ONOS is a Java based platform. In order to be able to compile the code, you need to install Oracle Java 1.8. The OpenJDK equivalent might work, but it's not suggested, since it has never been tested.
sudo apt-get install software-properties-common -y && \ sudo add-apt-repository ppa:webupd8team/java -y && \ sudo apt-get update && \ echo "oracle-java8-installer shared/accepted-oracle-license-v1-1 select true" | sudo debconf-set-selections && \ sudo apt-get install oracle-java8-installer oracle-java8-set-default -y |