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 + ",";
   }
}  
...

My question is , can this be done in a more elegant way in java?

EDIT: I would certainly use a StringBuilder/Buffer for efficiency but 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

Look here:

http://snippets.dzone.com/posts/show/91

for a full discussion of this topic.

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 ();

Answer 3

I would use a StringBuffer to implement this feature. String is immutable so everytime you concat two Strings, a new object is created.

More efficient is the use of StringBuffer:

String[] list = new String[] {"abc","def","ghi","jkl"};
StringBuffer str = new StringBuffer();
for (String s : list) {
   str.append(s);
   str.append(",");
}
str.deleteCharAt(str.length());
System.out.println(str); //automatically invokes StringBuffer.toString();

Answer 4

StringBuilder builder = new StringBuilder();
for (String st: list) {
    builder.append(st).append(',');
}
builder.deleteCharAt(builder.length());
String result = builder.toString();

Do not use '+' for string contacenations. It`s slow.

Answer 5

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

Answer 6

I'd bet that there are several classes named "StringUtil", "StringsUtil", "Strings" or anything along those lines on the classpath of any medium sized Java project. Most likely, any of them will provide a join function. Here are some examples I've found in my project:

org.apache.commons.lang.StringUtils.join(...)
org.apache.wicket.util.string.Wicket.join(...)
org.compass.core.util.StringUtils.arrayToDelimitedString(...)

As you might want to get rid of some external dependencies in the future, you may want to to something like this:

public static final MyStringUtils {
    private MyStringUtils() {}

    public static String join(Object[] list, String delim) {
        return org.apache.commons.lang.StringUtils.join(list, delim);
    }
}

Now that's what I call "elegant" ;)

Answer 7

using google-collections joiner class:

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

done.