Apache Cassandra was written in Java, and therefore requires a JVM to run. Make sure to download a Java Runtime Environment (JRE) or Java Development Kit (JDK) to include as a part of your Cassandra installation. Unless otherwise noted, the latest patch of version 8 of the Oracle JDK or OpenJDK should be used.
Adjustments to the JVM settings used by Cassandra can be made in the jvm.options configuration file. Many of those settings deal with the configuration and tuning of the garbage collector.