The lilbitcoinj library is a fork of the bitcoinj project. lilbitcoinj has much less surface area than bitcoinj and keeps the code that is useful for deriving keys and addresses from seeds.
To get started, it is best to have the latest JDK and Gradle installed. The HEAD of the master branch contains the latest development code and various production releases are provided on feature branches.
Build with JDK 11
gradle clean build
To perform a full build without unit/integration tests use:
gradle clean assemble
Alternatively, just import the project using your IDE. IntelliJ has Gradle integration built-in and has a free Community Edition. Simply use File | New | Project from Existing Sources and locate the build.gradle in the root of the cloned project source tree.