2 Ways to find duplicate elements in an Array - Java

Problem: You have given an array of objects, which could be an array of integers and or array of Strings or any object which implements the Comparable interface. How would you find duplicate elements from an array? Can you solve this problem in O(n) complexity? This is actually one of the frequently asked coding problems from Java interviews. There are multiple ways to solve this problem and you will learn two popular ways here, first the brute force way, which involves comparing each element with every other element and other which uses a hash table like data structure to reduce the time complexity of problem from quadratic to linear, of course by trading off some space complexity. This also shows that how by using a suitable data structure you can come up with a better algorithm to solve a problem. If you are preparing for programming job interviews, then I also suggest you take a look at Cracking the Coding Interview book, which contains 150 programming questions and solutions, good enough to do well on any programming job interviews e.g. Java, C++, Python or Ruby.

How to Sort List into Ascending and Descending Order in Java

ArrayList, Set Sorting in Ascending – Descending Order Java
Sorting List, Set and ArrayList in Java on ascending and descending order is very easy, You just need to know correct API method to do that. Collections.sort()  method will sort the collection passed to it,  doesn't return anything just sort the collection itself.  Sort() method of Collections class in Java is overloaded where another version takes a Comparator and sort all the elements of Collection on order defined by Comparator.If we  don't pass any Comparator than object will be sorted based upon there natural order like String will be sorted alphabetically or lexicographically. Integer will be sorted numerically etc. Default sorting order for an object is ascending order like Integer will be sorted  from low to high while descending order is just opposite. Collections.reverseOrder() returns a Comparator which will be used for sorting Object in descending order.

Top 10 Tricky Java interview questions and Answers

What is a tricky question? Well, tricky Java interview questions are those questions which have some surprise element on it. If you try to answer a tricky question with common sense, you will most likely fail because they require some specific knowledge. Most of the tricky Java questions comes from confusing concepts like function overloading and overriding, Multi-threading which is really tricky to master, character encoding, checked vs unchecked exceptions and subtle Java programming details like Integer overflow. Most important thing to answer a tricky Java question is attitude and analytical thinking, which helps even if you don't know the answer. Anyway in this Java article we will see 10 Java questions which are real tricky and requires more than average knowledge of Java programming language to answer them correctly. As per my experience, there is always one or two tricky or tough Java interview question on any core Java or J2EE interviews, so it's good to prepare tricky questions from Java in advance.

If I take an interview, I purposefully put this kind of question to gauge the depth of candidate's understanding in Java. Another advantage of asking such question is the surprising element, which is a key factor to put the candidate on some pressure during interviews.

Since these questions are less common, there is good chance that many Java developer doesn't know about it.  You won't find these questions even on popular Java interview books like Java Programming Interview exposed, which is nevertheless an excellent guide for Java interviews.

Btw, if you don't find these question tricky enough, then you should check Joshua Bloch's another classic book, Java Puzzlers for super tricky questions. I am sure you will find them challenging enough.

10 Tricky Java interview question - Answered

Here is my list of 10 tricky Java interview questions, Though I have prepared and shared lot of difficult core Java interview question and answers, But I have chosen them as Top 10 tricky questions because you can not guess answers of this tricky Java questions easily, you need some subtle details of Java programming language to answer these questions.

Question: What does the following Java program print?
public class Test {
    public static void main(String[] args) {
        System.out.println(Math.min(Double.MIN_VALUE, 0.0d));
Answer: This question is tricky because unlike the Integer, where MIN_VALUE is negative, both the MAX_VALUE and MIN_VALUE of the Double class are positive numbers. The Double.MIN_VALUE is 2^(-1074), a double constant whose magnitude is the least among all double values. So unlike the obvious answer, this program will print 0.0 because Double.MIN_VALUE is greater than 0. I have asked this question to Java developer having experience up to 3 to 5 years and surprisingly almost 70% candidate got it wrong.

What will happen if you put return statement or System.exit () on try or catch block? Will finally block execute?
This is a very popular tricky Java question and it's tricky because many programmers think that no matter what, but the finally block will always execute. This question challenge that concept by putting a return statement in the try or catch block or calling System.exit() from try or catch block. Answer of this tricky question in Java is that finally block will execute even if you put a return statement in the try block or catch block but finally block won't run if you call System.exit() from try or catch block.

Question: Can you override a private or static method in Java?
Another popular Java tricky question, As I said method overriding is a good topic to ask trick questions in Java. Anyway, you can not override a private or static method in Java, if you create a similar method with same return type and same method arguments in child class then it will hide the superclass method, this is known as method hiding.

Similarly, you cannot override a private method in sub class because it's not accessible there, what you do is create another private method with the same name in the child class. See Can you override a private method in Java or more details.

Question: What do the expression 1.0 / 0.0 will return? will it throw Exception? any compile time error?
Answer: This is another tricky question from Double class. Though Java developer knows about the double primitive type and Double class, while doing floating point arithmetic they don't pay enough attention to Double.INFINITY, NaN, and -0.0 and other rules that govern the arithmetic calculations involving them. The simple answer to this question is that it will not throw ArithmeticExcpetion and return Double.INFINITY.

Also, note that the comparison x == Double.NaN always evaluates to false, even if x itself is a NaN. To test if x is a NaN, one should use the method call Double.isNaN(x) to check if given number is NaN or not. If you know SQL, this is very close to NULL there.

Btw, If you are running out of time for your interview preparation, you can also check out Java Programming Interviews exposed for more of such popular questions,

tricky core Java interview questions and answers

Does Java support multiple inheritances?
This is the trickiest question in Java if C++ can support direct multiple inheritances than why not Java is the argument Interviewer often give. Answer of this question is much more subtle then it looks like, because Java does support multiple inheritances of Type by allowing an interface to extend other interfaces, what Java doesn't support is multiple inheritances of implementation. This distinction also gets blur because of default method of Java 8, which now provides Java, multiple inheritances of behavior as well. See why multiple inheritances are not supported in Java to answer this tricky Java question.

What will happen if we put a key object in a HashMap which is already there?
This tricky Java question is part of another frequently asked question, How HashMap works in Java. HashMap is also a popular topic to create confusing and tricky question in Java. Answer of this question is if you put the same key again then it will replace the old mapping because HashMap doesn't allow duplicate keys. The Same key will result in the same hashcode and will end up at the same position in the bucket.

 Each bucket contains a linked list of Map.Entry object, which contains both Key and Value. Now Java will take the Key object from each entry and compare with this new key using equals() method, if that return true then value object in that entry will be replaced by new value. See How HashMap works in Java for more tricky Java questions from HashMap.

Question: What does the following Java program print?
public class Test {
    public static void main(String[] args) throws Exception {
        char[] chars = new char[] {'\u0097'};
        String str = new String(chars);
        byte[] bytes = str.getBytes();

Answer: The trickiness of this question lies on character encoding and how String to byte array conversion works. In this program, we are first creating a String from a character array, which just has one character '\u0097', after that we are getting the byte array from that String and printing that byte. Since \u0097 is within the 8-bit range of byte primitive type, it is reasonable to guess that the str.getBytes() call will return a byte array that contains one element with a value of -105 ((byte) 0x97).

However, that's not what the program prints and that's why this question is tricky. As a matter of fact, the output of the program is operating system and locale dependent. On a Windows XP with the US locale, the above program prints [63], if you run this program on Linux or Solaris, you will get different values.

To answer this question correctly, you need to know about how Unicode characters are represented in Java char values and in Java strings, and what role character encoding plays in String.getBytes().

In simple word, to convert a string to a byte array, Java iterate through all the characters that the string represents and turn each one into a number of bytes and finally put the bytes together. The rule that maps each Unicode character into a byte array is called a character encoding. So It's possible that if same character encoding is not used during both encoding and decoding then retrieved value may not be correct. When we call str.getBytes() without specifying a character encoding scheme, the JVM uses the default character encoding of the platform to do the job.

The default encoding scheme is operating system and locale dependent. On Linux, it is UTF-8 and on Windows with a US locale, the default encoding is Cp1252. This explains the output we get from running this program on Windows machines with a US locale. No matter which character encoding scheme is used, Java will always translate Unicode characters not recognized by the encoding to 63, which represents the character U+003F (the question mark, ?) in all encodings.

If a method throws NullPointerException in the superclass, can we override it with a method which throws RuntimeException?
One more tricky Java questions from the overloading and overriding concept. The answer is you can very well throw superclass of RuntimeException in overridden method, but you can not do same if its checked Exception. See Rules of method overriding in Java for more details.

What is the issue with following implementation of compareTo() method in Java
public int compareTo(Object o){
   Employee emp = (Employee) o;
   return this.id - e.id;
where an id is an integer number.
Well, three is nothing wrong in this Java question until you guarantee that id is always positive. This Java question becomes tricky when you can't guarantee that id is positive or negative. the tricky part is, If id becomes negative than subtraction may overflow and produce an incorrect result. See How to override compareTo method in Java for the complete answer of this Java tricky question for an experienced programmer.

How do you ensure that N thread can access N resources without deadlock?
If you are not well versed in writing multi-threading code then this is a real tricky question for you. This Java question can be tricky even for the experienced and senior programmer, who are not really exposed to deadlock and race conditions. The key point here is ordering, if you acquire resources in a particular order and release resources in the reverse order you can prevent deadlock. See how to avoid deadlock in Java for a sample code example.

Question: Consider the following Java code snippet, which is initializing two variables and both are not volatile, and two threads T1 and T2 are modifying these values as following, both are not synchronized
int x = 0;
boolean bExit = false;

Thread 1 (not synchronized)
x = 1; 
bExit = true;

Thread 2 (not synchronized)
if (bExit == true) 
System.out.println("x=" + x);
Now tell us, is it possible for Thread 2 to print “x=0”?

Answer: It's impossible for a list of tricky Java questions to not contain anything from multi-threading. This is the simplest one I can get. Answer of this question is Yes, It's possible that thread T2 may print x=0.Why? because without any instruction to compiler e.g. synchronized or volatile, bExit=true might come before x=1 in compiler reordering. Also, x=1 might not become visible in Thread 2, so Thread 2 will load x=0. Now, how do you fix it?

 When I asked this question to a couple of programmers they answer differently, one suggests to make both threads synchronized on a common mutex, another one said make both variable volatile. Both are correct, as it will prevent reordering and guarantee visibility.

But the best answer is you just need to make bExit as volatile, then Thread 2 can only print “x=1”. x does not need to be volatile because x cannot be reordered to come after bExit=true when bExit is volatile.

What is difference between CyclicBarrier and CountDownLatch in Java
Relatively newer Java tricky question, only been introduced from Java 5. The main difference between both of them is that you can reuse CyclicBarrier even if Barrier is broken, but you can not reuse CountDownLatch in Java. See CyclicBarrier vs CountDownLatch in Java for more differences.

What is the difference between StringBuffer and StringBuilder in Java?
Classic Java questions which some people think tricky and some consider very easy. StringBuilder in Java was introduced in JDK 1.5 and the only difference between both of them is that StringBuffer methods e.g. length(), capacity() or append() are synchronized while corresponding methods in StringBuilder are not synchronized.

Because of this fundamental difference, concatenation of String using StringBuilder is faster than StringBuffer. Actually, it's considered the bad practice to use StringBuffer anymore, because, in almost 99% scenario, you perform string concatenation on the same thread. See StringBuilder vs StringBuffer for more differences.

Can you access a non-static variable in the static context?
Another tricky Java question from Java fundamentals. No, you can not access a non-static variable from the static context in Java. If you try, it will give compile time error. This is actually a common problem beginner in Java face when they try to access instance variable inside the main method. Because main is static in Java, and instance variables are non-static, you can not access instance variable inside main. See, why you can not access a non-static variable from static method to learn more about this tricky Java questions.

How many String objects are created by the following code?
Tricky Core Java questions with answers

Now, it's practice time, here are some questions for you guys to answer, these are contributed by readers of this blog, big thanks to them.
  1. When doesn't Singleton remain Singleton in Java?
  2. is it possible to load a class by two ClassLoader?
  3. is it possible for equals() to return false, even if contents of two Objects are same?
  4. Why compareTo() should be consistent to equals() method in Java?
  5. When do Double and BigDecimal give different answers for equals() and compareTo() == 0. 
  6. How does "has before" apply to volatile work?
  7. Why is 0.1 * 3 != 0.3,
  8. Why is (Integer) 1 == (Integer) 1 but (Integer) 222 != (Integer) 222 and which command arguments change this.
  9. What happens when an exception is thrown by a Thread?
  10. Difference between notify() and notifyAll() call?
  11. Difference between System.exit() and System.halt() method?
  12. Does following code legal in Java? is it an example of method overloading or overriding?
  13. public String getDescription(Object obj){
       return obj.toString;
    public String getDescription(String obj){
       return obj;
    public void getDescription(String obj){
       return obj;

This was my list of Some of the most common tricky questions in Java. It's not a bad idea to prepare tricky Java question before appearing for any core Java or J2EE interview. One or two open-ended or tricky question is quite common in Java interviews.

Further Reading
If you are looking for super challenging trick coding questions then you should check out Joshua Bloch another classic book, the Java Puzzlers, I am sure you ill find them really challenging to solve, I certainly did.

tricky core Java interview questions

Hungry for more Java Interview Question and Answer post, check out these articles

How to Print Pyramid Pattern in Java? Program Example

Pattern based exercises are a good way to learn nested loops in Java. There are many pattern based exercises and one of them is printing Pyramid structure as shown below:

* * 
* * * 
* * * * 
* * * * * 

You need to write a Java program to print above pyramid pattern. How many levels the pyramid triangle would have will be decided by the user input. You can print this kind of pattern by using print() and println() method from System.out object. System.out.print() just prints the String or character you passed to it, without adding a new line, useful to print stars in the same line. While, System.out.println() print characters followed by a newline character, which is useful to move to next line. You can also use Scanner class to get input from the user and draw pyramid up to that level only. For example in above diagram, the pyramid has 5 levels.

Java ArrayList Tutorial - The MEGA List

I have written several ArrayList tutorials, touching different ArrayList concepts and many how to do examples with ArrayList. In this tutorial, I am giving a summary of each of them. Why? So that any Java beginner who wants to learn ArrayList in detail, can go through the relevant tutorial and learn. It's also on request of many of my readers, who asked in past to share all the relevant tutorials in one place. Why should you learn ArrayList? Because it's the most important Collection class in Java. You will often find yourself using ArrayList and HashMap in tandem. It's your dynamic array which can resize itself as it grows. In another word, ArrayList is as much important as an array. When I started learning Java, my quest to ArrayList starts as a dynamic array, because there were many scenarios where we don't know the size of the array in advance. We end up either allocating less space or more space, both are not ideal. Btw, you should also check out Head First Java 2nd Edition if you are newbie and Effective Java 2nd Edition, if you know Java but wants to become a Java expert.

How to solve FizzBuzz in Java?

FizzBuzz is one of the most frequently asked questions on programming interviews and used to filter programmers who can't program. It looks extremely simple but it's tricky for those programmers or coder who struggle to structure their code. Fizzbuzz problem statement is very simple, write a program which return "fizz" if the number is a multiplier of 3, return "buzz" if its multiplier of 5 and return "fizzbuzz" if the number is divisible by both 3 and 5. If the number is not divisible by either 3 or 5 then it should just return the number itself. You can see nothing is fancy when it comes to thinking about the solution, but when you start coding, you will see a problem with structuring your code, particularly if else blocks.

How to convert float to int in Java? Examples

Even though both float and int are 32-bit wide data type, float has the higher range than integer primitive value. Since a float is a bigger than int, you can convert a float to an int by simply down-casting it e.g. (int) 4.0f will give you integer 4. By the way, you must remember that type casting just get rid of anything after the decimal point, they don't perform any rounding or flooring operation on the value. So if your float value is 3.999, down casting to an integer will produce 3, not 4. If you need rounding then consider using Math.round() method, which converts float to its nearest integer by adding +0.5 and then truncating it. Math.random() is overloaded for both float and double, so you can use this for converting double to long as well. Let's see an example of converting a float value to int in Java.

Top 5 FREE JavaScript Books - Download PDF or Read Online

The internet is full of good things and one of them is free eBooks and PDF. Programmers can almost run anything by these free books. Earlier I have shared some of the freely available Java Programming books and PDF and in this article, I am going to share 5 good books to learn JavaScript which are free to read online or allow you to download PDF for offline reading. JavaScript is probably another language I recommend someone who knows Java, Why? because it's really ubiquitous. There is almost no website which is not using JavaScript in one or other format. Some use JavaScript for client-side scripting and animation, while other use JavaScript on the server side with libraries like node.js, it forms the unique triangle with HTML and CSS.

How to declare ArrayList with values in Java? Examples

Sometimes you want to create an ArrayList with values, just like you initialize t at the time of declaration, as shown below:

int[] primes = {2, 3, 5, 7, 11, 13, 17};
String[] names = {"john", "Johnny", "Tom", "Harry"};

but unfortunately, ArrayList doesn't support such kind of declaration in Java. But don't worry, there is a workaround  to declare an ArrayList with values e.g. String, integers, floats or doubles by using Arrays.asList() method, which is nothing but a shortcut to convert an Array to ArrayList.

Java - How to convert from Integer to String?

This is the second part of data conversion tutorial, in the first part you have learned how to convert String to Integer and in this article, you will learn the opposite i.e. convert from Integer to String. Actually, you can apply all the tricks, which I had told you before about converting long to String, and autoboxing will take care of converting int to Integer in Java. But, if you care for performance and believe in not using auto-boxing when not needed then there are still a couple of ways which directly converts an Integer object to String e.g. Integer.toString() method, which returns a String object and doesn't have any auto-boxing overhead. Let's see a couple of more ways to convert an Integer to String in Java. Btw, if you are new to Java and just started learning, I suggest you to buy the Head First Java, 2nd Edition, one of the best books to learn Java.

Error: could not open 'C:\Java\jre8\lib\amd64\jvm.cfg'

A couple of weeks back I updated my laptop to Windows 10 but after trying for one day, I reverted back to Windows 8.1. Everything was alright until I open Eclipse, which was throwing "Error: could not open 'C:\Program Files\Java\jre8\lib\amd64\jvm.cfg', as soon as I launch it. It was quite bizarre because everything was fine earlier. I suspect Java installation problem, so I went to command prompt and typed Java, only to find the same error there as well. You can see below, I am just trying to run the "java" command from MS-DOS window. Here "java" command is picked from PATH environment variable.

Avoid ConcurrentModificationException while looping over Java ArrayList?

Apart from the NullPointerException and ClassNotFoundException, ConcurrentModificationException is another nightmare for Java developers. What makes this error tricky is the word concurrent, which always mislead Java programmers that this exception is coming because multiple threads are trying to modify the collection at the same time. Then begins the hunting, they spent countless hours to find the code which has the probability of concurrent modification. While in reality ConcurrentModficationException can also come on the single threaded environment. To give you an example, just loop over a list using for loop and try to remove one element, you will get the ConcurrentModificatoinExcetpion? Why? because you broke the rule of not modifying a Collection during iteration.

How to find highest repeating word from a text File in Java - Word Count Problem

How to find the word and their count from a text file is another frequently asked coding question from Java interviews. The logic to solve this problem is similar to what we have seen in how to find duplicate words in a String. In the first step you need to build a word Map by reading contents of a text File. This Map should contain word as a key and their count as value. Once you have this Map ready, you can simply sort the Map based upon values. If you don't know how to sort a Map on values, see this tutorial first. It will teach you by sorting HashMap on values. Now getting key and value in sorted should be easy, but remember HashMap doesn't maintain order, so you need to use a List to keep the entry in sorted order. Once you got this list, you can simply loop over the list and print each key and value from the entry. This way, you can also create a table of words and their count in decreasing order.  This problem is sometimes also asked as to print all word and their count in tabular format.