.