How to use Set<ANY> with String.join(iterableObj, separator)?

Question

I was curious about using Set<ANY> in the String.join(iterableObj, separator) method. When I try this code:

Set<String> mySet = new Set<String>();
mySet.add('One');
mySet.add('Two');
mySet.add('Three');
System.debug('mySet= ' + mySet);

String mySet_Joined_Exception = String.join(mySet, ', ');
System.debug('mySet_Joined_Exception= ' + mySet_Joined);

It fails to compile pointing to the line String mySet_Joined_Exception = String.join(mySet, ', '); with the following error:

Method does not exist or incorrect signature: void join(Set, String) from the type String

Which means that it's not possible to use directly Set<ANY> with the String.join() method. This happens because Set<ANY> does not implement Iterable interface, which clearly explained in another post "Do Apex collections implement iterable?".

Nevertheless, what are the workarounds to this issue?

Answer 1

There are quite a few options to solve this problem.

First, there is an idea Apex Code: Implement Iterable<T> on Set<T>., which when implemented would eliminate the need for the below workarounds.

Example on Set<String>

Set<String> mySet = new Set<String>();
mySet.add('One');
mySet.add('Two');
mySet.add('Three');
System.debug('mySet= ' + mySet);

Debug:

08:53:25.1 (2682641)|USER_DEBUG|[5]|DEBUG|mySet= {One, Three, Two}

Option 1 - cast it to Iterable<T>

String mySet_Joined = String.join((Iterable<String>)mySet, ', ');
System.debug('mySet_Joined= ' + mySet_Joined);

Result:

08:53:25.1 (2944980)|USER_DEBUG|[8]|DEBUG|mySet_Joined= One, Two, Three

UPDATE re- Option 1 - see this post for more info about why (Iterable<String>)mySet should not be used.

Option 2 - convert original Set into List

String mySet_Joined_List = String.join(new List<String>(mySet), ', ');
System.debug('mySet_Joined_List= ' + mySet_Joined_List);

Result:

08:57:13.1 (2812911)|USER_DEBUG|[11]|DEBUG|mySet_Joined_List= One, Two, Three

Answer 2

In the Summer '23 release, the Iterable interface was implemented on the Set data structure. You can find more information about this implementation in the following resources:

Idea: link

Release notes: link

To make use of this feature, you simply need to update the API version of your Apex class to 58 or higher and then the code posted in the question will work as expected.

Answer 3

The easiest solution for this is to use toString method as given below -

Set<String> mySet = new Set<String>();
mySet.add('One');
mySet.add('Two');
mySet.add('Three');
System.debug('mySet= ' + mySet);

String mySet_Joined_Exception = mySet.toString();
System.debug('mySet_Joined_Exception= ' + mySet_Joined); // output = {One, Two, Three}

You can remove the curly braces using the remove() method or you can use the substring(startIndex, endIndex) method of the String class.

Here, the only concern is the separator but that can be replaced by replace method.