NOTE
Comparing Java Strings
String identity, content equality, ordering, null safety, and case-insensitive comparison cautions.
This is a historical learning note and may contain outdated or incomplete understanding.
Use equals for exact content equality and compareTo for lexicographic ordering. == only checks whether two references identify the same object.
For null-safe equality, Objects.equals(a, b) is convenient.
Case-insensitive comparison is locale/Unicode-sensitive for some tasks. For user-facing linguistic rules, use locale-aware APIs rather than assuming ASCII-style case folding is universally correct.