The Java Development Kit (JDK) is a software development environment used for developing Java applications and applets. It includes the Java Runtime Environment (JRE), an interpreter/loader (java), a compiler (javac), an archiver (jar), a documentation generator (javadoc) and other tools needed in Java development.
The Java Development Kit (JDK) is required in order to compile SimStadt or a new SimStadt workflow. The current SimStadt version requires Java 8, and will not compile with newer versions.
Since January 2019, Oracle no longer issues public updates for commercial users of Java 8. You can download a full JDK 8 from Liberica instead.