Join a list of strings using `java.util.Optional`

Joiner.java

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public final class Joiner {
 
    public final static Optional<String> join(final String delimiter, final List<String> input) {
        final BiConsumer<StringBuilder, String> appender =
            (acc, next) -> {
                if (acc.length() == 0) acc.append(next);
                else acc.append(delimiter).append(next);
            };
        
        final Collector<String, StringBuilder, String> clctr =
            Collector.of(
                StringBuilder::new,
                appender,
                StringBuilder::append,
                StringBuilder::toString
            );
        
        final Function<List<String>, String> toStr = e -> e.stream().collect(clctr);
        
        return Optional.ofNullable(input).map(toStr);
    }
    
    public static void main(String []args){
        final List<String> empty = Collections.EMPTY_LIST;
        final List<String> single = Collections.singletonList("value");
        final List<String> list = new ArrayList<String>(){{
            add("value1");
            add("value2");
            add("value3");
        }};
        System.out.println("Join null: " + join(", ", null));
        System.out.println("Join an empty list: " + join(", ", null));
        System.out.println("Join a single element list: " + join(", ", single));
        System.out.println("Join a list of values: " + join(", ", list));
    }
}

/*
$java -Xmx128M -Xms16M Joiner
Join null: Optional.empty
Join an empty list: Optional.empty
Join a single element list: Optional[value]
Join a list of values: Optional[value1, value2, value3]
*/