It does not have to be a Java action. All can be done with microflows. Use indeed the find and substring actions. But may I ask why you want to do this? Because if it is for searching check the Lucine app in the appstore https://appstore.home.mendix.com/link/app/3099/FlowFabric-BV/Lucene-text-search
The best way to accomplish this is to create a custom java action that takes in the string as a parameter and does the .split() java function on a space (you would have to look up what a space is in regex). And have the java action return the list of separated strings.