What's the best way to build a string of delimited items in Java?

Pre Java 8:

Apache's commons lang is your friend here - it provides a join method very similar to the one you refer to in Ruby:

StringUtils.join(java.lang.Iterable,char)

Java 8:

Java 8 provides joining out of the box via StringJoiner and String.join(). The snippets below show how you can use them:

StringJoiner

StringJoiner joiner = new StringJoiner(",");
joiner.add("01").add("02").add("03");
String joinedString = joiner.toString(); // "01,02,03"

String.join(CharSequence delimiter, CharSequence... elements))

String joinedString = String.join(" - ", "04", "05", "06"); // "04 - 05 - 06"

String.join(CharSequence delimiter, Iterable<? extends CharSequence> elements)

List<String> strings = new LinkedList<>();
strings.add("Java");strings.add("is");
strings.add("cool");
String message = String.join(" ", strings);
//message returned is: "Java is cool"

You could write a little join-style utility method that works on java.util.Lists

public static String join(List<String> list, String delim) {

    StringBuilder sb = new StringBuilder();

    String loopDelim = "";

    for(String s : list) {

        sb.append(loopDelim);
        sb.append(s);            

        loopDelim = delim;
    }

    return sb.toString();
}

Then use it like so:

    List<String> list = new ArrayList<String>();

    if( condition )        list.add("elementName");
    if( anotherCondition ) list.add("anotherElementName");

    join(list, ",");

In the case of Android, the StringUtils class from commons isn't available, so for this I used

android.text.TextUtils.join(CharSequence delimiter, Iterable tokens)

http://developer.android.com/reference/android/text/TextUtils.html

The Google's Guava library has com.google.common.base.Joiner class which helps to solve such tasks.

Samples:

"My pets are: " + Joiner.on(", ").join(Arrays.asList("rabbit", "parrot", "dog")); 
// returns "My pets are: rabbit, parrot, dog"

Joiner.on(" AND ").join(Arrays.asList("field1=1" , "field2=2", "field3=3"));
// returns "field1=1 AND field2=2 AND field3=3"

Joiner.on(",").skipNulls().join(Arrays.asList("London", "Moscow", null, "New York", null, "Paris"));
// returns "London,Moscow,New York,Paris"

Joiner.on(", ").useForNull("Team held a draw").join(Arrays.asList("FC Barcelona", "FC Bayern", null, null, "Chelsea FC", "AC Milan"));
// returns "FC Barcelona, FC Bayern, Team held a draw, Team held a draw, Chelsea FC, AC Milan"

Here is an article about Guava's string utilities.

In Java 8 you can use String.join():

List<String> list = Arrays.asList("foo", "bar", "baz");
String joined = String.join(" and ", list); // "foo and bar and baz"

Also have a look at this answer for a Stream API example.