It works because a new object is returned. The variable 'x' does not change.
public String replace(char oldChar, char newChar)
Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.

You right

If

Benchmark                                                  Mode  Cnt           Score   Units
WillThereBeEnoughSpaceTest.testWillThereBeEnoughSpaceIf   thrpt   25  1437160896,811   ops/s

Max

Benchmark   
WillThereBeEnoughSpaceTest.testWillThereBeEnoughSpaceMax  thrpt   25  1542379975,592   ops/s

but slowly than "if" or "?"

The test does not work if English regional settings are not used.
English: assertEquals("5.000", String.format("%.3f", poly.circleDiameter()));
Non-English: assertEquals("5,000", String.format("%.3f", poly.circleDiameter()));