Java Advanced

☕ Java Advanced (target: JDK 25 LTS)

1. Language mastery

• OOP deeply: encapsulation, composition over inheritance, immutability
• Generics: bounded types, wildcards, PECS, type erasure
• Modern Java: var, records, sealed classes, pattern matching for switch/instanceof, record patterns, text blocks
• Streams & collectors (and when not to use them), Optional done right
• Functional interfaces, lambdas, method references
• Exceptions: checked vs unchecked, try-with-resources, exception design
• equals/hashCode contracts, Comparable vs Comparator
• Annotations & reflection (needed to understand Spring)
• Modules (JPMS): awareness level

2. Collections internals

• HashMap: hashing, buckets, treeification, resize, load factor
• ConcurrentHashMap: CAS + bin-level locking
• ArrayList vs LinkedList vs ArrayDeque; TreeMap (red-black); PriorityQueue (binary heap)
• LinkedHashMap access order → LRU in 10 lines

3. JVM

• Class loading, bytecode, JIT (C1/C2), tiered compilation
• Memory: heap (young/old), metaspace, stacks, direct memory
• GC: G1 (default), ZGC (generational, low pause), Parallel. How to choose; reading GC logs
• JFR + JDK Mission Control, async-profiler, heap dumps (Eclipse MAT)
• Container awareness: MaxRAMPercentage, CPU limits
• GraalVM native image / CDS / Project Leyden for startup time (awareness)

4. Concurrency → Concurrency

• JMM, happens-before, volatile, synchronized, java.util.concurrent
• Virtual threads (Project Loom), structured concurrency, scoped values
• CompletableFuture pipelines

5. Tooling & quality

• Maven and Gradle (Kotlin DSL); multi-module builds
• JUnit 5, AssertJ, Mockito, Testcontainers, ArchUnit, JMH
• Static analysis: SpotBugs / Error Prone / SonarLint

🧪 Labs (🟢 warm-up → 🟡 core → 🔴 hard → ⚫ boss)

• 🟢 JOL: print object layouts; compute the memory of 1M HashMap entries
• 🟡 Implement HashMap (with resize + treeify-lite) and ArrayList from scratch with tests
• 🟡 JMH: streams vs loops, boxing costs, escape analysis (-prof gc)
• 🔴 Platform vs virtual threads under a blocking load; then detect pinning with JFR
• 🔴 Read ConcurrentHashMap.putVal + AbstractQueuedSynchronizer; write a 1-page explanation
• ⚫ 1BRC in Java (W4 boss fight)

🧠 Cognitive tasks

• Predict → verify: GC pause p99 with G1 vs generational ZGC on the same load
• Code archaeology: ThreadPoolExecutor.execute(); draw its 3 branches
• Transfer: where does the JDK use the Decorator, Strategy, and Template Method patterns?

🛰️ Orbit integration

• orbit-kv + MiniThreadPool (W1–W2)
• orbit-api on virtual threads with sensible concurrency limits

Go deeper

⚙️ JVM Internals · Spring Internals · 🧩 GoF Patterns (Java & Go) · Concurrency Patterns

Resources

• Effective Java 3e (Bloch) ⭐ · Java Concurrency in Practice ⭐ · Modern Java in Action
• dev.java, Inside Java (podcast/blog), JEP index, José Paumard’s JEP Café (YouTube)
• Baeldung (reference), Optimizing Java (Evans et al.)

Interview questions interview-q

• How does HashMap work? What changed in Java 8?
• volatile vs synchronized vs AtomicInteger?
• When would virtual threads not help?
• How would you diagnose a memory leak in production?