`Strings` should not be concatenated using '+' in a loop.`RSPEC-1643`

Strings are immutable objects, so concatenation doesn’t simply add the new String to the end of the existing string. Instead, in each loop iteration, the first String is converted to an intermediate object type, the second string is appended, and then the intermediate object is converted back to a String. Further, performance of these intermediate operations degrades as the String gets longer. Therefore, the use of StringBuilder is preferred.

String str = "";
for (int i = 0; i < arrayOfStrings.length ; ++i) {
  str = str + arrayOfStrings[i];
}

StringBuilder bld = new StringBuilder();
  for (int i = 0; i < arrayOfStrings.length; ++i) {
    bld.append(arrayOfStrings[i]);
  }
  String str = bld.toString();

https://rules.sonarsource.com/java/RSPEC-1643

The issue provides Java examples and links to RSPEC-1643, but names no repository files or tests. Start by locating the existing Java static-analysis recipe conventions and their tests. Done means the rule handles string concatenation in loops according to the linked specification, with tests covering the examples.