Java

Quick Reference

nulls.md

collections.md

generics.md

concurrency.md

classes.md

streams.md

testing.md

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