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https://levelup.gitconnected.com/56-java-interview-questions-b4f25e3cf9a6 https://levelup.gitconnected.com/56-java-interview-questions-b4f25e3cf9a6 https://levelup.gitconnected.com/56-java-interview-questions-b4f25e3cf9a6 https://levelup.gitconnected.com/56-java-interview-questions-b4f25e3cf9a6 55 Java Interview Questions Marc Fichtel Marc Fichtel Follow Jan 27 · 8 min read For Junior and Intermediate Developers. Answers included! Image for post Photo by Joshua Ness on Unsplash Below you’ll find a list of Java interview questions and answers I’ve gathered during my own job hunts. Before you continue, please note that answers may not be 100% complete, up to date, or optimized, but are in my experience sufficient for interview situations. OOP Questions These questions apply not only to Java, but object oriented languages in general. 1. Compare interfaces and abstract classes Both are used to achieve abstraction. Neither can be instantiated. An abstract class can have concrete implementations, an interface cannot. An interface is like a contract — a structure that classes implementing it must adhere to. Abstract classes are used to hide internals and expose relevant functionality — one doesn’t have to know how an abstract class does something in order to use it. 2. Explain inheritance Making one object the child of another gives the child all properties and methods (with respect to access modifiers) of its parent and ancestors. That is inheritance — it is used to achieve code reusability and polymorphism. In Java, as in most modern languages, an object can only have one parent — Java does not support multiple inheritance. Inheritance creates tight coupling between the parent and its children, which is why dependency injection is often a preferred alternative, as it allows related pieces of code to be decoupled. 3. Explain polymorphism Polymorphism means an object can belong to two or more types. It can be achieved through inheritance and interfaces. For example, a Cat object extending Mammal and implementing LandAnimal is an instance of all three of those types. Method overloading and overriding is used to achieve flexible behavior when implementing a polymorphic class hierarchy. 4. Compare association, aggregation, and composition These terms are used to describe relational structures in a class hierarchy. Association describes how objects relate to/are associated with each other (1–1, 1-N, N-1, N-M). It can be achieved through aggregation and composition. Aggregation describes “is-a” relationships — Each object can exist independently. Composition describes “has-a” relationships — Objects cannot exist independently. 5. How would you model an ER Diagram for a social media website? There are many ways to answer this one. A first approach could look something like this: User * Id: Long * Name: String Post * Id: Long * Likes: Integer Thread * Id: Long * Post: ForeignKey Java Questions 6. What can you tell me about memory management and garbage collection in Java? Both are automatically handled by the JVM. The garbage collector periodically collects variables without any references. Programmers can tell the garbage collector to schedule garbage collection with System.gc(), but it’s not guaranteed when that will happen. The two most important memory areas in the JVM are the stack and the heap. They are used for different purposes. The stack is used to hold method frames and local variables, and is not shared between threads. Objects are always allocated memory from the heap, which is shared between all threads in the JVM. The stack is usually much smaller than heap memory. 7. What can you tell me about Generics in Java? Generics can be used in conjunction with classes and methods. They’re used to specify a single declaration for a set of related methods, or a single class declaration for a set of related types. Generics are checked at compile-time for type safety. Two examples are ArrayList and three classes representing daily, monthly, and yearly charts that extend an abstract Chart and can be specified with . 8. What can you tell me about Java Optionals? They encapsulate optional values, and are used to make code more readable, stable, and avoid having to deal with null values, thus avoiding NullPointerExceptions. 9. Explain the difference between stream()and parallelStream(). A regular stream is always synchronized, whereas a parallel stream can execute operations asynchronously across CPU cores. Unless running a heavy operation on a lot of objects, one should use regular streams, as parallel streams have a high overhead for setting up multithreading. 10. What is ClassLoader in Java? The part of the JVM that loads bytecodes for classes at runtime. 11. When is it appropriate to use a transient variable in Java? Use transient variables, when you want to make a variable non-serializable in a class that implements the Serializable interface. 12. Can private methods be overwritten in Java? No, because private methods are not visible in the subclass. 13. What is the difference between lists and sets in Java? They differ in how their items’ ordering and uniqueness. Lists are ordered and allow duplicate values. Sets are unordered and do not allow duplicate elements. 14. Which two methods do you have to override for an object to be usable as a key in a hash map? To be usable as a key in a hash map, an object needs to be comparable and define a hash function. You have to overwrite equals() and hashCode(). 15. What’s the difference between method overloading and overriding in Java? Overriding happens in a subclass. Overloading happens in the same class. 16. How do you prevent a class from being sub-classed in Java? You can either make the constructor of the class private, or mark the class as final. 17. What’s the difference between this and super in Java? this refers to the current instance of an object. super refers to an instance of the parent/superclass. 18. Will 3*0.1 == 0.3 return true or false? This will return false, because some floating point numbers cannot be represented exactly. 19. What is the right data type to represent a price in Java? BigDecimal if memory is not a concern and performance is not critical, otherwise double with a predefined precision. 20. Why is String immutable in Java? Because Java was designed on the assumption that strings will be heavily used. Making it immutable allows for some optimization around easily sharing the same string between multiple clients. 21. What are some ways that you could sort a collection? You could use an inherently sorted collection like TreeMap, or Collections.sort(), or the Stream API. 22. Write a Java program for the Fizz Buzz problem. Fizz buzz 23. Write an algorithm to check, if a string is a palindrome in Java. Palindromes 24. Write a Java program to check, if a number is even or odd. Even or Odd 25. Write a Java program with a memory overflow. Naive Fibonacci or Endless Loop 26. Write a Java program to check, if a number is prime. Primes 27. Implement a stack in Java. Stacks 28. In a stack, peek() is O(1). How would you achieve O(1) lookup for peek() in a linked list? Linked lists typically keep a reference to the head node. In peek()just return head.value. 29. Implement a queue in Java using a linked list. Queues using Linked Lists 30. Implement a queue in Java using arrays. Queues using Arrays 31. Implement a singly-linked list in Java. Singly-linked List 32. in Java, how fast is direct lookup in a hash map theoretically, and why is it often slower in reality? Lookup in hash maps is supposed to be constant time, O(1), as each element is mapped to a key, which is computed using the key object’s hash code. However, if the hash function returns the same result for two or more inputs, collisions occur. In this case, each key is essentially mapped to a linked list of N objects belonging to it, reducing lookup speed to O(N). 33. Implement binary search in Java. Arrays.binarySearch(sortedArray, key); if you’re being sassy, or from scratch: Binary search 34. Implement bubble sort in Java. Bubble Sort 35. Given a string like a**hf*kl9*, write a function that returns a string with all asterisks appearing first. Sort String Below you’ll find more general technical and nontechnical questions. How you respond to these depends entirely on you, your experience, and the position you’re applying for. General Technical Questions 36. How long have you been programming professionally? 37. Tell us the details of an interesting problem you worked on. What made it interesting? 38. What is something you’re proud to have contributed to? 39. What is something that didn’t go so well at work / while programming and how did you handle it / how would you handle it in the future? 40. How do you feel about ? What made you apply? 41. Tell us about a time when you had to make a trade-off between user experience and optimization / technical design? 42. What’s an example of a time, when you had make a decision quickly? What were the reasons behind that decision? Would you have done anything differently? 43. What is your favorite thing you’ve worked on recently? 44. What is the biggest technical challenge you have faced? 45. Describe a bug you fixed / a feature you implemented. 46. What are your favorite and least favorite (Java) frameworks/libraries/tools, and why? Nontechnical Questions 47. Tell us about yourself. What are your career goals and past projects? Where do you see yourself in 2 / 5 / 10 years? 48. Where did you hear about this role? 49. Which parts of our Creed / Mission / Vision resonate the most with you? Why us? What do you like about our company? 50. Which of our products / projects would you be excited to work on and why? Are there technologies you don’t want to work with and why not? 51. How do you use our and / or our competitors’ products? How would you improve on them? 52. What is your dream job? What’s your perfect work day like? 53. How do you work with different teams/departments? 54. Do you have references? 55. Do you have any questions for us? The last one is important. Here are some suggestions for questions you may want to ask your interviewer: How are you financed? (especially important for startups) What do you like about working here? How can I best prepare for this role before starting? Could you describe a typical work week? How big are teams? What would my immediate responsibilities be? Will there be opportunities to choose, what projects I work on? What does the career path look like for this role? How does your company promote personal growth? Do you feel there are any skills currently lacking on the team? What is the biggest change the company has gone through in the last year? What’s the style of leadership? What is the rhythm of work here? Is there a particular time of year, when it’s all hands on deck and we’re pulling long hours, or is it fairly consistent throughout the year? What type of background and experience are you looking for in this position? What would your ideal candidate be like? Is there anything that stands out to you that makes you think I might not be the right fit for this position? What is the timeline for making a decision on this position? When should I get back in touch with you?