Java Utililty Methods Collection Join

StringBuilder buff = new StringBuilder();
for (String str : coll) {
    if (buff.length() > 0) {
        buff.append(delimiter);
    buff.append(str);
return buff.toString();
...

return join(collection, ", ");

if (collection != null && collection.size() != 0) {
    StringBuffer sb = new StringBuffer();
    for (String item : collection) {
        sb.append(item);
    return sb.toString();
return null;
...

StringBuilder sb = new StringBuilder();
Iterator<String> it = collection.iterator();
while (it.hasNext()) {
    sb.append(it.next());
    if (it.hasNext()) {
        sb.append(separator);
return sb.toString();

String constructed = "";
if (collection.isEmpty()) {
    return constructed;
for (String item : collection) {
    constructed += item + separator;
return constructed.substring(0, constructed.length() - separator.length());
...

if (collection == null) {
    throw new NullPointerException("The collection parameter is null.");
if (separator == null) {
    throw new NullPointerException("The separator parameter is null.");
StringBuilder buf = new StringBuilder();
for (String s : collection) {
...

if (collection == null)
    return null;
if (collection.isEmpty())
    return "";
StringBuilder sb = new StringBuilder();
for (String s : collection) {
    if (sb.length() != 0)
        sb.append(separator);
...

assert elements != null;
assert elements.isEmpty() == false;
return String.join(", ", elements); 

String ret = null;
for (String s : in) {
    if (ret == null)
        ret = s;
    else
        ret += joinS + s;
if (ret == null)
...

String separator = ", ";
Iterator<String> iterator = items.iterator();
StringBuilder stringBuilder = new StringBuilder();
if (iterator.hasNext()) {
    stringBuilder.append(iterator.next());
    while (iterator.hasNext()) {
        stringBuilder.append(separator).append(iterator.next());
return stringBuilder.toString();