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 = ",";
}

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

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)

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.

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

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.

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 ;)

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

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

(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() ? "," : "");

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

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

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());
}

StringBuffer sb = new StringBuffer();

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

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

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);
}

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();

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.

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?"

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?

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

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