100 commonly asked interview questions for Java developer roles

Are you preparing for your first Java developer interview and unsure where to start? With so many concepts and technologies to cover, figuring out the key areas of focus can be overwhelming. What if you had a roadmap with a curated list of questions that not only tests your understanding but also helps you refine your skills?

This blog provides a comprehensive list of 100 commonly asked Java interview questions for freshers. From core Java concepts like OOP principles and JVM architecture to advanced topics such as multithreading, exception handling, and Java 8 features, we’ve got you covered. These questions are designed to help you gain confidence, master essential topics, and ace your interview.

Core Java and OOP Concepts

1. What is Java, and how does it work?
2. What are the JVM, JRE, and JDK, and how do they differ?
3. What are the four principles of Object-Oriented Programming (OOP)?
4. What is the difference between public, private, protected, and default (package-private) access modifiers?
5. Explain the concepts of class and object in Java.
6. What is polymorphism? Give an example of runtime and compile-time polymorphism.
7. What is method overloading and method overriding in Java?
8. What is the purpose of this keyword in Java?
9. What is the difference between an interface and an abstract class?
10. Explain the concept of inheritance in Java with an example.
11. What are constructors, and how do they work in Java?
12. What is the significance of the super keyword?
13. What is encapsulation in Java? How is it achieved?
14. What is an abstraction in Java? Provide an example.
15. What is the use of the static keyword in Java?
16. What is the difference between the == and .equals() method in Java?
17. What is the purpose of the final keyword in Java?
18. What is the difference between a class and an object?
19. What is the difference between an abstract class and a concrete class?
20. What are the different types of constructors in Java?

Also Read: Getting Started with Java: Essential Concepts and Techniques

Java Basics and Syntax

21. What are primitive data types in Java?
22. Explain autoboxing and unboxing in Java.
23. What are wrapper classes in Java?
24. What is the difference between String, StringBuffer, and StringBuilder?
25. What is a final variable, method, and class in Java?
26. What are the default values for instance variables in Java?
27. What is a package in Java? Why is it used?
28. What is the difference between import and static import in Java?
29. What is the purpose of a main method in Java?
30. What is a default constructor, and when is it used?
31. Explain the use of a try-catch-finally block in Java.
32. What are checked and unchecked exceptions in Java?
33. What is the difference between throw and throws?
34. What is the difference between break and continue in loops?
35. What is an enhanced for-each loop, and when is it used?
36. What is an enumeration (enum) in Java, and how is it used?
37. What is the significance of the synchronized keyword in Java?
38. What is the difference between shallow copy and deep copy in Java?
39. What are static blocks, and when are they executed?
40. Can we have multiple public classes in a single Java file? Why or why not?

Collections Framework

41. What is the Java Collections Framework?
42. What is the difference between ArrayList and LinkedList?
43. What is the difference between HashMap and TreeMap?
44. What is the difference between HashSet and LinkedHashSet?
45. Explain the internal workings of a HashMap in Java.
46. What is the difference between HashMap and Hashtable?
47. What is the difference between List, Set, and Map interfaces?
48. What is the difference between Iterator and ListIterator?
49. How does the ConcurrentHashMap differ from a HashMap?
50. What is the difference between fail-fast and fail-safe iterators in Java?
51. How does the LinkedHashMap maintain insertion order?
52. What are generics in Java? Explain with an example.
53. What is the difference between ArrayList and Vector in Java?
54. How does the TreeSet maintain sorted order?
55. What is the difference between Array and ArrayList in Java?
56. What is the role of the Comparator and Comparable interfaces in sorting collections?
57. What is the difference between Queue and Deque?
58. What is the difference between a PriorityQueue and a LinkedList in Java?
59. What is the purpose of Collections.sort() in Java?
60. What is the purpose of Collections.unmodifiableList() in Java?

Multithreading and Concurrency

61. What is multithreading in Java, and why is it important?
62. What is the difference between the Thread class and the Runnable interface?
63. What are the different states of a thread in Java?
64. What is synchronization, and why do we need it?
65. What is a deadlock, and how can you avoid it?
66. What is the difference between wait() and sleep() in Java?
67. What is the difference between notify() and notifyAll() in Java?
68. What is thread priority in Java, and how does it affect thread execution?
69. What is the volatile keyword in Java, and how is it used?
70. What is a thread pool, and how is it used in Java?
71. What is the purpose of the ExecutorService framework in Java?
72. Explain the difference between Callable and Runnable interfaces.
73. What is synchronized block and method in Java?
74. What is a CountDownLatch in Java, and when would you use it?
75. What is the ForkJoinPool in Java?

Exception Handling

76. What is an exception in Java? How is it different from an error?
77. What are the main differences between checked and unchecked exceptions?
78. What is the difference between throw and throws in Java?
79. What is the purpose of a finally block?
80. What is try-with-resources, and how does it work?
81. What is a custom exception, and how do you create one in Java?
82. What is NullPointerException, and how can you prevent it?
83. What is the RuntimeException class in Java?
84. Can we have multiple catch blocks in Java? How does it work?
85. What is the Exception class in Java, and how is it related to the Throwable class?

Java 8 Features

86. What are lambda expressions in Java 8, and how do they work?
87. What is the purpose of the Stream API in Java 8?
88. What are functional interfaces, and how are they used in Java 8?
89. What is a default method in an interface, and why was it introduced?
90. What is the Optional class in Java 8, and how is it used?
91. What is the difference between map() and flatMap() in the Stream API?
92. What is the purpose of the forEach() method in the Stream API?
93. What is the Predicate interface, and how is it used in Java 8?
94. Explain the difference between reduce() and collect() methods in the Stream API.
95. What is method reference in Java 8, and how does it differ from a lambda expression?
96. What is the Collectors class in Java 8?

SQL and Database Concepts

97. What is SQL, and how is it used in Java applications?
98. What is the difference between INNER JOIN and LEFT JOIN in SQL?
99. What is the purpose of a PRIMARY KEY in a database?
100. What is the difference between GROUP BY and ORDER BY in SQL?

Interview Preparation Tips:

1. Revise core Java and OOP concepts.
2. Practice coding using online platforms such as LeetCode, HackerRank, etc.
3. Understand projects you’ve worked on and be ready to explain them.
4. Mock interviews: Participate in practice interviews.
5. Read: Keep yourself updated with the latest Java features and best practices.

This comprehensive set of questions will help you cover the essential topics required for a fresher Java developer interview.

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Wrapping Up

Preparing for an interview isn’t just about memorizing questions, it’s about understanding the “why” behind them. Use this list to identify gaps in your knowledge, practice coding, and gain clarity on core Java concepts. Mock interviews and hands-on practice can further solidify your preparation.

Remember, your journey as a Java developer begins with mastering these fundamentals. Exploring the intricacies of the Collections Framework, or learning about Java 8 features, every step brings you closer to building a strong foundation. Good luck, and let these questions be your guide to success!