130+ Java Interview Questions Answers for 2 to 7 Year Experienced Programmers
Time is changing and so is Java interviews. Gone are the days, when knowing the difference between String and StringBuffer can help you to go through the second round of interview, questions are becoming more advanced and interviewers are asking more deep questions.
When I started my career, questions like Vector vs Array and HashMap vs, Hashtable was the most popular ones and just memorizing them gives you a good chance to do well in interviews, but not anymore.
Nowadays, you will get questions from the areas where not many Java programmer looks e.g. NIO, patterns, sophisticated unit testing or those which are hard to master e.g. concurrency, algorithms, data structures and coding.
Since I like to explore interview questions, I have got this huge list of questions with me, which contains lots and lots of questions on different topics.
I have been preparing this MEGA list for quite some time and now It's ready to share with you guys. It contains interview questions not only from classic topics like threads, collections, equals and hashcode, sockets but also from NIO, array, string, Java 8, and much more.
It has questions for both entry-level Java programmers and senior developers with years of experience. No matter whether you are a Java developer of 1, 2, 3, 4, 5, 6, 8, 9, or even 10 years of experience, you will find something interesting on this list.
It contains questions that are super easy to answer, and also, a question that is tricky enough for even seasoned Java programmers.
If you need more questions then you can also check out this Java Interview Guide: 200+ Interview Questions with Answers
Given the list is long and we have questions from everywhere, it's imperative that answers must be short, concise, and crisp, no fluffing at all. So apart from this paragraph, you will only hear from me is the questions and answers, no more context, no more feedback, and no more evaluation. For that, I have already written blog posts, where you can find my views on a particular the question, e.g. why I like that question, what makes them challenging, and what kind of answer
you should expect from candidates. This list is a little bit different and I encourage you to share questions and answers in a similar way so that it should be easy to revise. I hope this list can be a great use for both interviewer and candidates, the interviewer can, of course, but a little variety on questions to bring novelty and surprise element, which is important for a good interview. While a candidate, can expand and test their knowledge about key areas of Java programming language and platform. In 2018 and in coming years the focus will be more on advanced concurrency concept, JVM internals, 32-bit vs 64-bit JVM, unit testing, and clean code. I am sure, once you read through this MEGA list of Java interview questions, you should be able to do well on both telephonic and face-to-face programming interviews.
Important Topics for Java Interviews Apart from quantity, as you can see with a huge number of questions, I have worked hard to maintain quality as well. I have not only shared questions from all important topics but also ensured to include so-called advanced topics which many programmers do not prefer to prepare or just left out because they have not worked on that. Java NIO and JVM internals questions are the best examples of that.
You can keep design patterns also on the same list but a growing number of an experienced programmer is now well aware of GOF design patterns and when to use them. I have also worked hard to keep this list up-to-date to include what interviewers are asking in 2018 and what will be their core focus on the coming years. To give you an idea, this list of Java interview questions includes the following topics:
2. Date type conversion and fundamentals
19. Date, Time, and Calendar
20. XML Processing in Java
21. JUnit
22. Programming
You guys are also lucky that nowadays there are some good books available to prepare for Java interviews, one of them which I particularly find useful and interesting to read is Java Programming Interview Exposed by Markham. It will take you to some of the most important topics for Java and JEE interviews, worth reading even if you are not preparing for Java interview.
Top 120 Java Interview Questions Answers So now the time has come to introduce you to this MEGA list of 120 Java questions collected from various interviews of the last 5 years. I am sure you have seen many of these questions personally in your interviews and many of you would have answered them correctly as well. Multithreading, Concurrency, and Thread basics Questions
This is one of the tricky Java multi-threading questions you will see in senior Java developer Interview. Yes, you can make an array volatile in Java but only the reference which is pointing to an array, not the whole array. What I mean, if one thread changes the reference variable to points to another array, that will provide a volatile guarantee, but if multiple threads are changing individual array elements they won't be having happens before guarantee provided by the volatile modifier.
2) Can volatile make a non-atomic operation to atomic?
This another good question I love to ask on volatile, mostly as a follow-up of the previous question. This question is also not easy to answer because volatility is not about atomicity, but there are cases where you can use a volatile variable to make the operation atomic. One example I have seen is having a long field in your class. If you know that a long field is accessed by more than one thread e.g. a counter, a price field, or anything, you better make it volatile. Why? because reading to a long variable is not atomic in Java and done in two steps, If one thread is writing or updating long value, it's possible for another thread to see the half value (fist 32-bit). While reading/writing a volatile long or double (64 bit) is atomic.
3) What are the practical uses of a volatile modifier?
One of the practical use of the volatile variable is to make reading double and long atomic. Both double and long are 64-bit wide and they are read in two parts, first 32-bit first time and next 32- bit second time, which is non-atomic but volatile double and long-read are atomic in Java. Another use of the volatile variable is to provide a memory barrier, just like it is used in Disrupter framework. Basically, the Java Memory model inserts a write barrier after you write to a volatile variable and a read barrier before you read it. This means, if you write to a volatile field then it's guaranteed that any thread accessing that variable will see the value you wrote and anything you did before doing that right into the thread is guaranteed to have happened and any updated data values will also be visible to all threads because the memory barrier flushed all other writes to the cache.
4) What guarantee volatile variable provides?
volatile variables provide the guarantee about ordering and visibility e.g. volatile assignment cannot be re-ordered with other statements but in the absence of any synchronization instruction compiler, JVM or JIT are free to reorder statements for better performance. volatile also provides the happens-before guarantee which ensures changes made in one thread are visible to others. In some cases volatile also provide atomicity e.g. reading 64-bit data types like long and double are not atomic but read of volatile double or long is atomic.
5) Which one would be easy to write?
synchronization code for 10 threads or 2 threads? In terms of writing code, both will be of the same complexity because synchronization code is independent of a number of threads. Choice of synchronization though depends upon a number of threads because the number of thread present more contention, so you go for advanced synchronization technique e.g. lock stripping, which requires more complex code and expertise.
6) How do you call wait() method? using if block or loop? Why?
wait() method should always be called in the loop because it's possible that until the thread gets CPU to start running again the condition might not hold, so it's always better to check the condition in loop before proceeding. Here is the standard idiom of using wait and notify method in Java:
|
// The
standard idiom for using the wait method synchronized (obj) { while (condition does not
hold) obj.wait(); // (Releases lock,
and reacquires on wakeup) ... // Perform action
appropriate to condition } |
7) What is false sharing in the context of multi-threading?
false sharing is one of the well-known performance issues on multi-core systems, where each process has its local cache. false sharing occurs when threads on different processor modify variables that reside on the same cache line as shown in the following image:
False sharing is very hard to detect because the thread may be accessing completely different global variables that happen to be relatively close together in memory. Like many concurrency issues, the primary way to avoid false sharing is a careful code review and aligning your data structure with the size of a cache line.
8) What is a busy spin? Why should you use it?
Busy spin is one of the techniques to wait for events without releasing the CPU. It's often done to avoid losing data in CPU cached which is lost if the thread is paused and resumed in some other core. So, if you are working on a low latency system where your order processing thread currently doesn't have any order, instead of sleeping or calling wait(), you can just loop and then again check the queue for new messages. It's only beneficial if you need to wait for a very small amount of time e.g. in microseconds or nanoseconds. LMAX Disrupter framework, a high-performance inter-thread messaging library has a BusySpinWaitStrategy which is based on this concept and uses a busy spin loop for EventProcessors waiting on the barrier.
9) How do you take thread dump in Java?
You can take a thread dump of Java application in Linux by using kill -3 PID, where PID is the process id of Java process. In Windows, you can press Ctrl + Break. This will instruct JVM to print thread dump in standard out or err and it could go to console or log file depending upon your application configuration. If you have used Tomcat then when
10) is Swing thread-safe?
No, Swing is not thread-safe. You cannot update Swing components e.g. JTable, JList, or JPanel from any thread, in fact, they must be updated from GUI or AWT thread. That's why swings provide invokeAndWait() and invokeLater() method to request GUI update from any other threads. This method put update request in AWT threads queue and can wait till update or return immediately for an asynchronous update. You can also check the detailed answer to learn more.
11) What is a thread-local variable in Java?
Thread-local variables are variables confined to a thread, it's like the thread's own copy which is not shared between multiple threads. Java provides a ThreadLocal class to support thread-local variables. It's one of the many ways to achieve thread-safety. Though be careful while using a thread-local variable in the managed environment e.g. with web servers where the worker threads outlive any application variable. Any thread-local variable which is not removed once its work is done can potentially cause a memory leak in the Java application.
12) Write wait-notify code for the producer-consumer problem?
Please see the answer for a code example. Just remember to call wait() and notify() method from the synchronized block and test waiting for condition on the loop instead of if block.
13) Write code for thread-safe Singleton in Java?
Please see the answer for a code example and step by step guide to creating thread-safe singleton class in Java. When we say thread-safe, which means Singleton should remain singleton even if initialization occurs in the case of multiple threads. Using Java enum as Singleton class is one of the easiest ways to create a thread-safe singleton in Java.
14) The difference between sleep and wait in Java?
Though both are used to pause the currently running thread, sleep() is actually meant for short pause because it doesn't release the lock, while wait() is meant for the conditional wait and that's why it releases lock which can then be acquired by another thread to change the condition on which it is waiting.
15) What is an immutable object?
How do you create an Immutable object in Java? (answer) Immutable objects are those whose state cannot be changed once created. Any modification will result in a new object e.g. String, Integer, and other wrapper class. Please see the answer for step by step guide to creating an Immutable class in Java.
16) Can we create an Immutable object, which contains a mutable object?
Yes, it's possible to create an Immutable object which may contain a mutable object, you just need to be a little bit careful not to share the reference of the mutable component, instead, you should return a copy of it if you have to. The most common example is an Object which contains the reference of java.util.Date object. Date types and Basic Java Interview Questions
17) 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 predefined precision.
18) How do you convert bytes to String? (answer)
you can convert bytes to the string using string constructor which accepts byte[], just make sure that right character encoding otherwise platform's default character encoding will be used which may or may not be the same.
19) How do you convert bytes to long in Java?
The byte takes 1 byte of memory and the long takes 8 bytes of memory. Assignment 1 byte value to 8 bytes is done implicitly by the JVM.
byte –> short –> int –> long –> float –> double
The left-side value can be assigned to any right-side value and is done implicitly. The reverse requires explicit casting.
20) Can we cast an int value into byte variable?
what will happen if the value of int is larger than byte? Yes, we can cast but int is 32 bit long in java while byte is 8 bit long in java so when you cast an int to byte higher 24 bits are lost and a byte can only hold a value from -128 to 128.
22) Which class contains clone method? Cloneable or Object?
java.lang.Cloneable is marker interface and doesn't contain any method clone method is defined in the object class. It is also knowing that clone() is a native method means it's implemented in C or C++ or any other native language.
23) Is ++ operator is thread-safe in Java?
No, it's not a thread-safe operator because it involves multiple instructions like reading a value, incriminating it and storing it back into memory which can be overlapped between multiple threads.
24) Difference between a = a + b and a += b ?
The += operator implicitly cast the result of addition into the type of the variable used to hold the result. When you add two integral variables e.g. variable of type byte, short, or int then they are first promoted to int and them addition happens. If the result of the addition is more than the maximum value of a then a + b will give a compile-time error but a += b will be ok as shown below
|
byte a = 127;
byte b = 127; b = a + b;
// error : cannot convert from int to byte b += a; // ok |
25) Can I store a double value in a long variable without casting?
No, you cannot store a double value into a long variable without casting because the range of double is more than long and you need to typecast. It's not difficult to answer this question but many developers get it wrong due to confusion on which one is bigger between double and long in Java.
26) What will this return 3*0.1 == 0.3? true or false?
This is one of the really tricky questions. Out of 100, only 5 developers answered this question and only of them have explained the concept correctly. The short answer is false because some floating-point numbers can not be represented exactly.
27) Which one will take more memory, an int or Integer?
An Integer object will take more memory an Integer is the object and it stores metadata overhead about the object and int is primitive type so it takes less space.
28) Why is String Immutable in Java?
One of my favorite Java interview questions. The String is Immutable in java because java designer thought that string will be heavily used and making it immutable allow some optimization easy sharing the same String object between multiple clients. See the link for a more detailed answer. This is a great question for Java programmers with less experience as it gives them food for thought, to think about how things works in Java, what Jave designers might have thought when they created String class etc.
29) Can we use String in the switch case?
Yes from Java 7 onward we can use String in the switch case but it is just syntactic sugar. Internally string hash code is used for the switch. See the detailed answer for more explanation and discussion.
30) What is constructor chaining in Java?
When you call one constructor from another than it's known as constructor chaining in Java. This happens when you have multiple, overloaded constructors in the class.
JVM Internals and Garbage Collection Interview Questions In the year 2018, I have seen an increased focus on JVM internal and Garbage collection tuning, monitoring Java application, dealing with Java performance issues on various Java interviews. This actually becomes the prime topic for interviewing any experienced Java developer for a senior position e.g. technical lead, VP, or team lead. If you feel you are short of experience and knowledge in this area then you should read at least one book mentioned in my list of Java Performance books. My vote goes to Java Performance, The Definitive Guide by Scott.
31) What is the size of int in 64-bit JVM?
The size of an int variable is constant in Java, it's always 32-bit irrespective of platform. Which means the size of primitive int is the same in both 32-bit and 64-bit Java virtual machine.
32) The difference between Serial and Parallel Garbage Collector?
Even though both the serial and parallel collectors cause a stop-the-world pause during Garbage collection. The main difference between them is that a serial collector is a default copying collector that uses only one GC thread for garbage collection while a parallel collector uses multiple GC threads for garbage collection.
33) What is the size of an int variable in 32-bit and 64-bit JVM?
The size of int is the same in both 32-bit and 64-bit JVM, it's always 32 bits or 4 bytes.
34) A difference between WeakReference and SoftReference in Java?
Though both WeakReference and SoftReference helps garbage collector and memory efficiency, WeakReference becomes eligible for garbage collection as soon as the last strong reference is lost but SoftReference even though it can not prevent GC, can delay it until JVM absolutely need memory.
35) How does WeakHashMap works?
WeakHashMap works like a normal HashMap but uses WeakReference for keys, which means if the key object doesn't have any reference then both key/value mapping will become eligible for garbage collection.
36) What is -XX:+UseCompressedOops JVM option? Why use it?
When you go migrate your Java application from 32-bit to 64-bit JVM, the heap requirement suddenly increases, almost double, due to the increasing size of ordinary object pointer from 32 bit to 64 bit. This also adversely affects how much data you can keep in the CPU cache, which is much smaller than memory. Since the main motivation for moving to 64-bit JVM is to specify a large heap size, you can save some memory by using compressed OOP. By using - XX:+UseCompressedOops, JVM uses 32-bit OOP instead of 64-bit OOP.
37) How do you find if JVM is 32-bit or 64-bit from Java Program?
You can find that by checking some system properties like sun.arch.data.model or os. arch
38) What is the maximum heap size of 32-bit and 64-bit JVM?
Theoretically, the maximum heap memory you can assign to a 32-bit JVM is 2^32 which is 4GB but practically the limit is much smaller. It also varies between operating systems e.g. form 1.5GB in Windows to almost 3GB in Solaris. 64-bit JVM allows you to specify larger heap size, theoretically, 2^64 which is quite large but practically you can specify heap space up to 100GBs. There are even JVM e.g. Azul where heap space of 1000 gigs is also possible.
39) What is the difference between JRE, JDK, JVM, and JIT?
JRE stands for Java run-time and it's required to run Java application. JDK stands for Java development kit and provides tools to develop Java programs e.g. Java compiler. It also contains
JRE. The JVM stands for Java virtual machine and it's the process responsible for running Java application. The JIT stands for Just In Time compilation and helps to boost the performance of Java application by converting Java byte code into native code when the crossed certain threshold i.e. mainly hot code is converted into native code.
40) Explain Java Heap space and Garbage collection?
When a Java process is started using the java command, memory is allocated to it. Part of this memory is used to create heap space, which is used to allocate memory to objects whenever they are created in the program. Garbage collection is the process inside JVM which reclaims memory from dead objects for future allocation.
41) Can you guarantee the garbage collection process?
No, you cannot guarantee the garbage collection, though you can make a request using System.gc() or Runtime.gc() method.
42) How do you find memory usage from the Java program? How much percent of the heap is used?
You can use memory-related methods from java.lang.Runtime class to get the free memory, total memory, and maximum heap memory in Java. By using these methods, you can find out how many percents of the heap is used and how much heap space is remaining. Runtime.freeMemory() return amount of free memory in bytes, Runtime.totalMemory() returns total memory in bytes and Runtime.maxMemory() returns maximum memory in bytes.
43) What is the difference between stack and heap in Java?
Stack and heap are different memory areas in the JVM and they are used for different purposes. The stack is used to hold method frames and local variables while objects are always allocated memory from the heap. The stack is usually much smaller than heap memory and also didn't shared between multiple threads, but heap is shared among all threads in JVM.
Basic Java concepts Interview Questions
44) What's the difference between "a == b" and "a.equals(b)"?
The a = b does object reference matching if both a and b are an object and only return true if both are pointing to the same object in the heap space, on the other hand, a.equals(b) is used for logical mapping and its expected from an object to override this method to provide logically equality. For example, String class overrides this equals() method so that you can compare two Strings, which are the different object but contains same letters.
45) What is a.hashCode() used for? How is it related to a.equals(b)?
hashCode() method returns an int hash value corresponding to an object. It's used in hash-based collection classes e.g Hashtable, HashMap, LinkedHashMap and so on. It's very tightly related to equals() method. According to Java specification, two objects which are equal to each other using equals() method must have the same hash code.
46) Difference between final, finalize and finally?
The final is a modifier that you can apply to variables, methods, and classes. If you make a variable final it means its value cannot be changed once initialized. finalize is a method, which is called just before an object is a garbage collected, giving it last chance to resurrect itself, but the call to finalize is not guaranteed. finally is a keyword that is used in exception handling along with try and catch. the finally block is always executed irrespective of whether an exception is thrown from try block or not.
47) What is a compile-time constant in Java? What is the risk of using it?
public static final variables are also known as a compile-time constant, the public is optional there. They are replaced with actual values at compile time because compiler know their value up-front and also knows that it cannot be changed during run-time. One of the problem with this is that if you happened to use a public static final variable from some in-house or third party library and their value changed later than your client will still be using old value even after you deploy a new version of JARs. To avoid that, make sure you compile your program when you upgrade dependency JAR files. Java Collections Framework Interview Questions It also contains Data structure and algorithm Interview question in Java, questions on array, linked list, HashMap, ArrayList, Hashtable, Stack, Queue, PriorityQueue, LinkedHashMap and ConcurrentHashMap.
48) The difference between List, Set, Map, and Queue in Java?
The list is an ordered collection that allows duplicates. It also has an implementation that provides constant time index-based access, but that is not guaranteed by the List interface. Set is an unordered collection which
49) Difference between poll() and remove() method?
Both poll() and remove() take out the object from the Queue but if poll() fails then it returns null but if remove fails it throws Exception.
50) The difference between LinkedHashMap and PriorityQueue in Java?
PriorityQueue guarantees that the lowest or highest priority element always remains at the head of
the queue, but LinkedHashMap maintains the order in which elements are inserted. When you iterate over a PriorityQueue, the iterator doesn't guarantee any order but iterator of LinkedHashMap does guarantee the order in which elements are inserted.
51) Difference between ArrayList and LinkedList in Java?
The obvious difference between them is that ArrayList is backed by array data structure, supports random access and LinkedList is backed by linked list data structure and doesn't support random access. Accessing an element with the index is O(1) in ArrayList but its O(n) in LinkedList. See the answer for a more detailed discussion.
52) What are a couple of ways that you could sort a collection?
You can either use the Sorted collection like TreeSet or TreeMap or you can sort using the an ordered collection like a list and using Collections.sort() method.
53) How do you print Array in Java?
You can print an array by using the Arrays.toString() and Arrays.deepToString() method. Since array doesn't implement toString() by itself, just passing an array to System.out.println() will not print its contents but Arrays.toString() will print each element.
54) LinkedList in Java is doubly or singly linked list?
It's a doubly linked list, you can check the code in JDK. In Eclipse, you can use the shortcut, Ctrl + T to directly open this class in Editor.
55) Which kind of tree is used to implement TreeMap in Java?)
A Red Black tree is used to implement TreeMap in Java.
56) What is the difference between Hashtable and HashMap?
There are many differences between these two classes, some of them are following: a) Hashtable is a legacy class and present from JDK 1, HashMap was added later. b) Hashtable is synchronized and slower but HashMap is not synchronized and faster. c) Hashtable doesn't allow null keys but HashMap allows one null key. See the answer for more differences between HashMap and Hashtable in Java.
57) How HashSet works internally in Java?
HashSet is internally implemented using a HashMap. Since a Map needs key and value, a default value is used for all keys. Similar to HashMap, HashSet doesn't allow duplicate keys and only one null key, I mean you can only store one null object in HashSet.
58) Write code to remove elements from ArrayList while iterating?
The key here is to check whether the candidate uses ArrayList's remove() or Iterator's remove(). Here is the sample code which uses right way o remove elements from ArrayList while looping over and avoids ConcurrentModificationException.
59) Can I write my own container class and use it in the for-each loop?
Yes, you can write your own container class. You need to implement the Iterable interface if you want to loop over advanced for loop in Java, though. If you implement Collection then you by default get that property.
60) What is default size of ArrayList and HashMap in Java?
As of Java 7 now, default size of ArrayList is 10 and default capacity of HashMap is 16, it must be power of 2. Here is code snippet from ArrayList and HashMap class :
|
// from
ArrayList.java JDK 1.7 private static final int
DEFAULT_CAPACITY = 10; //from HashMap.java JDK 7 static final int DEFAULT_INITIAL_CAPACITY =
1 << 4; // aka 16 |
61) Is it possible for two unequal objects to have the same hashcode?
ConversionConversion EmoticonEmoticon