Builder: StringBuilder.reverse mutates in place

reverse() rewrites the buffer's own characters back to front and returns the same StringBuilder instance rather than a new one, which is why the reference equality check afterwards is true. Any other reference to that builder would see the reversed content too.

Code

StringBuilder sb = new StringBuilder("Java");
StringBuilder same = sb.reverse();
System.out.println("Reversed: " + sb);
System.out.println("Same instance: " + (sb == same));

Output

Reversed: avaJ
Same instance: true