Java

Write robust Java avoiding null traps, equality bugs, and concurrency pitfalls.

Quick Reference

Topic	File
Nulls, Optional, autoboxing	nulls.md
Collections and iteration traps	collections.md
Generics and type erasure	generics.md
Concurrency and synchronization	concurrency.md
Classes, inheritance, memory	classes.md
Streams and CompletableFuture	streams.md
Testing (JUnit, Mockito)	testing.md
JVM, GC, modules	jvm.md

Critical Rules

• == compares references, not content — always use .equals() for strings
• Override equals() must also override hashCode() — HashMap/HashSet break otherwise
• Optional.get() throws if empty — use orElse(), orElseGet(), or ifPresent()
• Modifying while iterating throws ConcurrentModificationException — use Iterator.remove()
• Type erasure: generic type info gone at runtime — can't do new T() or instanceof List<String>
• volatile ensures visibility, not atomicity — count++ still needs synchronization
• Unboxing null throws NPE — Integer i = null; int x = i; crashes
• Integer == Integer uses reference for values outside -128 to 127 — use .equals()
• Try-with-resources auto-closes — implement AutoCloseable, Java 7+
• Inner classes hold reference to outer — use static nested class if not needed
• Streams are single-use — can't reuse after terminal operation
• thenApply vs thenCompose — compose for chaining CompletableFutures
• Records are implicitly final — can't extend, components are final
• serialVersionUID mismatch breaks deserialization — always declare explicitly