Interviews
A useful Java interview asks how the candidate models types, concurrency, and failure, then watches them write a small class another engineer can change. The questions below are for a junior, a mid-level engineer, and a senior. Each one says what a good answer covers. After the conversation, watch the candidate code.
A junior answer names the rule and the bug it prevents.
== compares references. equals is the value comparison, and it must agree with hashCode when the object is used as a hash key. Compare strings with equals. Two equal strings are not required to be the same object.
A class can implement several interfaces and extend one class. An interface names a capability, and it can include default methods. An abstract class can hold instance state that subclasses share. Use an interface unless that shared state is real.
A method's local primitives and references live in its stack frame. Objects are allocated on the heap, and the local variable holds a reference. When the method returns, the frame is gone. The object can be collected once nothing reachable points at it.
A checked exception must be caught or declared. An unchecked exception extends RuntimeException and is not part of the signature. Use a checked exception for a recoverable condition the caller should know about. Do not use exceptions for ordinary branching.
A final variable cannot be assigned again. A final method cannot be overridden. A final class cannot be extended. final on a reference does not make the object immutable. Fields of that object can still change unless they are guarded too.
String is immutable, so repeated concatenation can copy the growing text. StringBuilder appends into a buffer. The compiler rewrites a simple concatenation in one expression. A loop driven by data still wants a builder.
A mid-level answer names the mechanism and a failure mode.
The map hashes the key, picks a bucket, then uses equals inside the bucket. If a field used by hashCode or equals changes after insertion, lookup searches the wrong bucket and misses the entry. Keys should be immutable in those fields. A high load resizes and rehashes.
A synchronized method locks the instance, or the Class object if it is static, so one thread at a time runs that block. It does not make two separately locked actions atomic. Locking on an object other code can also lock invites contention. Prefer a private lock, or a java.util.concurrent class, for shared state.
ArrayList is a growable array: index access is cheap, and an insert in the middle shifts elements. LinkedList is a doubly linked list with poor random access, and it is rarely faster in practice. For a queue, ArrayDeque is the usual choice. Use ArrayList unless a measurement says otherwise.
The compiler checks the type argument and erases it. At runtime the list is a list of objects, with casts inserted where values are read. Two methods cannot be overloaded so that they differ only by a type argument. A raw type turns the check off.
int compares values. Integer is an object, so == compares references. Boxing reuses instances for at least -128 through 127, so == can pass in that range and fail outside it. Compare with equals, or unbox to int.
try-with-resources calls close when the block ends, including on an exception. The resource must implement AutoCloseable. If close also throws, that exception is attached to the original as a suppressed exception, so the first failure is not lost.
A senior answer names the tradeoff, the failure, and what to measure.
Without a happens-before relationship, a thread can keep seeing a stale value written by another thread. volatile, locks, and the concurrent collections create that relationship. Final fields are visible after the constructor finishes, as long as this does not escape during construction. A plain write is not enough.
Read the collector log. Note the pause, how full the heap was before and after, and the allocation rate. A pause that tracks a growing old generation points at a leak or an unbounded cache. A pause where the heap recovers, with a high allocation rate, points at short-lived garbage. Change one thing and compare the same log.
Finish the object, then publish one reference. Final fields are safely published when the constructor returns, if this does not escape while the constructor is still running. Handing out the reference and then calling setters lets another thread observe defaults.
ConcurrentHashMap allows concurrent reads and updates without locking the whole map, and a retrieval sees the most recently completed update. synchronizedMap locks the entire map, and a compound action such as check-then-act still needs an external lock. Use compute for a compound update on the concurrent map.
Virtual threads, since Java 21, make a blocking I/O call cheap in operating-system threads, so a thread per request can wait on many calls. They do not make CPU-bound work faster. A scarce resource, such as a database, still needs a limit. On Java 21–23, avoid blocking inside synchronized, which pins the carrier thread; JDK 24 removed that limit (JEP 491).
The caller cannot tell "not found" from "the database failed," and null then spreads. Catch the failure you can handle. Log an unexpected failure with its stack and let it surface as an error. Returning null deletes the cause.
These questions show how a candidate talks about Java. They do not show the candidate writing the next change with no assistant. How to tell if a developer can code without AI explains the watched session.
What the candidate shows us is proctored tests, watched by a real person: a coding test with no AI allowed, a 2-hour build project with AI allowed, the Fire Drill (a live pair-programming session on a real-world problem), an English test, a thinking-skills test and a typing test. It also covers recorded reference checks. The menu is on Proctored tests. You pay for the tests you order. A real person watches.
When the answers are strong, Hire backend developers. Arca.ph is a network of remote engineers. Your own AI agent, such as Grok, Claude, or ChatGPT, searches and sorts that network. On the marketplace, the fee is a one-time 15% fee of first-year pay, only on hire, with a 3-month free replacement. Done for you is 35% of first-year pay, charged once on hire. Details are on pricing and how it works.
Data, errors, the GIL, and what a good Python answer covers at each level.
Values, types, and the event loop, with what a good answer covers at each level.
State, effects, and rendering, with what a good React answer covers at each level.
The event loop, APIs, and failures, with what a good Node.js answer covers at each level.
Ask two or three questions from the level you are hiring, then ask what would break. A useful interview is a conversation about a real constraint. It is not a recital of every question on this page. Use the watched coding check before you decide to hire.
A junior answer names the rule and the common bug. A mid-level answer names the mechanism and one failure mode. A senior answer names a tradeoff, a failure they have seen, and what they would measure. Hire for the work the role will own, not for a perfect score on every question.
Questions do not show the candidate writing code. What the candidate shows us is proctored tests, watched by a real person: a coding test with no AI allowed, a 2-hour build project with AI allowed, the Fire Drill (a live pair-programming session on a real-world problem), an English test, a thinking-skills test and a typing test. It also covers recorded reference checks. The guide How to tell if a developer can code without AI explains the watched session.
Open the backend developer hire page linked on this page. Java is one of the stacks listed for that role. Your own AI agent searches and sorts the Arca.ph network. You choose who to test and who to interview. Profiles are free to view once you sign up and sign the Arca.ph Client Agreement.
Arca.ph is a network of remote engineers. Your own AI agent, such as Grok, Claude, or ChatGPT, searches and sorts that network. On the marketplace, the fee is a one-time 15% fee of first-year pay, only on hire, with a 3-month free replacement. Done for you is 35% of first-year pay, charged once on hire.
Profiles are free to view once you sign up and sign the Arca.ph Client Agreement.