When should you use StringBuilder?

StringBuilder should be used in Java when you need to modify or concatenate strings multiple times, especially inside loops. This is because strings in Java are immutable, and each concatenation operation creates a new String object, which reduces performance and increases memory load.

Example of using StringBuilder:

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 1000; i++) {
    sb.append(i).append(", ");
}
String result = sb.toString();

This approach is more efficient than sequentially using the + operator for strings in a loop.