The job of supplier() is to return (at every call) a Supplier of an empty mutable result container.

In our case, the result container is of the Map<Boolean, List<Melon>> type, and so supplier() can be implemented as follows:

@Override
public Supplier<Map<Boolean, List<Melon>>> supplier() {

  return () -> {
    return new HashMap<Boolean, List<Melon>> () {
      {
        put(true, new ArrayList<>());
        put(false, new ArrayList<>());
      }
    };
  };
}

In parallel execution, this method may be called multiple times.