Essential Built-in Libraries (`pkg13libs`)

Runnable demos: `pkg13libs`. Run with java pkg13libs/libs1DateTimeApi.java.

These are the high-value standard-library APIs every Java developer should know — all part of the JDK, no dependencies.

Files in this package

#	File	Package	What it covers
1	libs1DateTimeApi	java.time	dates, times, zones, Period/Duration, formatting
2	libs2RegexPattern	java.util.regex	match/find/groups/replace/split
3	libs3CryptoAndHashing	java.security, javax.crypto	SHA-256, HMAC, Base64, SecureRandom
4	libs4RandomAndUuid	java.util	Random, ThreadLocalRandom, SecureRandom, UUID
5	libs5BigNumbersAndMath	java.math	BigInteger, BigDecimal, Math
6	libs6Reflection	java.lang.reflect	inspect/invoke types at runtime
7	libs7Annotations	java.lang.annotation	custom annotations + a mini test runner
8	libs8Logging	java.util.logging	levels, handlers, formatters

Quick rules of thumb

• Dates: use java.time (immutable, thread-safe). Never Date/Calendar.
• Money/precision: use BigDecimal with explicit scale + RoundingMode. Never double.
• Regex: compile Pattern once, reuse it; prefer named groups.
• Security: SecureRandom for tokens/salts/keys; for passwords use a slow KDF (PBKDF2/bcrypt/Argon2), not a plain hash.
• Reflection: powerful but slow and unsafe — the basis of Spring/JUnit/Jackson; use sparingly in app code.
• Annotations: RetentionPolicy.RUNTIME makes them readable via reflection (how frameworks discover your code).
• Logging: JUL ships with the JDK; most apps prefer SLF4J + Logback/Log4j2, but the concepts (levels/handlers/formatters) are identical.