WireframeSketcher is a Java application, however you don't need to install Java on your system, and WireframeSketcher's installer won't install Java on your system either. WireframeSketcher is bundled with a private Java VM which is only used by WireframeSketcher and is not installed in any way on your system. So from user's point it looks and behaves like a regular native application. Give it a try.
I hope this helps.
William Bell31 Mar 2020
OK, thank you for your quick response - much appreciated!