What's the most elegant way to concatenate a list of values with delimiter in Java?

Question

I have never found a neat(er) way of doing the following.

Say I have a list/array of strings.

abc
def
ghi
jkl

And I want to concatenate them into a single string delimited by a comma as follows:

abc,def,ghi,jkl

In Java, if I write something like this (pardon the syntax),

String[] list = new String[] {"abc","def","ghi","jkl"};
String str = null;
for (String s : list)
{
   str = str + s + "," ;
}

System.out.println(str);

I'll get

abc,def,ghi,jkl,  //Notice the comma in the end

So I have to rewrite the above for loop as follows

...
for (int i = 0; i < list.length; i++)
{
   str = str + list[i];
   if (i != list.length - 1)
   {
     str = str + ",";
   }
}
...

Can this be done in a more elegant way in Java?

I would certainly use a StringBuilder/Buffer for efficiency, but I wanted to illustrate the case in point without being too verbose. By elegant, I mean a solution that avoids the ugly(?) if check inside the loop.

Answer 1

Using Guava's (formerly google-collections) joiner class:

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

Done.

Answer 2

Here is my version: Java Tricks: Fastest Way to Collecting Objects in a String

StringBuilder buffer = new StringBuilder ();
String delim = "";
for (Object o: list)
{
    buffer.append (delim);
    delim = ", "; // Avoid if(); assignment is very fast!
    buffer.append (o);
}
buffer.toString ();

As an additional bonus: If your code in the loop is more complex, then this approach will produce a correct results without complex if()s.

Also note that with modern CPUs, the assignment will happen only in the cache (or probably only in a register).

Conclusion: While this code looks odd at first glance, it has many advantages.