Clearest 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?

Answer 1

Java 8 and later

Using StringJoiner class :

StringJoiner joiner = new StringJoiner(",");
for (Item item : list) {
    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 = ",";
}

Answer 2

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

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;.

Answer 4

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

Joiner.

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)

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.

Answer 7

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

Answer 8

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

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

Answer 9

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

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);
}

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.

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 ;)

Answer 13

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

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

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() ? "," : "");

Answer 15

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

Answer 16

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());
}

Answer 17

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

Answer 18

StringBuffer sb = new StringBuffer();

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

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

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);
}

Answer 20

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.

Answer 21

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.

Answer 22

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.

Answer 23

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?"

Answer 24

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;
}

Answer 25

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?

Answer 26

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();
}

Answer 27

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, ','));
    }

}

Answer 28

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();

Answer 29

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

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