Question

While working in a Java app, I recently needed to assemble a comma-delimited list of values to pass to another web service without knowing how many elements there would be in advance. The best I could come up with off the top of my head was something like this:

public String appendWithDelimiter( String original, String addition, String delimiter ) {     if ( original.equals( '' ) ) {         return addition;     } else {         return original + delimiter + addition;     } }  String parameterString = ''; if ( condition ) parameterString = appendWithDelimiter( parameterString, 'elementName', ',' ); if ( anotherCondition ) parameterString = appendWithDelimiter( parameterString, 'anotherElementName', ',' ); 

I realize this isn’t particularly efficient, since there are strings being created all over the place, but I was going for clarity more than optimization.

In Ruby, I can do something like this instead, which feels much more elegant:

parameterArray = []; parameterArray << 'elementName' if condition; parameterArray << 'anotherElementName' if anotherCondition; parameterString = parameterArray.join(','); 

But since Java lacks a join command, I couldn’t figure out anything equivalent.

So, what’s the best way to do this in Java?

Answer 1

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)

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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' 

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

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

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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'