Question 1

How does HashMap work internally in Java 8+?

Accepted Answer

A lazily-created array of buckets indexed by a spread hash; collisions chain into a linked list that becomes a red-black tree only when the bin holds 8+ nodes AND the table is at least 64 slots.

Question 2

What happens when two keys have the same hash code?

Accepted Answer

Equal objects must return the same hash code. Unequal objects are free to collide — that is legal and normal, not a bug. Break the first rule and hash-based collections lose your data silently.

Question 3

What is the difference between String, StringBuilder and StringBuffer?

Accepted Answer

String is immutable, so every change allocates a new one. StringBuilder is a growable buffer you mutate in place. StringBuffer is the same thing with every method synchronized, which you almost never need. Concatenating in a loop is the only case where the difference is dramatic — and it is dramatic.

Question 4

How does ConcurrentHashMap achieve thread safety?

Accepted Answer

Since Java 8 there are no segments. It locks one bin at a time — a CAS to install the first node in an empty bin, and synchronized on the bin's head node otherwise — while reads never lock at all. The catch is that per-key atomicity does not make your sequence of calls atomic.

Question 5

What is the difference between volatile, synchronized and AtomicInteger?

Accepted Answer

volatile guarantees visibility and ordering: a write is seen by any later read, and operations are not reordered across it. It never guarantees atomicity, so volatile count++ still loses updates — it is three operations, and volatile makes each of them visible without making the trio indivisible.

Question 6

How does the Java Memory Model work?

Accepted Answer

A guarantee, not a statement about time: if A happens-before B, everything A wrote is visible to B. Without an edge between two threads there is no guarantee at all, whatever the clock says. The practical payoff is the opposite of what people expect — most fields crossing an existing edge need no volatile.

Question 7

What is ForkJoinPool and how does work-stealing work?

Accepted Answer

It runs on one ForkJoinPool shared by the whole JVM, so a slow task in one place starves parallel work everywhere else — including anything a library does. Add the usual problems of shared mutable state and non-associative reductions, and the honest default is not to use it until you have measured.

Question 8

How would you detect and troubleshoot a deadlock?

Accepted Answer

Two threads each holding a lock the other needs, in a cycle nobody can break. The practical cause is almost always inconsistent lock ordering, and the practical fix is a global order every thread follows. When ordering is impossible, tryLock with a timeout turns a permanent hang into a recoverable failure.

Question 9

What happens when you create too many threads in a Java application?

Accepted Answer

Small. Connections needed equals arrival rate times how long each request holds one, so a pool of ten usually beats a pool of a hundred — past the point where the database can run more statements at once, extra connections add contention, not throughput. The number that decides everything is hold time, and the fastest way to ruin a pool is to hold a connection across a call you do not control.

Question 10

What happens internally when a Java thread calls wait()?

Accepted Answer

A guarantee, not a statement about time: if A happens-before B, everything A wrote is visible to B. Without an edge between two threads there is no guarantee at all, whatever the clock says. The practical payoff is the opposite of what people expect — most fields crossing an existing edge need no volatile.

Question 11

What causes OutOfMemoryError versus StackOverflowError?

Accepted Answer

Every call keeps a frame alive until it returns, and the stack is about a megabyte — roughly twenty thousand frames, not millions. Java performs no tail-call elimination, so even a recursion whose last action is the recursive call still consumes a frame per level. Recursion is for problems shaped like a tree; depth proportional to n belongs in a loop.

Twenty advanced Java questions worth revising

Collections and strings

Concurrency

JVM, memory and garbage collection

What this list is worth