The most sophisticated way for creating comma-separated Strings from a Collection/Array/List?

During my work with databases I noticed that I write query strings and in this strings I have to put several restrictions in the where-clause from a list/array/collection. Should look like this:

select * from customer 
where customer.id in (34, 26, ..., 2);

You can simplify this by reducing this to the question that you have collection of strings and want to create a comma-separated list of this strings in just one string.

My approach I have used so far is something like that:

String result = "";
boolean first = true;
for(String string : collectionOfStrings) {
    if(first) {
        result+=string;
        first=false;
    } else {
        result+=","+string;
    }
}

But this is as you can see very ugly. You cannot see what happens there on the first look, especially when the constructed strings (like every SQL query) is getting complicated.

What is your (more) elegant way?

Use the Google Guava API's join method:

Joiner.on(",").join(collectionOfStrings);

Note: This answers was good when it was written 11 years ago, but now there are far better options to do this more cleanly in a single line, both using only Java built-in classes or using a utility library. See other answers below.

Since strings are immutable, you may want to use the StringBuilder class if you're going to alter the String in the code.

The StringBuilder class can be seen as a mutable String object which allocates more memory when its content is altered.

The original suggestion in the question can be written even more clearly and efficiently, by taking care of the redundant trailing comma:

    StringBuilder result = new StringBuilder();
    for(String string : collectionOfStrings) {
        result.append(string);
        result.append(",");
    }
    return result.length() > 0 ? result.substring(0, result.length() - 1): "";

I just looked at code that did this today. This is a variation on AviewAnew's answer.

collectionOfStrings = /* source string collection */;
String csList = StringUtils.join(collectionOfStrings.toArray(), ",");

The StringUtils ( <-- commons.lang 2.x, or commons.lang 3.x link) we used is from Apache Commons.