Your way of doing it is not really incorrect.
Tho I find it highly strange that you do the same thing in 2 very different ways in those 2 halves of the if statement.. The else block has a bunch of unnecessary array/list conversions.
You can avoid all the conversions using an approach like such:
String[] itemArr = new String[Constants.ARMOR.length + Constants.TOKENS.length];
System.arrayCopy(Constants.ARMOR, 0, itemArr, 0, Constants.TOKENS.length);
System.arrayCopy(Constants.TOKENS, 0, itemArr, Constants.ARMOR.length, Constants.TOKENS.length);
return itemArr;
(wrote that out in hand, mite have some typos). Do what you want tho, the difference in performance wont be noticable if its just gonna be used once