Z2 has the following Java Version requirements

Z2 Version Min. Java version required Max Java version supported
2.1 - 2.3.1 Java 6 Java 7
2.4 - master Java 8 Java 8

Note: Most samples suggest to use the master branch. You may choose another version branch (please check the respective repository).
Make sure you have a corresponding Java Development Kit (JDK) or Java Runtime Environment (JRE) installed. If in doubt, go to Download Java SE.

Note: Running v2.1-v2.3.1 on Java 8 is supported by specifying

com.zfabrik.java.level=7

(or 6, if that is your desired compilation language level) in <home>/run/bin/runtime.properties. By this the Java compiler version detection does not fall back to a lower level.