Java & the JVM · topic 6 of 6

Advanced Java

Concurrency, JVM tuning, memory model, virtual threads.

Questions below are grouped from foundations to advanced. Work top to bottom if the topic is new to you, or jump to a level if you are brushing up.

Questions: 28
Difficulty mix: 12 medium · 16 hard

Topics covered

Concurrency2
Virtual Threads2
Class Loading2
Serialization2
Generics2
Executors2
JVM Internals1
Memory Model1
Memory1
Garbage Collection1
References1
Reflection1
Dynamic Proxy1
Annotations1
Ordering1
Object Contract1
Immutability1
Collections1
CompletableFuture1
ForkJoin1
ThreadLocal1
Modern Java1

Core concepts

Medium · 12 questions

The questions most interviews spend their time on: how it works and when to use it.

What is synchronization and why do we need it?
ConcurrencyMedium
What is a thread and how is it different from a process?
ConcurrencyMedium
Beyond the definitions, what actually happens between javac and native execution across the JDK, JRE and JVM?
JVM InternalsMedium
How do JVM thread stacks work, and what's the difference between StackOverflowError and OutOfMemoryError?
MemoryMedium
What is the difference between Serializable and Externalizable, and when would you implement Externalizable?
SerializationMedium
What does the transient keyword do, and what are its gotchas?
SerializationMedium
What is the Reflection API, and what are its costs and risks?
ReflectionMedium
How do Java annotations work — retention policies, targets, and how are they processed?
AnnotationsMedium
Beyond final fields, how do you guarantee deep immutability with defensive copies and immutable collections?
ImmutabilityMedium
How do you safely modify a collection while iterating it, and what causes ConcurrentModificationException?
CollectionsMedium
Where do exceptions go when a task fails in an executor, and why does submit() swallow them?
ExecutorsMedium
What are Records and Sealed Classes, and how do they enable data-oriented programming?
Modern JavaMedium

Advanced

Hard · 16 questions

Internals, trade-offs and edge cases — useful for senior rounds and follow-ups.

What are Virtual Threads (Project Loom) in Java 21?
Virtual ThreadsHard
Explain the ClassLoader hierarchy (Bootstrap, Platform, Application) and how you'd write a custom class loader.
Class LoadingHard
When and how do you deliberately break the parent-delegation model?
Class LoadingHard
How do you reason about safe publication and reordering with the Java Memory Model in lock-free code?
Memory ModelHard
How do the modern collectors (Parallel, G1, ZGC, Shenandoah) trade off throughput vs latency, and how do you choose?
Garbage CollectionHard
How do you build a leak-free cache or cleanup mechanism with soft, weak and phantom references?
ReferencesHard
How do Java dynamic proxies work, and how do frameworks use them for AOP?
Dynamic ProxyHard
How do you compose Comparators safely, and why must ordering be consistent with equals?
OrderingHard
How does the equals()/hashCode() contract break under inheritance, and with ORM proxies?
Object ContractHard
How do bounded wildcards and the PECS rule let you design flexible generic APIs?
GenericsHard
What is heap pollution with generic varargs, and when is @SafeVarargs appropriate?
GenericsHard
How do you shut down an ExecutorService cleanly and handle rejected tasks?
ExecutorsHard
How do you handle errors, timeouts, and combine multiple CompletableFutures without blocking?
CompletableFutureHard
How do you write a correct ForkJoin task, and why is the common pool a shared resource to respect?
ForkJoinHard
What are ThreadLocals legitimately good for, and how do ScopedValues improve on them?
ThreadLocalHard
In production, how do virtual threads pin, why shouldn't you pool them, and how does structured concurrency help?
Virtual ThreadsHard

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Topics covered

Core concepts

What is synchronization and why do we need it?

What is a thread and how is it different from a process?

Beyond the definitions, what actually happens between javac and native execution across the JDK, JRE and JVM?

How do JVM thread stacks work, and what's the difference between StackOverflowError and OutOfMemoryError?

What is the difference between Serializable and Externalizable, and when would you implement Externalizable?

What does the transient keyword do, and what are its gotchas?

What is the Reflection API, and what are its costs and risks?

How do Java annotations work — retention policies, targets, and how are they processed?

Beyond final fields, how do you guarantee deep immutability with defensive copies and immutable collections?

How do you safely modify a collection while iterating it, and what causes ConcurrentModificationException?

Where do exceptions go when a task fails in an executor, and why does submit() swallow them?

What are Records and Sealed Classes, and how do they enable data-oriented programming?

Advanced

What are Virtual Threads (Project Loom) in Java 21?

Explain the ClassLoader hierarchy (Bootstrap, Platform, Application) and how you'd write a custom class loader.

When and how do you deliberately break the parent-delegation model?

How do you reason about safe publication and reordering with the Java Memory Model in lock-free code?

How do the modern collectors (Parallel, G1, ZGC, Shenandoah) trade off throughput vs latency, and how do you choose?

How do you build a leak-free cache or cleanup mechanism with soft, weak and phantom references?

How do Java dynamic proxies work, and how do frameworks use them for AOP?

How do you compose Comparators safely, and why must ordering be consistent with equals?

How does the equals()/hashCode() contract break under inheritance, and with ORM proxies?

How do bounded wildcards and the PECS rule let you design flexible generic APIs?

What is heap pollution with generic varargs, and when is @SafeVarargs appropriate?

How do you shut down an ExecutorService cleanly and handle rejected tasks?

How do you handle errors, timeouts, and combine multiple CompletableFutures without blocking?

How do you write a correct ForkJoin task, and why is the common pool a shared resource to respect?

What are ThreadLocals legitimately good for, and how do ScopedValues improve on them?

In production, how do virtual threads pin, why shouldn't you pool them, and how does structured concurrency help?