The simplest way to comma-delimit a list?

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?

Unhopedfor

Possible duplicate of What's the best way to build a string of delimited items in Java? – Dichotomy

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 = ",";
}

Chondroma

A very surprising way to store state! It's almost OO polymorphic. I don't like the unnecessary assignment every loop, but I bet it's more efficient than an if. Most solutions repeat a test we know won't be true - though inefficient, they're probably the clearest. Thanks for the link! – Unhopedfor

I like this, but because it's clever and surprising, I thought it wouldn't be appropriate to use (surprising is not good!). However, I couldn't help myself. I'm still not sure about it; however, it is so short that it is easy to work out provided there's a comment to tip you off. – Unhopedfor

@13ren: You and Uncle Bob need to have a talk about code that is "clever and surprising." – Ehtelehud

I feel like I wasted years not knowing this^^; – Subterfuge

Why Java needs to turn such task into an ugly code? – Laflamme

@eduardo - I know. Clojure has a much simpler syntax: (clojure.string/join "," my list) – Chondroma

Thank you for introducing me to StringJoiner..love it – Schroder

Answer 2

org.apache.commons.lang3.StringUtils.join(list,",");

Leaved

Found the source: svn.apache.org/viewvc/commons/proper/lang/trunk/src/java/org/… The join method has 9 overloadings. The iteration-based ones are like "Sample solution 3"; the index-based ones use: if (i > startIndex) { <add separator> } – Unhopedfor

I'm really after implementations though. It's a good idea to wrap it up in a function, but in practice, my delimiter is sometimes a newline, and each line is also indented to some specified depth. Unless... join(list, "\n"+indent) ...will that always work? Sorry, just thinking aloud. – Unhopedfor

In my opinion, this is the prettiest and shortest solution – Graubert

probably, list.iterator()? – Interphone

Answer 3

Java 8 provides several new ways to do this:

• The join method on String.
• A joining collector for streams from the Collectors class.
• The StringJoiner class.

Example:

// Util method for strings and other char sequences
List<String> strs = Arrays.asList("1", "2", "3");
String listStr1 = String.join(",", strs);

// For any type using streams and collectors
List<Object> objs = Arrays.asList(1, 2, 3);
String listStr2 = objs.stream()
    .map(Object::toString)
    .collect(joining(",", "[", "]"));

// Using the new StringJoiner class
StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.setEmptyValue("<empty>");
for (Integer i : objs) {
  joiner.add(i.toString());
}
String listStr3 = joiner.toString();

The approach using streams assumes import static java.util.stream.Collectors.joining;.

Entity

This is IMHO, if you are using java 8, the best answer. – Jenisejenkel

Answer 4

Joiner.on(",").join(myList)

Joiner.

Ehtelehud

I came here looking for something that does exactly this. Thanks! +1 – Abstracted

Answer 5

If you use the Spring Framework you can do it with StringUtils:

public static String arrayToDelimitedString(Object[] arr)
public static String arrayToDelimitedString(Object[] arr, String delim)
public static String collectionToCommaDelimitedString(Collection coll)
public static String collectionToCommaDelimitedString(Collection coll, String delim)

Colugo

Thanks, but how is it implemented? – Unhopedfor

Answer 6

Based on Java's List toString implementation:

Iterator i = list.iterator();
for (;;) {
  sb.append(i.next());
  if (! i.hasNext()) break;
  ab.append(", ");
}

It uses a grammar like this:

List --> (Item , )* Item

By being last-based instead of first-based, it can check for skip-comma with the same test to check for end-of-list. I think this one is very elegant, but I'm not sure about clarity.

Unhopedfor

Answer 7

There is a pretty way to achieve this using Java 8:

List<String> list = Arrays.asList(array);
String joinedString = String.join(",", list);

Stamey

TextUtils.join(",", list); – Denice

Answer 8

for(int i=0, length=list.size(); i<length; i++)
  result+=(i==0?"":", ") + list.get(i);

Balanchine

Yes it's a bit cryptic, but an alternative to what's already been posted. – Balanchine

I think the code looks fine, IMHO its more pretty than your other answer and its hardly 'cryptic'. Wrap it up in a method called Join and your laughing, still I would expect the language to have a better way of solving what is really a common problem. – Marmoreal

@tarn: you think it looks good because you have a Python/perl background ;) I like it as well – Balanchine

Answer 9

String delimiter = ",";
StringBuilder sb = new StringBuilder();
for (Item i : list) {
    sb.append(delimiter).append(i);
}
sb.toString().replaceFirst(delimiter, "");

Escritoire

Answer 10

One option for the foreach loop is:

StringBuilder sb = new StringBuilder();
for(String s:list){
  if (sb.length()>0) sb.append(",");
  sb.append(s);
}

Antihero

Answer 11

StringBuffer result = new StringBuffer();
for(Iterator it=list.iterator; it.hasNext(); ) {
  if (result.length()>0)
    result.append(", ");
  result.append(it.next());
}

Update: As Dave Webb mentioned in the comments this may not produce correct results if the first items in the list are empty strings.

Balanchine

meh.. still has the if in the loop. – Began

You can use a foreach loop to make it shorter. – Cowart

That won't work if the first item in the list is an empty string. – Gunnar

@Dave Webb: Yes, thanks. The question does not have such a requirement though. – Balanchine

@cherovium: The question doesn't say the none of the items will be the empty string so it's an implicit requirement. If you're creating a CSV file empty fields can be pretty common. – Gunnar

Turns out Jon Skeet gave this answer, too (meaning you're in good company): #286023 – Unhopedfor

For heavens sake, use a StringBuilder – Jenisejenkel

Answer 12

I usually do this :

StringBuffer sb = new StringBuffer();
Iterator it = myList.iterator();
if (it.hasNext()) { sb.append(it.next().toString()); }
while (it.hasNext()) { sb.append(",").append(it.next().toString()); }

Though I think I'll to a this check from now on as per the Java implementation ;)

Recto

That's same logic as Sample 3, I like it because it does nothing unnecessary. However, I don't know if it is the clearest. BTW: it's slightly more efficient if you put the while inside the if, by avoiding two tests when the list is empty. It also makes it slighly clearer, to me. – Unhopedfor

For heavens sake, use a StringBuilder – Jenisejenkel

Answer 13

If you can use Groovy (which runs on the JVM):

def list = ['a', 'b', 'c', 'd']
println list.join(',')

Ureter

thanks, but I'm interested in implementations. How is this one implemented? – Unhopedfor

Answer 14

(Copy paste of my own answer from here.) Many of the solutions described here are a bit over the top, IMHO, especially those that rely on external libraries. There is a nice clean, clear idiom for achieving a comma separated list that I have always used. It relies on the conditional (?) operator:

Edit: Original solution correct, but non-optimal according to comments. Trying a second time:

int[] array = {1, 2, 3};
StringBuilder builder = new StringBuilder();
for (int i = 0 ;  i < array.length; i++)
       builder.append(i == 0 ? "" : ",").append(array[i]);

There you go, in 4 lines of code including the declaration of the array and the StringBuilder.

2nd Edit: If you are dealing with an Iterator:

    List<Integer> list = Arrays.asList(1, 2, 3);
    StringBuilder builder = new StringBuilder();
    for (Iterator it = list.iterator(); it.hasNext();)
        builder.append(it.next()).append(it.hasNext() ? "," : "");

Jap

This and #669452 above are similar. It's short and clear, but if you only have an iterator, you'd first need to convert. Inefficient, but maybe worth it for the clarity of this approach? – Unhopedfor

Ugh, string concatenation creating a temporary StringBuilder within use of a StringBuilder. Inefficient. Better (more likes of Java, but better running code) would be "if (i > 0) { builder.append(','); }" followed by builder.append(array[i]); – Ruddy

Yes, if you look in the bytecode, you are right. I can't believe such a simple question can get so complicated. I wonder if the compiler can optimize here. – Jap

Provided 2nd, hopefully better solution. – Jap

Might be better to split them (so people can vote on one or other). – Unhopedfor

I'd assumed this test-every-time-style only worked with arrays, not iterators - good one! BTW: You have two "hasNext()" tests - they can be combined like this: #669452 Shorter is nice, but which is clearer? – Unhopedfor

That is, a single hasNext() can both break out of the loop, and omit the last comma. – Unhopedfor

Answer 15

I usually use something similar to version 3. It works well c/c++/bash/... :P

Began

Answer 16

This is very short, very clear, but gives my sensibilities the creeping horrors. It's also a bit awkward to adapt to different delimiters, especially if a String (not char).

for (Item i : list)
  sb.append(',').append(i);
if (sb.charAt(0)==',') sb.deleteCharAt(0);

Inspired by: Last iteration of enhanced for loop in java

Unhopedfor

Answer 17

I didn't compile it... but should work (or be close to working).

public static <T> String toString(final List<T> list, 
                                  final String delim)
{
    final StringBuilder builder;

    builder = new StringBuilder();

    for(final T item : list)
    {
        builder.append(item);
        builder.append(delim);
    }

    // kill the last delim
    builder.setLength(builder.length() - delim.length());

    return (builder.toString());
}

Extortion

I like this way, i would just prefer an other formatting. But to mention one small fix: If the list is empty, this will throw an IndexOutOfBounds exception. So, either do a check before the setLength, or use Math.max(0,...) – Anhedral

Answer 18

StringBuffer sb = new StringBuffer();

for (int i = 0; i < myList.size(); i++)
{ 
    if (i > 0) 
    {
        sb.append(", ");
    }

    sb.append(myList.get(i)); 
}

Irritant

I find this one to be very easy to embed into a static utility class, while being very easy to read and understand as well. It also does't require any extra variables or state other than the list itself, the loop and the delimiter. – Higgle

This will provide something like: Item1Item2, Item3, Item4, – Musculature

For heavens sake, use a StringBuilder – Jenisejenkel

Answer 19

I somewhat like this approach, which I found on a blog some time ago. Unfortunately I don't remember the blog's name/URL.

You can create a utility/helper class that looks like this:

private class Delimiter
{
    private final String delimiter;
    private boolean first = true;

    public Delimiter(String delimiter)
    {
        this.delimiter = delimiter;
    }

    @Override
    public String toString()
    {
        if (first) {
            first = false;
            return "";
        }

        return delimiter;
    }
}

Using the helper class is simple as this:

StringBuilder sb = new StringBuilder();
Delimiter delimiter = new Delimiter(", ");

for (String item : list) {
    sb.append(delimiter);
    sb.append(item);
}

Armilla

Just a note: it should be "delimiter", not "delimeter". – Burchell

Answer 20

I like this solution:

String delim = " - ", string = "";

for (String item : myCollection)
    string += delim + item;

string = string.substring(delim.length());

I assume it can make use of StringBuilder too.

Bevbevan

Answer 21

Because your delimiter is ", " you could use any of the following:

public class StringDelim {

    public static void removeBrackets(String string) {
        System.out.println(string.substring(1, string.length() - 1));
    }

    public static void main(String... args) {
        // Array.toString() example
        String [] arr = {"Hi" , "My", "name", "is", "br3nt"};
        String string = Arrays.toString(arr);
        removeBrackets(string);

        // List#toString() example
        List<String> list = new ArrayList<String>();
        list.add("Hi");
        list.add("My");
        list.add("name");
        list.add("is");
        list.add("br3nt");
        string = list.toString();
        removeBrackets(string);

        // Map#values().toString() example
        Map<String, String> map = new LinkedHashMap<String, String>();
        map.put("1", "Hi");
        map.put("2", "My");
        map.put("3", "name");
        map.put("4", "is");
        map.put("5", "br3nt");
        System.out.println(map.values().toString());
        removeBrackets(string);

        // Enum#toString() example
        EnumSet<Days> set = EnumSet.allOf(Days.class);
        string = set.toString();
        removeBrackets(string);
    }

    public enum Days {
        MON("Monday"),
        TUE("Tuesday"),
        WED("Wednesday"),
        THU("Thursday"),
        FRI("Friday"),
        SAT("Saturday"),
        SUN("Sunday");

        private final String day;

        Days(String day) {this.day = day;}
        public String toString() {return this.day;}
    }
}

If your delimiter is ANYTHING else then this isn't going to work for you.

Bevbevan

Answer 22

In my opinion, this is the simplest to read and understand:

StringBuilder sb = new StringBuilder();
for(String string : strings) {
    sb.append(string).append(',');
}
sb.setLength(sb.length() - 1);
String result = sb.toString();

Orman

Answer 23

public static String join (List<String> list, String separator) {
  String listToString = "";

  if (list == null || list.isEmpty()) {
   return listToString;
  }

  for (String element : list) {
   listToString += element + separator;
  }

  listToString = listToString.substring(0, separator.length());

  return listToString;
}

Tommie

I think you meant to say listToString.substring(0, listToString.length()-separator.length()); – Unhopedfor

Answer 24

In Python its easy

",".join( yourlist )

In C# there is a static method on the String class

String.Join(",", yourlistofstrings)

Sorry, not sure about Java but thought I'd pipe up as you asked about other languages. I'm sure there would be something similar in Java.

Marmoreal

IIRC, you get a trailing , in the .Net version. :-( – Hipparchus

Just tested, no trailing delimiter in .NET or Python – Marmoreal

thanks! I found the source for it: svn.python.org/view/python/branches/release22-branch/Objects/… search for "Catenate everything". It omits the separator for the last item. – Unhopedfor

@Unhopedfor Nicely done! Does it help? I had a lock and quickly remembered why I choose not to program in C these days :P – Marmoreal

@tarn, yeah, it's an interesting example because it only adds the comman if it is not the last item: if (i < seqlen - 1) { <add separator> } – Unhopedfor

Answer 25

You can also unconditionally add the delimiter string, and after the loop remove the extra delimiter at the end. Then an "if list is empty then return this string" at the beginning will allow you to avoid the check at the end (as you cannot remove characters from an empty list)

So the question really is:

"Given a loop and an if, what do you think is the clearest way to have these together?"

Wishywashy

Answer 26

if (array.length>0)          // edited in response Joachim's comment
  sb.append(array[i]);
for (int i=1; i<array.length; i++)
  sb.append(",").append(array[i]);

Based on Clearest way to comma-delimit a list (Java)?

Using this idea: Does the last element in a loop deserve a separate treatment?

Unhopedfor

For this to work, you need to know that there is at least 1 element in the list, otherwise your first for-loop will crash with an IndexOutOfBoundsException. – Higgle

@Joachim thanks, you're right of course. What an ugly solution I wrote! I'll change for (int i=0; i<1; i++) to if (array.length>0). – Unhopedfor

Answer 27

public String toString(List<Item> items)
{
    StringBuilder sb = new StringBuilder("[");

    for (Item item : items)
    {
        sb.append(item).append(", ");
    }

    if (sb.length() >= 2)
    {
        //looks cleaner in C# sb.Length -= 2;
        sb.setLength(sb.length() - 2);
    }

    sb.append("]");

    return sb.toString();
}

Stoop

Just noticed it's similar to the answer TofuBeer gave – Stoop

Answer 28

None of the answers uses recursion so far...

public class Main {

    public static String toString(List<String> list, char d) {
        int n = list.size();
        if(n==0) return "";
        return n > 1 ? Main.toString(list.subList(0, n - 1), d) + d
                  + list.get(n - 1) : list.get(0);
    }

    public static void main(String[] args) {
        List<String> list = Arrays.asList(new String[]{"1","2","3"});
        System.out.println(Main.toString(list, ','));
    }

}

Rossiya

Answer 29

private String betweenComma(ArrayList<String> strings) {
    String united = "";
    for (String string : strings) {
        united = united + "," + string;
    }
    return united.replaceFirst(",", "");
}

Meadows

Answer 30

Method

String join(List<Object> collection, String delimiter){
    StringBuilder stringBuilder = new StringBuilder();
    int size = collection.size();
    for (Object value : collection) {
        size --;
        if(size > 0){
            stringBuilder.append(value).append(delimiter);
        }
    }

    return stringBuilder.toString();
}

Usage

Given the array of [1,2,3]

join(myArray, ",") // 1,2,3

Grig