Java backend interview quiz (10 questions)

Answer

ArrayList - ArrayList is the best default choice when order matters, duplicates are allowed, and indexed reads are common.

Explanation

ArrayList preserves insertion order, allows duplicates, and gives O(1) indexed access. HashSet removes duplicates. TreeMap is a key-value structure, not a List. PriorityQueue orders by priority rather than preserving insertion order.

Common interview notes

Interviewers often follow up by asking when LinkedList is preferable. A strong answer mentions queue-like operations or frequent inserts/removes at known positions, not random access.

Answer

Visibility of writes across threads - volatile gives you visibility of writes across threads.

Explanation

A volatile write becomes visible to other threads that read the same field. It does not make compound operations like count++ atomic, and it does not replace locking when multiple operations must happen together safely.

Common interview notes

A common follow-up is how volatile differs from synchronized or AtomicInteger.

Answer

String literals may be interned in the string pool - String literals may be interned in the string pool.

Explanation

String is immutable regardless of how it is constructed. equals compares content, while == compares references. In loops, StringBuilder is usually preferred over repeated + because it avoids many intermediate String allocations.

Common interview notes

Strong candidates can explain when intern() is useful and why overusing it is rarely necessary in application code.

Answer

record - record is the best fit for a compact immutable data carrier.

Explanation

Records are ideal for simple data-focused models because they generate accessors, constructor, equals, hashCode, and toString automatically. enum models a fixed set of constants. interface and abstract class solve different design problems.

Common interview notes

A good follow-up is when not to use a record, for example when the type has substantial mutable state or complicated lifecycle rules.

Answer

It propagates to the caller - The exception propagates to the caller until some frame handles it or the thread terminates.

Explanation

Runtime exceptions do not need to be declared, but they still follow normal stack unwinding. finally blocks run during that unwinding, yet they do not magically swallow the exception unless code explicitly does so.

Common interview notes

Interviewers may ask you to compare checked exceptions, runtime exceptions, and Error.

Answer

ExecutorService - ExecutorService is the standard starting point for managing a pool of worker threads and submitted tasks.

Explanation

ExecutorService separates task submission from thread management and supports fixed pools, futures, graceful shutdown, and queueing. synchronized controls access to shared state, not task execution. The other choices are unrelated.

Common interview notes

Good answers often mention shutdown(), Future, and the difference between CPU-bound and IO-bound workloads.

Answer

(no modifier / default) - Package-private visibility is the default when you do not write an access modifier.

Explanation

public is visible everywhere, private only inside the declaring class, and protected is visible to the package plus subclasses. The default visibility sits between private and protected for package-level API design.

Common interview notes

A useful follow-up is why reducing visibility can make APIs safer and easier to evolve.

Answer

Use entrySet() iteration or forEach((k, v) -> ...) - entrySet() iteration or Map.forEach is the clearest way to access both keys and values together.

Explanation

entrySet() gives you direct access to both pieces of data in one pass. values() loses the key. Converting to arrays adds unnecessary work. Stream.of(map) does not flatten the map into entries.

Common interview notes

This kind of question often checks whether you reach for the simplest readable collection view first.

Answer

StringBuilder - StringBuilder is the best default choice for repeated concatenation in single-threaded code.

Explanation

StringBuilder is mutable, so it avoids creating many intermediate String objects. StringBuffer is synchronized and usually unnecessary unless multiple threads truly share the same builder. Raw char arrays are too low-level for most application code.

Common interview notes

Interviewers may ask when StringBuffer is still relevant or how the compiler handles simple constant concatenation.

Answer

It may never run, so you should not rely on it for cleanup - You should assume finalize may never run and must not be relied on for resource cleanup.

Explanation

finalize is deprecated because it is unpredictable and harmful for correctness and performance. Use try-with-resources, explicit close methods, and well-defined ownership of resources instead.

Common interview notes

A strong answer usually mentions AutoCloseable and try-with-resources immediately.