A quick and easy way to join array elements with a separator (the opposite of split) in Java [duplicate]

Answer 1

You can use replace and replaceAll with regular expressions.

String[] strings = {"a", "b", "c"};

String result = Arrays.asList(strings).toString().replaceAll("(^\\[|\\]$)", "").replace(", ", ",");

Because Arrays.asList().toString() produces: "[a, b, c]", we do a replaceAll to remove the first and last brackets and then (optionally) you can change the ", " sequence for "," (your new separator).

A stripped version (fewer chars):

String[] strings = {"a", "b", "c"};

String result = ("" + Arrays.asList(strings)).replaceAll("(^.|.$)", "").replace(", ", "," );

Regular expressions are very powerful, specially String methods "replaceFirst" and "replaceAll". Give them a try.

Answer 2

Using Java 8 you can do this in a very clean way:

String.join(delimiter, elements);

This works in three ways:

1) directly specifying the elements

String joined1 = String.join(",", "a", "b", "c");

2) using arrays

String[] array = new String[] { "a", "b", "c" };
String joined2 = String.join(",", array);

3) using iterables

List<String> list = Arrays.asList(array);
String joined3 = String.join(",", list);

Answer 3

This options is fast and clear:

  public static String join(String separator, String... values) {
    StringBuilder sb = new StringBuilder(128);
    int end = 0;
    for (String s : values) {
      if (s != null) {
        sb.append(s);
        end = sb.length();
        sb.append(separator);
      }
    }
    return sb.substring(0, end);
  }

Answer 4

"I'm sure there is a certified, efficient way to do it (Apache Commons?)"

yes, apparenty it's

StringUtils.join(array, separator)

http://www.java2s.com/Code/JavaAPI/org.apache.commons.lang/StringUtilsjoinObjectarrayStringseparator.htm

Answer 5

All of these other answers include runtime overhead... like using ArrayList.toString().replaceAll(...) which are very wasteful.

I will give you the optimal algorithm with zero overhead; it doesn't look as pretty as the other options, but internally, this is what they are all doing (after piles of other hidden checks, multiple array allocation and other crud).

Since you already know you are dealing with strings, you can save a bunch of array allocations by performing everything manually. This isn't pretty, but if you trace the actual method calls made by the other implementations, you'll see it has the least runtime overhead possible.

public static String join(String separator, String ... values) {
  if (values.length==0)return "";//need at least one element
  //all string operations use a new array, so minimize all calls possible
  char[] sep = separator.toCharArray();

  // determine final size and normalize nulls
  int totalSize = (values.length - 1) * sep.length;// separator size
  for (int i = 0; i < values.length; i++) {
    if (values[i] == null)
      values[i] = "";
    else
      totalSize += values[i].length();
  }

  //exact size; no bounds checks or resizes
  char[] joined = new char[totalSize];
  int pos = 0;
  //note, we are iterating all the elements except the last one
  for (int i = 0, end = values.length-1; i < end; i++) {
    System.arraycopy(values[i].toCharArray(), 0, 
      joined, pos, values[i].length());
    pos += values[i].length();
    System.arraycopy(sep, 0, joined, pos, sep.length);
    pos += sep.length;
  }
  //now, add the last element; 
  //this is why we checked values.length == 0 off the hop
  System.arraycopy(values[values.length-1].toCharArray(), 0,
    joined, pos, values[values.length-1].length());

  return new String(joined);
}

Answer 6

I prefer Google Collections over Apache StringUtils for this particular problem:

Joiner.on(separator).join(array)

Compared to StringUtils, the Joiner API has a fluent design and is a bit more flexible, e.g. null elements may be skipped or replaced by a placeholder. Also, Joiner has a feature for joining maps with a separator between key and value.