A method to reverse effect of java String.split()?

There's no method in the JDK for this that I'm aware of. Apache Commons Lang has various overloaded join() methods in the StringUtils class that do what you want.

I got the following example here

/*
7) Join Strings using separator >>>AB$#$CD$#$EF

 */

import org.apache.commons.lang.StringUtils;

public class StringUtilsTrial {
  public static void main(String[] args) {

    // Join all Strings in the Array into a Single String, separated by $#$
    System.out.println("7) Join Strings using separator >>>"
        + StringUtils.join(new String[] { "AB", "CD", "EF" }, "$#$"));
  }
}

Google also provides a joiner class in their Google Collections library:

Joiner API

Google Collections

There are several examples on DZone Snippets if you want to roll your own that works with a Collection. For example:

public static String join(AbstractCollection<String> s, String delimiter) {
    if (s == null || s.isEmpty()) return "";
    Iterator<String> iter = s.iterator();
    StringBuilder builder = new StringBuilder(iter.next());
    while( iter.hasNext() )
    {
        builder.append(delimiter).append(iter.next());
    }
    return builder.toString();
}

For Android: in android.text.TextUtils there are methods:

public static String join (CharSequence delimiter, Iterable tokens)
public static String join (CharSequence delimiter, Object[] tokens)

Returns a string containing the tokens joined by delimiters.

tokens -- an array objects to be joined. Strings will be formed from the objects by calling object.toString().

For the sake of completeness, I'd like to add that you cannot reverse String#split in general, as it accepts a regular expression.

"hello__world".split("_+"); Yields ["hello", "world"].
"hello_world".split("_+"); Yields ["hello", "world"].

These yield identical results from a different starting point. splitting is not a one-to-one operation, and is thus non-reversible.

This all being said, if you assume your parameter to be a fixed string, not regex, then you can certainly do this using one of the many posted answers.

Below code gives a basic idea. This is not best solution though.

public static String splitJoin(String sourceStr, String delim,boolean trim,boolean ignoreEmpty){
    return join(Arrays.asList(sourceStr.split(delim)), delim, ignoreEmpty);
}


public static String join(List<?> list, String delim, boolean ignoreEmpty) {
    int len = list.size();
    if (len == 0)
        return "";
    StringBuilder sb = new StringBuilder(list.get(0).toString());
    for (int i = 1; i < len; i++) {
        if (ignoreEmpty && !StringUtils.isBlank(list.get(i).toString())) {
            sb.append(delim);
            sb.append(list.get(i).toString().trim());
        }
    }
    return sb.toString();
}