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

Question

Using a comma is just to make it specific - it could be any delimiter, so using List's default toString() doesn't help. (The question could be rephrased as: "best way to implement List's toString?")

This is a well-known problem, and I know several ways of doing it, but I'm wondering what the best way is (where "best" means clearest and/or shortest, not most efficient).

The problem as a story: I have a list. 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).

The problem as a grammar (edit: added second alternative):

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: http://stackoverflow.com/questions/285523/last-iteration-of-for-loop-in-java

And another: http://stackoverflow.com/questions/156650/does-the-last-element-in-a-loop-deserve-a-separate-treatment

Answer 1

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 2

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 3

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

Answer 4

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

Answer 5

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

Answer 6

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 7

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 8

See also #285523

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

Answer 9

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 10

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: http://stackoverflow.com/questions/285523/last-iteration-of-for-loop-in-java/669221#669221

Answer 11

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

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

Answer 12

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 13

(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 14

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 15

for (int i=0; i<1; i++)
  sb.append(array[i]);
for (int i=1; i<array.length; i++)
  sb.append(",").append(array[i]);

Based on http://stackoverflow.com/questions/668952/clearest-way-to-comma-delimit-a-list-java/669831#669831

Using this idea: http://stackoverflow.com/questions/156650/does-the-last-element-in-a-loop-deserve-a-separate-treatment

Answer 16

StringBuffer sb = new StringBuffer();

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

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

Answer 17

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 18

One option for the foreach loop is:

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