Java & the JVM · topic 1 of 6

Core Java

OOP, Collections, Strings, Exceptions, JVM internals.

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: 29
Difficulty mix: 8 easy · 15 medium · 6 hard

Topics covered

OOP7
Collections6
Strings4
Exceptions3
Memory/GC3
Primitives2
JVM/Class loading2
Generics1
Serialization1

Foundations

Easy · 8 questions

Definitions and building blocks — start here if the topic is new to you.

What is a HashMap?
CollectionsEasy
How does an ArrayList grow internally?
CollectionsEasy
What is a Queue and when would you use one?
CollectionsEasy
What is a Stack and how does it differ from a Queue?
CollectionsEasy
Why does comparing two Strings with == sometimes pass and sometimes fail in production?
StringsEasy
Why is String immutable in Java, and how does that help a high-throughput service?
StringsEasy
Checked vs unchecked exceptions — which do you use in a service layer, and why?
ExceptionsEasy
How can autoboxing silently slow down a hot loop or cause a subtle bug?
PrimitivesEasy

Core concepts

Medium · 15 questions

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

What is the difference between HashMap and Hashtable?
CollectionsMedium
Why must you override equals() and hashCode() together?
OOPMedium
How does a LinkedList work, and when beats ArrayList?
CollectionsMedium
How does String interning work, and when would you actually use intern() in production?
StringsMedium
How does try-with-resources prevent JDBC connection and file-handle leaks?
ExceptionsMedium
StringBuilder vs StringBuffer — does the difference still matter in modern code?
StringsMedium
Comparable vs Comparator — how do you sort domain objects by multiple fields?
OOPMedium
How does generic type erasure affect runtime behavior, and what breaks because of it?
GenericsMedium
Why prefer enums over int/String constants, and how do you put behavior in an enum?
OOPMedium
Is Java pass-by-value or pass-by-reference? Prove it.
OOPMedium
How do you design a correct immutable class for a multi-threaded service?
OOPMedium
How can static initialization order cause a subtle bug or even a deadlock?
JVM/Class loadingMedium
What are the exception anti-patterns that cause silent failures in production?
ExceptionsMedium
What's the real cost of excessive object creation, and how do IntStream/LongStream avoid the autoboxing tax Stream<Integer> pays?
PrimitivesMedium
Records vs Lombok @Value for DTOs — what do you actually gain and lose?
OOPMedium

Advanced

Hard · 6 questions

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

A long-running service's heap keeps growing until OutOfMemoryError — how do you diagnose it?
Memory/GCHard
How can ThreadLocal cause a memory leak in a thread-pool / application server?
Memory/GCHard
What are the pitfalls of Java serialization in distributed/enterprise systems?
SerializationHard
Why is finalize() discouraged, and what replaces it for resource cleanup?
Memory/GCHard
How should equals() and hashCode() be implemented for JPA/Hibernate entities?
OOPHard
What causes a ClassLoader leak on application redeploy, and how do you fix it?
JVM/Class loadingHard

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Topics covered

Foundations

What is a HashMap?

How does an ArrayList grow internally?

What is a Queue and when would you use one?

What is a Stack and how does it differ from a Queue?

Why does comparing two Strings with == sometimes pass and sometimes fail in production?

Why is String immutable in Java, and how does that help a high-throughput service?

Checked vs unchecked exceptions — which do you use in a service layer, and why?

How can autoboxing silently slow down a hot loop or cause a subtle bug?

Core concepts

What is the difference between HashMap and Hashtable?

Why must you override equals() and hashCode() together?

How does a LinkedList work, and when beats ArrayList?

How does String interning work, and when would you actually use intern() in production?

How does try-with-resources prevent JDBC connection and file-handle leaks?

StringBuilder vs StringBuffer — does the difference still matter in modern code?

Comparable vs Comparator — how do you sort domain objects by multiple fields?

How does generic type erasure affect runtime behavior, and what breaks because of it?

Why prefer enums over int/String constants, and how do you put behavior in an enum?

Is Java pass-by-value or pass-by-reference? Prove it.

How do you design a correct immutable class for a multi-threaded service?

How can static initialization order cause a subtle bug or even a deadlock?

What are the exception anti-patterns that cause silent failures in production?

What's the real cost of excessive object creation, and how do IntStream/LongStream avoid the autoboxing tax Stream<Integer> pays?

Records vs Lombok @Value for DTOs — what do you actually gain and lose?

Advanced

A long-running service's heap keeps growing until OutOfMemoryError — how do you diagnose it?

How can ThreadLocal cause a memory leak in a thread-pool / application server?

What are the pitfalls of Java serialization in distributed/enterprise systems?

Why is finalize() discouraged, and what replaces it for resource cleanup?

How should equals() and hashCode() be implemented for JPA/Hibernate entities?

What causes a ClassLoader leak on application redeploy, and how do you fix it?