Below are 25 advanced Java interview questions and answers tailored for experienced developers. These questions delve into deeper concepts, best practices, and advanced features of Java.
1. What is the difference between HashMap and ConcurrentHashMap?
Answer:
HashMap: Not thread-safe. Suitable for single-threaded applications.ConcurrentHashMap: Thread-safe. Allows concurrent read and write operations without locking the entire map.
2. What is the difference between synchronized and ReentrantLock?
Answer:
synchronized: A keyword that provides intrinsic locking. Easy to use but less flexible.ReentrantLock: A class that provides more advanced features like fairness, try-lock, and interruptible locks.
3. What is the difference between wait() and sleep()?
Answer:
wait(): Releases the lock and waits for another thread to notify or interrupt. Part of theObjectclass.sleep(): Pauses the current thread without releasing the lock. Part of theThreadclass.
4. What is the difference between notify() and notifyAll()?
Answer:
notify(): Wakes up a single waiting thread.notifyAll(): Wakes up all waiting threads.
5. What is the difference between volatile and atomic variables?
Answer:
volatile: Ensures visibility of changes across threads but does not guarantee atomicity.atomic: Ensures both visibility and atomicity (e.g.,AtomicInteger,AtomicReference).
6. What is the difference between Callable and Runnable?
Answer:
Runnable: Represents a task that does not return a result or throw checked exceptions.Callable: Represents a task that returns a result and can throw checked exceptions.
7. What is the difference between ExecutorService and ForkJoinPool?
Answer:
ExecutorService: A general-purpose thread pool for executing tasks.ForkJoinPool: A specialized thread pool for divide-and-conquer algorithms (e.g., parallel streams).
8. What is the difference between Stream and ParallelStream?
Answer:
Stream: Processes elements sequentially.ParallelStream: Processes elements in parallel using multiple threads.
9. What is the difference between Optional and null?
Answer:
null: Represents the absence of a value and can lead toNullPointerException.Optional: A container object that may or may not contain a non-null value, reducing the risk ofNullPointerException.
10. What is the difference between CompletableFuture and Future?
Answer:
Future: Represents the result of an asynchronous computation. Limited functionality.CompletableFuture: Provides more advanced features like chaining, combining, and exception handling.
11. What is the difference between ClassNotFoundException and NoClassDefFoundError?
Answer:
ClassNotFoundException: Thrown when the classloader cannot find a class at runtime.NoClassDefFoundError: Thrown when the JVM cannot find a class definition that was available at compile time.
12. What is the difference between equals() and hashCode()?
Answer:
equals(): Compares the content or values of objects.hashCode(): Returns an integer representation of the object, used in hash-based collections likeHashMap.
13. What is the difference between Comparable and Comparator?
Answer:
Comparable: A single sorting sequence defined within the class using thecompareTo()method.Comparator: Multiple sorting sequences defined outside the class using thecompare()method.
14. What is the difference between ArrayList and LinkedList?
Answer:
ArrayList: Uses a dynamic array. Faster for random access but slower for insertions/deletions in the middle.LinkedList: Uses a doubly linked list. Faster for insertions/deletions but slower for random access.
15. What is the difference between HashSet and TreeSet?
Answer:
HashSet: Stores elements in a hash table. Does not maintain order.TreeSet: Stores elements in a red-black tree. Maintains sorted order.
16. What is the difference between String, StringBuilder, and StringBuffer?
Answer:
String: Immutable (cannot be changed after creation).StringBuilder: Mutable and not thread-safe (faster for single-threaded operations).StringBuffer: Mutable and thread-safe (slower due to synchronization).
17. What is the difference between checked and unchecked exceptions?
Answer:
- Checked Exceptions: Checked at compile-time (e.g.,
IOException,SQLException). Must be handled usingtry-catchorthrows. - Unchecked Exceptions: Checked at runtime (e.g.,
NullPointerException,ArithmeticException). Handling is optional.
18. What is the difference between throw and throws?
Answer:
throw: Used to explicitly throw an exception.throws: Used in method signatures to declare that a method might throw an exception.
19. What is the difference between final, finally, and finalize?
Answer:
final: Used to restrict modification (e.g., final variable, method, or class).finally: A block used in exception handling to execute code regardless of whether an exception occurs.finalize: A method called by the garbage collector before an object is reclaimed.
20. What is the difference between static and non-static methods?
Answer:
staticmethods: Belong to the class and can be called without creating an instance.non-staticmethods: Belong to an instance of the class and require an object to be called.
21. What is the difference between instanceof and isInstance()?
Answer:
instanceof: A keyword used to check if an object is an instance of a specific class or interface.isInstance(): A method in theClassclass that performs the same check at runtime.
22. What is the difference between Enum and EnumSet?
Answer:
Enum: A special data type for defining a set of constants.EnumSet: A specialized set implementation for enum types, providing better performance.
23. What is the difference between WeakHashMap and HashMap?
Answer:
HashMap: Strongly references its keys, preventing them from being garbage collected.WeakHashMap: Weakly references its keys, allowing them to be garbage collected when no longer in use.
24. What is the difference between Stream and Iterator?
Answer:
Stream: Provides a functional approach to process collections (e.g., map, filter, reduce).Iterator: Provides an imperative approach to traverse collections.
25. What is the difference between JVM, JRE, and JDK?
Answer:
JVM(Java Virtual Machine): Executes Java bytecode and makes Java platform-independent.JRE(Java Runtime Environment): Provides the libraries and JVM required to run Java programs.JDK(Java Development Kit): Includes tools for developing Java applications (e.g., compiler, debugger).
These advanced questions and answers should help experienced developers prepare for Java interviews effectively. Make sure to practice coding examples and understand the underlying concepts thoroughly!
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