Avoid Java Pitfalls: Error Codes, Stream toMap, Leap Year Bugs & More

Alibaba Cloud's "Developer Treasury" program offers 200 free technical e‑books, including the newly released "Java Development Handbook (Taishan Edition)" with 34 new rules and 90 description updates.

The handbook shares real‑world Java pitfalls, such as using stream.Collectors.toMap() which can cause duplicate‑key exceptions and NullPointerExceptions, and provides concrete solutions.

Error Code Unification

A five‑character string error code is introduced, divided into a source identifier (A: user, B: system, C: third‑party) and a four‑digit number, with a three‑level hierarchy (macro, secondary, tertiary) to enable rapid tracing and standardized communication.

Leap‑Year Bug

Hard‑coding 365 days for a year leads to cache‑expiration issues in leap years; using LocalDate to obtain the correct number of days avoids this problem.

Table Alias

When multiple tables have columns with the same name, using table aliases prevents column‑name conflicts in multi‑table SQL queries.

Code Formatting

For methods longer than ten lines, inserting a blank line after the closing brace of a return/throw block improves readability and maintainability.

Ternary Operator NPE

The ternary operator can trigger automatic unboxing; if the selected expression is null, a NullPointerException occurs. Example code demonstrates this behavior.

Developers are encouraged to download the handbook to avoid these common pitfalls and benefit from the unified error‑code scheme.