Top 25 Core Java Interview Questions for Experienced Developers
Top 25 Core Java Interview Questions for Experienced Developers Work through these 15 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
When should you use CopyOnWriteArrayList?
Use for subscriber lists in pub/sub where adds are rare and iteration is constant. Avoid for high-frequency add/remove — use ConcurrentLinkedQueue or synchronized collections....
Explain the difference between == and .equals() in Java.
For primitives, == compares values. For objects, == asks if two references point to the same instance. equals() compares content when overridden — String overrides it for...
What is the difference between ArrayList and LinkedList?
ArrayList: dynamic array, great for get(i) and iteration, amortized append. LinkedList: node chain, costly random access, only wins for frequent insert/remove in middle with...
What is the difference between Comparable and Comparator?
Comparable built into the type — Employee implements compareTo by id. Comparator external — sort by name or salary without changing Employee. Use...
Explain method overloading vs overriding.
Overload print(int) and print(String) in one class — compiler picks at compile time. Override toString in subclass — JVM calls subclass version via virtual method table....
What is pattern matching for switch (Java 21)?
Instead of if (obj instanceof String s) { ... }, use switch (obj) { case String s when s.length() > 5 -> ...; case String s -> ...; case null -> ...; default -> ... }. Works...
volatile vs synchronized — when to use each?
volatile for single-writer flags (shutdown, initialized) where no read-modify-write. synchronized or java.util.concurrent locks when multiple threads update shared state. For...
Why is ThreadLocal problematic with virtual threads?
Libraries storing per-request state in ThreadLocal allocate one entry per virtual thread — catastrophic memory with 100k concurrent requests. Prefer ScopedValue for request...
Explain the happens-before relationship in the Java Memory Model.
Thread A writes x=1 without sync; Thread B reads x may see 0 forever. volatile x or synchronized block creates happens-before. Classic DCL singleton needs volatile instance...
HashMap vs ConcurrentHashMap — when and why?
HashMap for single-threaded or external synchronization. ConcurrentHashMap for shared concurrent maps — finer-grained locking than Hashtable. Cannot store null keys/values in...
Explain immutability and why String is immutable.
Immutable objects cannot change after build — thread-safe without synchronization. String immutability allows intern pool, caching hashCode, and security when used as...
What is an ExecutorService and why use it?
Instead of new Thread per task, submit Runnable to fixed pool sized to CPU/IO profile. Handle Future results and exceptions. On shutdown hook call shutdown() and...
What are virtual threads in Java 21 and when should you use them?
Virtual threads excel when code blocks on IO — HTTP clients, JDBC, file reads — allowing one platform thread to serve thousands of concurrent requests. For CPU-heavy parallel...
Explain sealed classes and their purpose in Java 17+.
sealed interface Shape permits Circle, Rectangle limits who implements Shape. Compiler verifies switch (Shape s) covers all cases — no default needed. Use for domain variants...
How do CompletableFuture and virtual threads compare for concurrency?
Use virtual threads when business logic is naturally blocking — call DB, call HTTP, return. Use CompletableFuture when composing multiple independent async results with...
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