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?
A comma-separated list is produced for a structure array when you access one field from multiple structure elements at a time. For instance if S is a 5-by-1 structure array then S.name is a five-element comma-separated list of the contents of the name field.
In Java, we can use String. join(",", list) to join a List String with commas.
Use the join() Function to Convert a List to a Comma-Separated String in Python. The join() function combines the elements of an iterable and returns a string. We need to specify the character that will be used as the separator for the elements in the string.
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();
See also #285523
String delim = ""; for (Item i : list) { sb.append(delim).append(i); delim = ","; }
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