Java

Quick Reference

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
• 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