A Java 8 approach to do strings join

JoinStringsTest.java

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

import java.util.*;
import java.util.concurrent.atomic.AtomicInteger;


/*
 * Test for joining collection of String's into one
 * like ["a", "aaa", "bbbb", "Dda"] to "a,aaa,bbbb,Dda"
 *
 * */

class JoinStringsTest {

    @Test
    void happy() {
        Collection<String> collection = new ArrayList<>();
        String delim = ",";
        Assertions.assertEquals(joinString(collection, delim), Optional.of(""));
    }

    @Test
    void nullCheckArgs() {
        Collection<String> collection = new ArrayList<>();
        String delim = ",";
        Assertions.assertEquals(joinString(collection, delim), Optional.of(""));
    }

    @Test
    void onlyOneElement() {
        Collection<String> collection = new ArrayList<>();
        collection.add("a");
        String delim = ",";
        Assertions.assertEquals(joinString(collection, delim), Optional.of("a"));
    }

    @Test
    void twoElements() {
        Collection<String> collection = new ArrayList<>();
        collection.add("a");
        collection.add("aa");
        String delim = ",";
        Assertions.assertEquals(joinString(collection, delim), Optional.of("a,aa"));
    }


    private static Optional<String> joinString(Iterable<String> collection, String delim) {
        Optional<String> result;
        if (collection == null || delim == null || !collection.iterator().hasNext()) {
            result = Optional.of("");
        } else {
            AtomicInteger finalCount = new AtomicInteger(0);
            Iterator<String> strings = collection.iterator();
            collection.forEach(s -> finalCount.getAndIncrement());
            if (finalCount.get() == 1) {
                result = Optional.of(strings.next());
            } else {
                StringBuilder buffer = new StringBuilder();
                while (strings.hasNext()) {
                    buffer.append(strings.next()).append(delim);
                }
                result = Optional.of(buffer.deleteCharAt(buffer.length() - 1).toString());
            }
        }
        return result;
    }
}