I fail to see how is that relevant in their solution, but you also used it in your own solution.

Not only the local StringBuilder instance doesn't leave the scope of the method, they don't even create threads in their solution to worry about this.

However, concatenation operation AFAIK creates StringBuilder instances each time they are called on two String instances, and return StringBuilder.toString().

You can select Java 17 on CodeWars for your solution.

That's Java 11 or later

Is checking for null or returning string smaller/equal 4 must have?
I did code like this and it has no error with empty string and properly showing 1-4 character strings

You could use the string repeat function tocreatedthe masked part

here better to use StringBuilder.

put str.kength() in an integer variable for more neat code

String concatenations are expensive.

Compiler won't do StringBuilder magic on this one most likely. In general that loop gets turned into:

String result = "";
for (int i = 0; i < str.length()-4; i++) {
result = new StringBuilder().append(result).append('#').toString();
}

worth to put str.length()-4 into a variable for better performance

That's a ton of String objects created and thrown away (or hope for the compilers StringBuilder replacement magic)