What are best practices for working with custom collectors?

        import java.util.*;
        import java.util.stream.Collector;
        import java.util.stream.Collectors;

        public class CustomCollectorExample {
            public static void main(String[] args) {
                List names = Arrays.asList("Alice", "Bob", "Charlie", "David");

                // Using a custom collector to join names in a specific format
                String collectedNames = names.stream().collect(Collectors.joining(", ", "[", "]"));
                System.out.println(collectedNames); // Output: [Alice, Bob, Charlie, David]
            }
        }

        // Creating a custom collector
        public class ToUpperCaseCollector {
            public static Collector> toUpperCaseList() {
                return Collectors.mapping(String::toUpperCase, Collectors.toList());
            }
        }