String vs StringBuilder vs StringBuffer — when to use which?

Answer

`String` is immutable, so repeated concatenation creates many objects. `StringBuilder` is mutable and fast for building strings in a single thread; `StringBuffer` is similar but synchronized (rarely needed today).

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 3; i++) {
  sb.append(i).append(",");
}
System.out.println(sb.toString());