50+ Java Interview Questions for Programmers
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Hello, guys! Recently, I have been sharing a lot of Java Interview questions and discussion individually, and many of my readers requested to bring them together so that they can have them in the same page and prepare better and this post is the result of that.
This article contains more than 50 Java Interview questions covering all important topics like core Java fundamentals, Java Collection Framework, Java Multithreading and Concurrency, Java IO, JDBC, JVM Internals, Coding Problems, Object-Oriented programming, etc.
The questions are also picked up from various interviews and they are, by no means, very difficult, and you might have seen them already in your telephonic or face-to-face round of interview.
The questions are also very good to revise important topics like multithreading and collections as I have also shared some useful resources for further learning and improvement like The Complete Java MasterClass to brush up and fill gaps in your Java skills.
So what are we waiting for here is the list of some of the frequently asked Java questions from interviews from both beginner and experienced Java developer of 2 to 5 years experience:
Java Interview Questions and Answers
1) How Java achieves platform independence? (answer)
hint: bytecode and Java Virtual Machine
2) What is ClassLoader in Java? (answer)
hint: part of JVM that loads bytecodes for classes. You can write your own.
3) Write a Java program to check if a number is Even or Odd? (answer)
hint: you can use bitwise operator, e.g. bitwise AND &, remember, even the number has zero at the end in binary format and an odd number has 1 in the end.
4) Difference between ArrayList and HashSet in Java? (answer)
hint: all differences between List and Set are applicable here, e.g. ordering, duplicates, random search, etc.
5) What is double checked locking in Singleton? (answer)
hint: two-time check whether instances is initialized or not, first without locking and second with locking.
6) How do you create thread-safe Singleton in Java? (answer)
hint: many ways, e.g. using Enum or by using double-checked locking pattern or using a nested static class.
7) When to use volatile variable in Java? (answer)
hint: when you need to instruct the JVM that a variable can be modified by multiple threads and give hint to JVM that does not cache its value.
8) When to use a transient variable in Java? (answer)
hint: when you want to make a variable non-serializable in a class, which implements the Serializable interface. In other words, you can use it for a variable whose value you don't want to save. See The Complete Java MasterClass to learn about transient variables in Java.
9) Difference between the transient and volatile variable in Java? (answer)
hint: totally different, one used in the context of serialization while the other is used in concurrency.
10) Difference between Serializable and Externalizable in Java? (answer)
hint: Externalizable gives you more control over the Serialization process.
11) Can we override the private method in Java? (answer)
hint: No, because it's not visible in the subclass, a primary requirement for overriding a method in Java.
12) Difference between Hashtable and HashMap in Java? (answer)
hint: several but most important is Hashtable, which is synchronized, while HashMap is not. It's also legacy and slow as compared to HashMap.
13) Difference between Listand Set in Java? (answer)
hint: List is ordered and allows duplicate. Set is unordered and doesn't allow duplicate elements.
14) Difference between ArrayList and Vector in Java (answer)
hint: Many, but most important is that ArrayList is non-synchronized and fast while Vector is synchronized and slow. It's also legacy class like Hashtable.
15) Difference between Hashtable and ConcurrentHashMap in Java? (answer)
hint: more scalable
16) How does ConcurrentHashMap achieve scalability? (answer)
hint: by dividing the map into segments and only locking during the write operation.
17) Which two methods you will override for an Object to be used as Key in HashMap? (answer)
hint: equals and hashcode
18) Difference between wait and sleep in Java? (answer)
hint: The wait() method releases the lock or monitor, while sleep doesn't.
19) Difference between notify and notifyAll in Java? (answer)
hint: notify notifies one random thread is waiting for that lock while notifyAll inform to all threads waiting for a monitor. If you are certain that only one thread is waiting then use notify, or else notifyAll is better. See Threading Essentials Mini-Course by Java Champion Heinz Kabutz to learn more about threading basics.
20) Why you override hashcode, along with equals() in Java? (answer)
hint: to be compliant with equals and hashcode contract, which is required if you are planning to store your object into collection classes, e.g. HashMap or ArrayList.
21) What is the load factor of HashMap means? (answer)
hint: The threshold that triggers the re-sizing of HashMap is generally 0.75, which means HashMap resize itself if it's 75 percent full.
22) Difference between ArrayList and LinkedList in Java? (answer)
hint: same as an array and linked list, one allows random search while other doesn't. Insertion and deletion easy on the linked list but a search is easy on an array. See Java Fundamentals: Collections Richard Warburton course on Pluralsight to learn more about essential Collection data structure in Java.
23) Difference between CountDownLatch and CyclicBarrier in Java? (answer)
hint: You can reuse CyclicBarrier after the barrier is broken but you cannot reuse CountDownLatch after the count reaches to zero.
24) When do you use Runnable vs Thread in Java? (answer)
hint: always
25) What is the meaning of Enum being type-safe in Java? (answer)
hint: It means you cannot assign an instance of different Enum type to an Enum variable. e.g. if you have a variable like DayOfWeek day then you cannot assign it value from DayOfMonth enum.
26) How does Autoboxing of Integer work in Java? (answer)
hint: using valueOf() method
27) Difference between PATH and Classpath in Java? (answer)
hint: PATH is used by the operating system while Classpath is used by JVM to locate Java binary, e.g. JAR files or Class files. See Java Fundamentals: The Core Platform to learn more about PATH, Classpath, and other Java environment variable.
28) Difference between method overloading and overriding in Java? (answer)
hint: Overriding happens at subclass while overloading happens in the same class. Also, overriding is a runtime activity while overloading is resolved at compile time.
29) How do you prevent a class from being sub-classed in Java? (answer)
hint: just make its constructor private
30) How do you restrict your class from being used by your client? (answer)
hint: make the constructor private or throw an exception from the constructor
31) Difference between StringBuilder and StringBuffer in Java? (answer)
hint: StringBuilder is not synchronized while StringBuffer is synchronized.
32) Difference between Polymorphism and Inheritance in Java? (answer)
hint: Inheritance allows code reuse and builds the relationship between class, which is required by Polymorphism, which provides dynamic behavior. See Java Fundamentals: Object-Oriented Design to learn more about OOP features.
33) Can we override static method in Java? (answer)
hint: No, because overriding resolves at runtime while static method call is resolved at compile time.
34) Can we access the private method in Java? (answer)
hint: yes, in the same class but not outside the class
35) Difference between interface and abstract class in Java? (answer)
hint: from Java 8, the difference is blurred. However, a Java class can still implement multiple interfaces but can only extend one class.
36) Difference between DOM and SAX parser in Java? (answer)
hint: DOM loads whole XML File in memory while SAX doesn't. It is an event-based parser and can be used to parse a large file, but DOM is fast and should be preferred for small files.
37) Difference between throw and throws keyword in Java? (answer)
hint: throws declare what exception a method can throw in case of error but throw keyword actually throws an exception. See Java Fundamentals: Exception Handling to learn more about Exception handling in Java.
38) Difference between fail-safe and fail-fast iterators in Java? (answer)
hint: fail-safe doesn't throw ConcurrentModificationException while fail-fast does whenever they detect an outside change on the underlying collection while iterating over it.
39) Difference between Iterator and Enumeration in Java? (answer)
hint: Iterator also gives you the ability to remove an element while iterating while Enumeration doesn't allow that.
40) What is IdentityHashMap in Java? (answer)
hint: A Map, which uses the == equality operator to check equality instead of the equals() method.
41) What is String pool in Java? (answer)
hint: A pool of String literals. Remember it's moved to heap from perm gen space in JDK 7.
42) Can a Serializable class contain a non-serializable field in Java? (answer)
hint: Yes, but you need to make it either static or transient.
43) Difference between this and super in Java? (answer)
hint: this refers to the current instance while super refers to an instance of the superclass.
44) Difference between Comparator and Comparable in Java? (answer)
hint: Comparator defines custom ordering while Comparable defines the natural order of objects, e.g. the alphabetic order for String. See The Complete Java MasterClass to learn more about sorting in Java.
45) Difference between java.util.Date and java.sql.Date in Java? (answer)
hint: former contains both date and time while later contains only date part.
46) Why wait and notify method are declared in Object class in Java? (answer)
hint: because they require lock which is only available to an object.
47) Why Java doesn't support multiple inheritances? (answer)
hint: It doesn't support because of bad experience with C++, but with Java 8, it does in some sense — only multiple inheritances of Type are not supported in Java now.
48) Difference between checked and unchecked Exception in Java? (answer)
hint: In case of checked, you must handle exception using catch block, while in case of unchecked, it's up to you; compile will not bother you.
49) Difference between Error and Exception in Java? (answer)
hint: I am tired of typing please check the answer
50) Difference between race condition and deadlock in Java? (answer)
hint: both are errors that occur in a concurrent application, one occurs because of thread scheduling while others occur because of poor coding.
Additional Resources
130+ Java Interview Questions of Last 5 Years
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