noob java question: is the difference between

DataFetcher<CompletableFuture<String>>

and

DataFetcher<CompletableFuture<Boolean>>

the return type (the first returns a string and second returns a boolean)?

It is, DataFetcher is an interface. More on interface here - https://www.javatpoint.com/interface-in-java. DataFetcher docs here: https://javadoc.io/doc/com.graphql-java/graphql-java/11.0/graphql/schema/DataFetcher.html