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Install JDK version 8. If you want you can do it from https://adoptopenjdk.net/index.html?variant=openjdk8&jvmVariant=hotspot
Install Robocode from here: https://sourceforge.net/projects/robocode/files/robocode/1.9.3.5/
Open editor, create robot and plaa plaa plaa. The robocode runtime has built-in editor and compiler, and the package contains example robots.