Java Advanced Interview Questions
When you prepare for a job interview, Java is a crucial part of the interview process. Candidates must pass the Java advanced interview questions to prove they have a Java expertise. Predictably, the difficulty of such Java advanced interview questions is directly related to how much of an expert you are in fundamental Java concepts, data structures, concurrency, and design patterns. The majority of the Java advanced interview questions relate to the following main topics: the differences between HashMap and TreeMap, the application of the final keyword, and the Singleton pattern implementation.
On top of gaining practice in the Java advanced interview questions, you not only study the theoretical part but also get hands-on practice which will help you to solve real-world problems efficiently. By practising these Java advanced interview questions, you can also gauge your practical experience in the handling of certain complex cases such as memory management or exception handling. Also, facing Java advanced interview questions about newer Java features like Streams API, Optional, and CompletableFuture will keep you up to date with the latest improvements, in addition to it.
Perfecting these Java advanced interview questions will set you aside as a smart and capable candidate. In this way, it will not only prepare you for the process, but will also make it easy for you to succeed in your next Java interview.
1. What is the difference between == and equals() in Java?
Answer:
- ==: Compares memory addresses (references) for objects.
- .equals(): Compares the content of two objects. The default implementation of the Object class compares memory references. But this method is usually overridden (e.g. in String).
2. What is a Singleton design pattern and how is it implemented in Java?
Answer: Singleton pattern defines a way to ensure that a class has only one instance and that it can be accessed globally through a single point.
Example implementation:
public class Singleton {
private static Singleton instance;
private Singleton() { }
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
3. When discussing ArrayList, what is the relation to LinkedList?
Answer:
- ArrayList: The list of items changes in size while providing good access to all types of items. However, it becomes a bit slow in case of insertion or deletion.
- LinkedList: By using a doubly linked list data structure, you can add and delete elements as fast as possible, but you have to sacrifice random access.
4. What exactly is Java Memory Model (JMM)?
Answer:
- The Java Memory Model (JMM) depicts the way the interactions between threads take place by memory and what activities are allowed when multiple threads read and write shared variables.
- It encompasses the visibility, atomicity, and ordering of variables between threads in the program.
5. What is the difference between final, finally, and finalize() in Java?
Answer:
- final: It is a keyword in Java that is used to define constants, prevent method overriding, and prevent inheritance of classes (final class).
- finally: This is a block used after try-catch to ensure cleanup (always executes, regardless of exception).
- finalize(): A method in Object class calling before the garbage collector reclaims an object’s memory (deprecated in later versions).
6. What is a HashMap and how does it work internally?
Answer:
A HashMap is a collection that maps keys to values using a hash table. Internally, it stores key-value pairs in buckets and uses the hash code of the key to determine the bucket index.
- Hashing: A key’s hash code is used to determine its bucket index.
- Collision handling: When multiple keys have the same hash code, a linked list or tree is used to store elements in the same bucket.
7. What is the volatile keyword in Java?
Answer:
- The volatile keyword is used to ensure that a variable is all the time read from and written to the main memory.
- The change is then visible to all other threads. It thus avoids is mixing of the variable in CPU registers or thread-local memory.
8. Explain synchronized keyword in Java.
Answer:
- The synchronized keyword guarantees that only one thread at a time can enter the code block or access the method.
- This is done so race conditions cannot be the result of instructions in the program are executed by more than one thread simultaneously.
- Example:
public synchronized void method() {
// thread-safe code
}
9. What is the difference between StringBuilder and StringBuffer?
Answer:
- StringBuilder: Mutable, but not thread-safe (faster than StringBuffer).
- StringBuffer: Mutable and thread-safe (slower due to synchronization).
10. What are Java annotations and how are they used?
Answer:
- Annotations are pieces of meta-information that supply additional information to programs.
- They are ignored by program logic but can be read by compilers or tools.
- Example:
@Override
public void run() { // method overriding
// code here
}
11. What is the difference between == and .equals() for string comparison?
Answer:
- == concerns the reference of two String objects, which is to say it checks whether they both point to the same memory location.
- .equals() is just another way of comparing the content of two String objects. i.e., By using it you get to know whether the two strings are alike or not.
12. What is the distinction, if any, between HashMap and TreeMap?
Answer:
- HashMap: It stores elements in an unordered manner, it propagates the changes to the data structure maintaining its uniqueness and is speedier in operations of get() and put().
- TreeMap: Stores elements in either a natural order that is based on the natural ordering of the keys or an external comparator.
13. What is the try-with-resources statement in Java?
Answer:
- The try-with-resources statement closes resources (for instance streams, sockets, etc.) automatically when the try block completes its execution. To do so, you need resources to implement the AutoCloseable interface.
- Example:
try (BufferedReader br = new BufferedReader(new FileReader(“file.txt”))) {
// file reading code
} catch (IOException e) {
// handle the exception
}
14. How does super() and this() to Java once a time?
Answer:
- super(): The superclass constructor is selected by it. That is, it should be the first statement in a subclass constructor.
- this(): Invoking another constructor of the same class is also another one of its roles. It also must be the initial expression in the constructor.
15. What is the Enum type in Java?
Answer:
- It is a special data type in Java that is used as an enumeration to represent a fixed set of constants.
- With it, values like days of the week, months, etc. can be mimicked.
- Example:
enum Day { SUNDAY, MONDAY, TUESDAY }
16. What are the benefits of using LinkedHashMap?
Answer:
- LinkedHashMap keeps the order of insertion or access (in case the access order is used).
- It is ideal for collecting the resources in order while simultaneously giving constant-time traffic to the core operations.
17. What is the purpose of transient keyword in Java?
Answer:
- The transient keyword is the keyword used for the purpose of stopping the serialization of the particular variables during the process of java objects serialization.
- The will not be saved if the variable simply has transient.
18. What are wait(), notify(), and notifyAll() in Java?
Answer:
- wait(): Puts the current thread on hold and be notified later to continue execution of the rest of the threads.
- notify(): Wakes up the thread which is sleeping on the object’s monitor in Java.
- notifyAll(): It will wake up all the threads which are waiting on that object’s monitor in Java.
19. Explain the concept of Garbage Collection in Java.
Answer:
- Garbage Collection (GC) is a system in which the JVM reclaims the memory utilized by the objects that are no more used by any of the live threads, on autopilot, of course.
- The JVM has several garbage collectors including Serial GC, Parallel GC, CMS, and G1 GC.
20. What is the difference between ArrayList and Vector?
Answer:
- ArrayList: No synchronization, higher access speed, but it is not thread-safe either.
- Vector: By nature, a vector is synchronized thus the processing speed is exceeded as compared to that of ArrayList in the absence of synchronization technique but it is a thread-safe data structure.
21. What is Callable in Java?
Answer:
- The Callable interface is nearly identical to Runnable but has the ability to return a result or throw an exception.
- Callable is employed with ExecutorService for the purpose of getting tasks done in parallel with the result.
22. Explain the concept of Reflection in Java.
Answer:
- Reflection in Java is a utility that enables applications to examine and control classes, interfaces, constructors, methods, and fields at runtime.
- This capability can be advantageous in developing adaptable frameworks and libraries.
23. What is the difference between throw and throws in Java?
Answer:
- throw: It is used to write statements that explicitly throw an exception in the code.
- throws: It is used as part of a method signature to notify that a method may throw an exception.
24. How are default methods in Java interfaces used effectively?
Answer:
- Introduced in Java 8, default methods in Java interfaces are methods that have bodies.
- In this way, it is now possible to add new methods to interfaces without forcing the change of classes that implement them.
25. What is the CompletableFuture in Java 8?
Answer:
- CompletableFuture is a new class in Java 8 that functions as a promise/Future to facilitate the non-blocking code handling.
- It may carry its tasks at the same time, eliminating the necessity of the programmer to order the sequence of the tasks to be done, hence, programming asynchronically, and it can run only if the previous operation completed successfully.
26. What is Stream in Java 8?
Answer:
- A Stream in Java 8 can be either processed in parallel or sequentially.
- It is a sequence of elements that supports many operations like filtering, mapping, and reducing, a powerful tool for functional-style programming.
27. What is Optional in Java 8?
Answer:
- An Optional is a container object that may or may not carry a non-null value.
- It allows for an alternative to NullPointerException when you utilize functions like Through different methods such as isPresent(), ifPresent(), and orElse().
28. What is the purpose of hashCode() method in Java?
Answer:
- The hashCode() method is a method that returns an integer number which is the memory address of the object.
- It is by hash-based collections such as HashMap and HashSet so that the system can regroup and get the elements efficiently.
29. What is the difference between ArrayList and CopyOnWriteArrayList?
Answer:
- ArrayList: A direct consequence of being non-thread-safe.
- CopyOnWriteArrayList: Also known as a thread-safe version of ArrayList, it is defined as a copy that can be modified, which ensures that read operations remain thread safe.
30. What is the purpose of the enum keyword in Java?
Answer:
- enum is a special kind of data type, which makes it possible for a variable to possess a collection of predetermined characteristics.
- It offers type safety and thus, the code also becomes more readable and more maintainable.