Types that quietly promise the wrong thing

Most of the bugs in this section are not logic errors. They are a type or a signature that made a promise the code could not keep: a wrapper used where a plain field belonged, a record treated as a struct, a view mistaken for a copy, two strings that render identically and are not equal, an interface widened for the second case and wrong by the third. The compiler is satisfied in every one of them, which is exactly why they survive review.

• Two strings that look identical and are not equal: Unicode normalisation in Java
  20 September 2026 · equals() compares code points, not glyphs. A filename from macOS and the same name typed on Windows can differ, and the map lookup that fails will not tell you why.
• unmodifiableList gives a view, List.copyOf gives a copy, and the bug is the difference
  19 September 2026 · One of them keeps changing under you. The other rejects nulls in a place you did not expect.
• Java records are not structs, and the difference will bite you
  16 September 2026 · Value semantics, mutable components, and the equals() trap.
• The abstraction you added for the second case is usually wrong
  15 September 2026 · Two examples is not a pattern. It is two examples.
• Optional is a return type, not a field type
  08 September 2026 · Where Optional earns its cost, and the three places it does not.