At present, the no-parameter toLowerCase and toUpperCase methods of String are based on the default locale.
I checked all the uses of this method in OpenJDK, and found that most of the use cases are suspicious, and even there are some hidden bugs. For example, I just found that jdk.incubator.vector.LaneType will trigger assertion failure on Turkish locale, and opened a PR for this problem: https://github.com/openjdk/panama-vector/pull/210 In addition to such obvious problems, some use cases behave suspiciously after calling Locale.setDefault. I am investigating these problems and preparing to open a PR to solve these hidden bugs. In all the third-party libraries I have used, I have never seen the correct use of these two methods. Although the behavior of modifying API methods is destructive, I think it is worthwhile to consider whether to modify its behavior for such a suspicious method. If users need locale-sensitive case conversion, it may be better to explicitly use Locale.getDefault(). Using Locale.ROOT as the default value also helps to keep the behavior of these two methods consistent with equalsIgnoreCase, Character.toLowerCase/toUpperCase and other methods. This is my rough idea. I hope to get your suggestions.
