Question about String.toUpperCase behaviour

[Сергей Цыпанов]

Hello,

current implementation of e.g. String.toUpperCase() returns the object it was called on in case all code points are in upper case.

E.g. this test 

@Test
public void upperCase() {
  String str = "AZ";                // English
  assert str == str.toUpperCase();
  
  str = "АЯ";                       // Russian
  assert str == str.toUpperCase();
}

passes for both Latin-1 and UTF-16. This assumption allows to simplify this:

boolean isUpperCase = str.toUpperCase().equals(str); 

to

boolean isUpperCase = str.toUpperCase() == str;

Could this transformation be legal assuming that existing behaviour of String.toUpperCase is not documented?

Regards,
Sergey Tsypanov