Question

What is the clearest way to comma-delimit a list in Java?

I know several ways of doing it, but I’m wondering what the best way is (where ‘best’ means clearest and/or shortest, not the most efficient.

I have a list and I want to loop over it, printing each value. I want to print a comma between each item, but not after the last one (nor before the first one).

List --> Item ( , Item ) * List --> ( Item , ) * Item 

Sample solution 1:

boolean isFirst = true; for (Item i : list) {   if (isFirst) {     System.out.print(i);        // no comma     isFirst = false;   } else {     System.out.print(', '+i);   // comma   } } 

Sample solution 2 – create a sublist:

if (list.size()>0) {   System.out.print(list.get(0));   // no comma   List theRest = list.subList(1, list.size());   for (Item i : theRest) {     System.out.print(', '+i);   // comma   } } 

Sample solution 3:

  Iterator<Item> i = list.iterator();   if (i.hasNext()) {     System.out.print(i.next());     while (i.hasNext())       System.out.print(', '+i.next());   } 

These treat the first item specially; one could instead treat the last one specially.

Incidentally, here is how List toString is implemented (it’s inherited from AbstractCollection), in Java 1.6:

public String toString() {     Iterator<E> i = iterator();     if (! i.hasNext())         return '[]';      StringBuilder sb = new StringBuilder();     sb.append('[');     for (;;) {         E e = i.next();         sb.append(e == this ? '(this Collection)' : e);         if (! i.hasNext())             return sb.append(']').toString();         sb.append(', ');     } } 

It exits the loop early to avoid the comma after the last item. BTW: this is the first time I recall seeing ‘(this Collection)’; here’s code to provoke it:

List l = new LinkedList(); l.add(l); System.out.println(l); 

I welcome any solution, even if they use unexpected libraries (regexp?); and also solutions in languages other than Java (e.g. I think Python/Ruby have an intersperse function – how is that implemented?).

Clarification: by libraries, I mean the standard Java libraries. For other libraries, I consider them with other languages, and interested to know how they’re implemented.

EDIT toolkit mentioned a similar question: Last iteration of enhanced for loop in java

And another: Does the last element in a loop deserve a separate treatment?

Answer 1

Java 8 and later

Using StringJoiner class, and forEach method :

StringJoiner joiner = new StringJoiner(","); list.forEach(item -> joiner.add(item.toString()); return joiner.toString(); 

Using Stream, and Collectors:

return list.stream().        map(Object::toString).        collect(Collectors.joining(",")).toString(); 

Java 7 and earlier

See also #285523

String delim = ""; for (Item i : list) {     sb.append(delim).append(i);     delim = ","; }