Different ways of sorting an User Object

There are many ways to sort a java object but it is very hard to figure out which one is more efficient. Here is an example which describes different ways of executing sorting mechanism for User object based on age.

Try to run this application in you local machine to see which method is more efficient and good to use in our regular programming life.

package com.malliktalksjava.java8;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

import static java.util.stream.Collectors.toList;

public class SortingExampleUser {

    public static void main(String[] args) {
        List<User> userList = new ArrayList<>();
        userList.add(new User("Ram", 28));
        userList.add(new User("Raj", 35));
        userList.add(new User("Rakesh", 31));
        userList.add(new User("Peter", 30));
        userList.add(new User("John", 25));
        userList.add(new User("Sri", 55));

        long starttime = System.currentTimeMillis();
        System.out.println("sortListUsingCollections : " + sortListUsingCollections(userList));
        System.out.println("Time Taken in Millis : " + (System.currentTimeMillis() - starttime));

        long starttime2 = System.currentTimeMillis();
        System.out.println("sortListUsingCollections : " + sortListUsingStreams(userList));
        System.out.println("Time Taken in Millis  2: " + (System.currentTimeMillis() - starttime2));

        long starttime3 = System.currentTimeMillis();
        System.out.println("sortListUsingCollections : " + sortUsingLambda(userList));
        System.out.println("Time Taken in Millis  2: " + (System.currentTimeMillis() - starttime3));


    //using Collections.sort
    private static List<User> sortListUsingCollections(List<User> list){

        Collections.sort(list, Comparator.comparingInt(User::getAge));
        //Collections.reverse(list); // un comment if for descending order

        return list;

    //using streams and comparator
    private static List<User> sortListUsingStreams(List<User> list){

        return list.stream()
                //.sorted(Comparator.comparingInt(User::getAge).reversed()) //-- for reverse order uncomment this line and comment above line

    //using lambda expressions
    private static List<User> sortUsingLambda(List<User> list){

        return list.stream()
                .sorted((User user1, User user2) -> user1.getAge() > user2.getAge() ? 1: 0)
                //.sorted((User user1, User user2) -> user1.getAge() < user2.getAge() ? 1: 0) - uncomment if reverse order needed


class User{
    private String name;
    private int age;

    public User(String name, int age) {
        this.name = name;
        this.age = age;

    public String getName() {
        return name;

    public void setName(String name) {
        this.name = name;

    public int getAge() {
        return age;

    public void setAge(int age) {
        this.age = age;

    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +

Selenium Automation – Open URL in multiple browsers

This example shows how to open a url in multiple browsers for browser based testing using Selenium and WebdriverManager.

package seleniumProjects;

import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.edge.EdgeDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.safari.SafariDriver;

import io.github.bonigarcia.wdm.WebDriverManager;
import io.github.bonigarcia.wdm.config.DriverManagerType;

public class StartBrowser{
	static WebDriver driver = null;
	static String[] appList= {"chrome","firefox","edgedriver", "safari"};
	public static void main(String[] args) throws Exception {
		for(int i=0;i<appList.length;i++) {

	public static void browserStart(String appName, String appUrl)
			throws InstantiationException, IllegalAccessException {

		if (appName.equals("chrome")) { //Run Chrome browser
			driver = new ChromeDriver();
		} else if (appName.equals("firefox")) { //Run in Firefox broweser
			driver = new FirefoxDriver();
		} else if (appName.equals("edgedriver")) { // Run in Edge browser
			driver = new EdgeDriver();
		} else if (appName.equals("safari")) { //Run in Safari browser
            //For Safari browser, you need enable 
			//'Allow Remote Automation' under develop menu
			DriverManagerType safari = DriverManagerType.SAFARI;
			driver = new SafariDriver();



	public static void browserClose() {

Apache FreeMarker for transformation between data formats

In this post, we will learn how to use Apache FreeMarker for data format transformations

What is Apache FreeMarker?

Apache FreeMarker is a template engine: a Java library to generate text output (HTML web pages, e-mails, configuration files, XML, JSON, source code, etc.) based on templates and changing data. Templates are written in the FreeMarker Template Language (FTL), which is a simple, specialized language (not a full-blown programming language like PHP). Usually, a general-purpose programming language (like Java) is used to prepare the data (issue database queries, do business calculations). Then, Apache FreeMarker displays that prepared data using templates. In the template you are focusing on how to present the data, and outside the template you are focusing on what data to present.

If your project needs you to transform between data formats like XML to JSON or vice versa. Such transformations can be accomplished using FreeMarker

Apache FreeMarker for Data Transformations

XML TO JSON Transformation using FreeMarker

We will use SpringBoot project created using Spring Initilizer. https://start.spring.io/

FreeMarker Transformations – Project Structure

Firstly add dependencies to pom.xml




Add XML to transform in src/main/resources folder – test.xml

<?xml version="1.0" encoding="UTF-8"?>

Add FTL Template in src/main/resources/templates folder – FTL file: xml2json.ftl

<#assign data = xml['child::node()']>
    "employee": {
        "id": ${data.employee.id},
        "name": "${data.employee.name}",
        "location": "${data.employee.location}"

Create FmtManager to load and process template as below

package com.mvtechbytes.fmt;

import java.io.IOException;
import java.io.StringWriter;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Map;

import freemarker.cache.StringTemplateLoader;
import freemarker.ext.beans.BeansWrapperBuilder;
import freemarker.template.Configuration;
import freemarker.template.Template;

public class FmtManager {

    private Configuration freemarkerConfig;
    private static final String TEMPLATE_DIRECTORY = "src/main/resources/templates/";

    public FmtManager() {
        freemarkerConfig = new Configuration(Configuration.VERSION_2_3_23);
        freemarkerConfig.setObjectWrapper(new BeansWrapperBuilder(Configuration.VERSION_2_3_23).build());
        freemarkerConfig.setTemplateLoader(new StringTemplateLoader());

    private Template loadTemplate(String templateName, String templatePath) {
        try {
            String templateContent = new String(Files.readAllBytes(Paths.get(templatePath)));
            ((StringTemplateLoader) freemarkerConfig.getTemplateLoader()).putTemplate(templateName, templateContent);
            return freemarkerConfig.getTemplate(templateName);
        } catch (IOException e) {
            throw new RuntimeException(e);

    public String processTemplate(String templateName, Map<String, Object> data) {
        Template template = loadTemplate(templateName, TEMPLATE_DIRECTORY + templateName + ".ftl");
        try (StringWriter writer = new StringWriter()) {
            template.process(data, writer);
            return writer.toString();
        } catch (Exception e) {
            throw new RuntimeException(e);

After adding all the code in the respective folders. Execution of use case can be done using below static method with FmtManager bean injected

public static void xmlToJson(FmtManager templateManager) throws Exception {

        String xmlString = new String(Files.readAllBytes(Paths.get("src/main/resources/test.xml")));
        NodeModel xmlNodeModel = NodeModel.parse(new InputSource(new StringReader(xmlString)));

        Map<String, Object> data = new HashMap<>();
        data.put("xml", xmlNodeModel);

        String json = templateManager.processTemplate("xml2json", data);


Execution Log Output:

12:48:44.926 [main] DEBUG freemarker.cache - TemplateLoader.findTemplateSource("xml2json"): Found
12:48:44.929 [main] DEBUG freemarker.cache - Loading template for "xml2json"("en_US", UTF-8, parsed) from "xml2json"
"employee": {
"id": 101,
"name": "Vikas",
"location": "Toronto"

JSONTOXML Transformation using FreeMarker

Add JSON to transform in src/main/resources folder – test.json

  "data": {
    "employee": {
      "empid": 2012,
      "empname": "Virat",
      "location": "Hyderabad"

Add FTL Template in src/main/resources/templates folder – FTL file: json2xml.ftl

<#-- @ftlvariable name="JsonUtil" type="de.consol.jbl.util.JsonUtil" -->
<#assign body = JsonUtil.jsonToMap(input)>
<?xml version="1.0" encoding="UTF-8"?>

Create FmtJSONUtil – This to convert json to Java object

package com.mvtechbytes.fmt;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

public class FmtJsonUtil {
    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();

    public static Map<String, Object> jsonToMap(String json) throws IOException {
        return OBJECT_MAPPER.readValue(json, new TypeReference<HashMap<String, Object>>(){});

After adding all the code in the respective folders. Execution of use case can be done using below static method with FmtManager bean injected

private static void jsonToXml(FmtManager templateManager) throws IOException, TemplateModelException {
		 String input = new String(Files.readAllBytes(Paths.get("src/main/resources/test.json")));

	        Map<String, Object> data = new HashMap<>();
	        data.put("input", input);

	        TemplateHashModel staticModels = new BeansWrapperBuilder(Configuration.VERSION_2_3_23).build().getStaticModels();
	        data.put("JsonUtil", staticModels.get(FmtJsonUtil.class.getName()));

	        String output = templateManager.processTemplate("json2xml", data);


Execution Log Output:

<?xml version="1.0" encoding="UTF-8"?>

Full sourcecode is available in below github link



Algorithms in Java Interviews

In this post, we will see algorithm problems with their solutions which are asked during Java interviews.

How to check if a number is Palindrome?

void checkPalindrome(int n){
  int temp, sum = 0;
  int input=n;

  while(n>0) {
     temp = n%10;
     sum = (sum*10) + temp;
     n = n/10;

  if(input == sum){
  } else {
   System.out.println("Not Palindrome");

How to check if a number is Prime in Java8?

void checkPrime(int n) {
if(n > 1 && IntStream.range(2, n).noneMatch(i -> i%n==0)) {
} else {

How to sort objects in reverse order in Java8?

Student student1 = new Student(372,"Venkat",1);
Student student2 = new Student(2,"Sachin",4);
Student student3 = new Student(2345,"Ganguly",6);
Student student4 = new Student(72,"Karthik",2);
List studlist = new CopyOnWriteArrayList();

// Iterate in Java8
studlist.forEach(s -> System.out.println(s.name));

// Sort by Ids
studlist.sort((Student s1,Student s2) -> s1.getId() - s2.getId());

// Sort by Rank in reverse Order
studlist.sort((Student s1,Student s2) -> s2.getRank() - s1.getRank());

Find second highest number in an Array?

int arr[] = {45,89, 29,1, 9, 100};
int highest = 0, secondHighest = 0;

for(int i=0; i<arr.length;i++) {   if(arr[i] > highest) {
     highest = arr[i];
  } else if(arr[i] > secondHighest) {
     secondHighest = arr[i];

Find Nth highest Salary from a SQL Table?

                               FROM EMPLOYEE ORDER BY SALARY desc);

Print Only Numerics from a String?

String sampleStr = "fdsha3430d3kdjafl0737434833";
String numericsOnlyStr = sampleStr.replaceAll("[^0-9]", "");

Print Duplicates in an Array?

for(int i=0;i<arr.length;i++) {
  for(int j=i+1; j< arr.length; j++) {
     if(arr[i] == arr[j]) {

Fetch Frequency of Elements repeated in an Array?

  Map<Integer, Integer> mp = new HashMap<>(); 
        // Iterating through array elements 
        for (int i = 0; i < n; i++) 
            if (mp.containsKey(arr[i]))  { 
                mp.put(arr[i], mp.get(arr[i]) + 1); 
            } else { 
                mp.put(arr[i], 1); 
        // Iterating through Map and Printing frequencies 
        for (Map.Entry<Integer, Integer> entry : mp.entrySet()) { 
            System.out.println(entry.getKey() + " " + entry.getValue()); 

Find Triplets in an array whose sum is equal to n?

public class Triplets {
public static List<List> findTriplets(int[] numbers, int sum) {
List<List> tripletsCombo = new ArrayList<List>();
HashSet set = new HashSet();
List triplets = new ArrayList();

if (numbers.length == 0 || sum <= 0) {
   return tripletsCombo;


for (int i = 0; i < numbers.length - 2; i++) {
int j = i + 1;
int k = numbers.length - 1;

while (j < k) {
   if (numbers[i] + numbers[j] + numbers[k] == sum) {
      String str = numbers[i] + "," + numbers[j] + "," +       numbers[k];
      // Check for the unique Triplet
      if (!set.contains(str)) {
               triplets = new ArrayList();
} else if (numbers[i] + numbers[j] + numbers[k] < sum) {    j++; } else { // numbers[i] + numbers[j] + numbers[k] > sum

return tripletsCombo;

public static void main(String[] args) {
int[] numbers = { 2, 3, 1, 5, 4 };
int sum = 9;
List<List> triplets = findTriplets(numbers, sum);

if (triplets.isEmpty()) {
   System.out.println("No triplets are found");
} else {

How to check if two strings are Anagrams?

Two strings are called Anagrams if they contain same set of characters but in different order.  Examples:  “Astronomer – Moon starer”, “A gentleman – Elegant man”, “Dormitory – Dirty Room”, “keep – peek”.

void isAnagram(String input1, String input2) {
   //Removing all white spaces from s1 and s2
   String s1_nonSpaces = input1.replaceAll("\\s", "");
   String s2_nonSpaces = input2.replaceAll("\\s", "");

   boolean status = true;
   if(s1_nonSpaces.length() != s2_nonSpaces.length()) {
      status = false;
   } else {
      char[] s1Array = s1_nonSpaces.toLowerCase().toCharArray();
      char[] s2Array = s2_nonSpaces.toLowerCase().toCharArray();
      status = Arrays.equals(s1Array, s2Array);

Swap numbers without using temp/third variable?

void swapWithoutTemp(int a, int b) {
 a = a+b;
 b = a-b;
 a = a-b;

Find number of combinations for Sum of Two Elements from two arrays is equal to N?

We have two arrays of numbers, suppose we take one element from first array and another element from second array. Their sum should be equal to N(given number).

sumOfTwoElementsInTwoArrays() {
  int arr1[] = {4,8,10,12,7};
  int arr2[] = {6,90,34,45};

  int sumValue = 44; 
  HashSet complements = new HashSet();
  int pairCount = 0;

  for(int i=0;i<arr1.length;i++) {
    complements.add(arr1[i] - sumValue);

  for(int j=0;j<arr1.length;j++) {
    if(complements.contains(arr2[j])) {

System.out.print("Number of pairs is "+pairCount);

First non repeated character in a String?

String str = "BANANA";
char firsNonRepeatedCharacter;
HashMap<Character, Integer> hmp = new HashMap<Character, Integer>();

for(int z=0;z<s.length();z++) {
  if(hmp.containsKey(str.charAt(z))) {
    hmp.put(str.charAt(z), hmp.get(str.charAt(z))+1);
  } else {
     hmp.put(str.charAt(z), 1);

Set characterSet = hmp.keySet();
for(Character c:characterSet){
  if(hmp.get(c).toString()equals("1")) {
    firsNonRepeatedCharacter = c;

Find the number of occurrence of an element in an array using Java8?

int b[] = {1,2,34,1};

List bList = Arrays.stream(b).boxed().collect(Collectors.toList());

System.out.println(bList.stream().filter(z -> z.toString().equalsIgnoreCase("1")).count());

100 doors toggle open/close

There are 100 doors in a row, all doors are initially closed. A person walks through all doors multiple times and toggle (if open then close, if close then open) them in following way:

In first walk, the person toggles every door, In second walk, the person toggles every second door, i.e., 2nd, 4th, 6th, 8th, …, In third walk, the person toggles every third door, i.e. 3rd, 6th, 9th, …

Find in nth walk, what will be the status of all doors

doorsOpenClosed(int no_of_walks) {
  int door_id, walk_id;
  int doors[] = new int[101];
  for(int i=0;i<100;i++) {
   doors[i] = 0;

for (walk_id = 1; walk_id <= no_of_walks; walk_id++) {
  for (door_id = walk_id; door_id <= 100; door_id += walk_id) {
    if(door_id%walk_id == 0) {
      doors[door_id]=(doors[door_id] == 0)?1:0;

for (int j = 0; j <= 100; j++) {
 if(doors[j] == 1) {
   System.out.println("Open Door number::::"+j);


Core Java and Java 8 Concepts

In this post, you will see some important Core Java/Java 8 concepts related to Collections, Exception Handling, Multi-threading, Concurrency etc.

Comparable Vs Comparator

Comparable Comparator
Comparable provides a single sorting sequence. In other words, Sorting of  collection is based on a single property of a class such as ID, ItemName or quantity etc. The Comparator provides multiple sorting sequences. In other words, sorting of collection can based of multiple properties such as ID, ItemName, and quantity etc.
Comparable affects the original class, i.e., the actual class is modified. Comparator doesn’t affect the original class, i.e., the actual class is not modified.
Comparable provides compareTo() method to sort elements. Comparator provides compare() method to sort elements.
Comparable is from  java.lang package. A Comparator is from java.util package.
Sorting list of Objects-Comparable type can be done using Collections.sort(List) method. Sorting list of Objects-Comparator type by Collections.sort(List, Comparator) method.

JVM Architecture

Different types of Class Loaders?

  • Bootstrap class Loader
  • Extensions class Loader
  • System class Loader

Boostrap class loader loads the classes from jdk/jre/lib/rt.jar. Extension class loader loads the classes from jdk/lib/ext folder jars. System class loader loads the classes from CLASSPATH.

Difference between ClassNotFoundException and NoClassDefFoundError

  • ClassNotFoundException is an Exception, while NoClassDefFoundError is an Error.
  • ClassNotFoundException occurs when CLASSPATH does not get updated with required JAR files while NoClassDefFoundError occurs when required class definition is not present at runtime.

Example for NoClassDefFoundError :

class Shape {
  public void draw() {
     System.out.println("Drawing Shape!");

public class DrawingApp {
  public void draw() {
     System.out.println("Drawing Shape!");
  public static void main(String[] args) {
     Shape shape = new Shape();

After compilation, Shape.class and DrawApp.class are generated, If Shape.class is deleted and DrawApp is run then NoClassDefFoundError is thrown.

Difference between ConcurrentHashMap and SynchronizedMap

  • ConcurrentHashMap is designed for concurrency and improves performance while Collections.synchronizedMap(map) which is non-synchronized by sort can be synchronized by applying a wrapper using Collections.synchronizedMap(map).
  • ConcurrentHashMap doesn’t support null keys or null values while synchronized HashMap supports one null key.
  • Locking in SynchronizedMap is at object level, so read/write operations performance is slower.
  • Locking in ConcurrentHashMap is at a much finer granularity at a hashmap bucket level.

Differences betwen equals() and hashcode() methods

equals() and hashCode() are methods present in Object class and hashCode method should not be used to check if two object references are same. Reason: hashCode just returns int value for an Object, even two different objects can have same hashCode integer. The value returned by hashCode() is the object’s hash code, which is the object’s memory address in hexadecimal. equals() checks if the two object references are same. If two objects are equal then their hashCode must be the same, but the reverse is not true.

O(1) vs O(n) vs O(log n)

These are measures of time complexity of running a piece of code.

O(1) – if execution time is constant, it requires the same amount of time regardless of the size. Example:  array – accessing any element int i = a[0];

O(n) – if execution time is directly proportional to the size.  Example: Linear search for an element has a time complexity of O(n).

O(log n) – if execution time is proportional to the logarithm of the input size. Example: Performing Binary Search on array of elements

Changes to HashMap in Java8

  • In case of Hash collision entry objects are stored as a node in a LinkedList and equals() method is used to compare keys. That comparison to find the correct key with in a linked-list is a linear operation so in a worst case scenario the complexity becomes O(n).
  • To address this issue, Java 8 hash elements use Balanced Tree instead of LinkedList after a certain threshold is reached. Which means HashMap starts with storing Entry objects in linked list but after the number of items in a hash becomes larger than a certain threshold, the hash will change from using a LinkedList to a Balanced Tree, which will improve the worst case performance from O(n) to O(log n).

Fail Fast Vs Fail Safe Iterators

Fail-Fast Iterators Fail-Safe Iterators
Fail-Fast iterators doesn’t allow  modifications of a collection while iterating over it. Fail-Safe iterators allow modifications of a collection while iterating over it.
Concurrent Modification Exception is thrown if a collection is modified while iterating over it. These iterators don’t throw any exceptions if a collection is modified while iterating over it.
They use original collection to traverse over the elements of the collection. They use copy of the original collection to traverse over the elements of the collection.
These iterators don’t require extra memory. These iterators require extra memory to clone the collection.
Ex : Iterators returned by ArrayList, Vector, HashMap. Ex : Iterator returned by CopyOnWriteArrayList, ConcurrentHashMap.

Difference between map() and flatmap() in Java8

Lets suppose we are applying map and flatmap on stream of streams. Example given below

Stream<List<Character>> stream = Stream.of({'a','b'},{'c','d'})

with map:  For input Stream of two lists {‘a’,’b’} and {‘c’,’d’}, output will be {{‘a’,’b’},{‘c’,’d’}} .Here two lists are placed inside a list, so the output will be list containing lists

With flat map: For input Stream of two lists {‘a’,’b’} and {‘c’,’d’}, output will be {{a,b,c,d}} .Here two lists are flattened and only the values are placed in list, so the output will be list containing only elements

What are Functional interfaces how we can define them?

Functional interfaces are interfaces which have only one single abstract method in it. Example:  Runnable Interface since it has only single abstract method, run().

From Java8, we can use @FunctionalInterface to define a functional interface. Although this annotation is optional, once it is used then declaring more than one abstract method will throw compile time error.

Rules of Method Overloading and Method Overriding

There are specific rules while we implement method overloading and overriding in Java with regards to increasing/decreasing visibility of methods of parent class in child class and throwing Checked Exceptions in child class. Complete rules are posted in this below link


Exception Handling flow having return statements in try/catch/finally blocks

  • Once try block encounters a return statement, the flow immediately transfers to finally block. Let say,it prints “print statement from finally”.
  • Upon the completion of finally block execution, control goes back to the return statement in the try block and returns “returning from try block”.
  • If finally block has a return statement, then the return statements from try/catch blocks will be overridden.

Exception Handling flow while exceptions thrown in catch/finally blocks

  • If the catch block completes normally, then the finally block is executed. Then there is a choice:
  • If the finally block completes normally, then the try statement completes normally. If the finally block completes abruptly for any reason, then the try statement completes abruptly for the same reason.
  • If the catch block completes abruptly for reason R, then the finally block is executed. Then there is a choice:
    If the finally block completes normally, then the try statement completes abruptly for reason R.
    If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S (and reason R is discarded).

FixedThreadPool vs CachedThreadPool vs ScheduledThreadPool

  • newCachedThreadPool(): creates an expandable thread pool executor. New threads are created as needed, and previously constructed threads are reused when they are available. Idle threads are kept in the pool for one minute. This executor is suitable for applications that launch many short-lived concurrent tasks.
  • newFixedThreadPool(int n): creates an executor with a fixed number of threads in the pool. This executor ensures that there are no more than n concurrent threads at any time. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread becomes available. If any thread terminates due to failure during execution, it will be replaced by a new one. The threads in the pool will exist until it is explicitly shutdown. Use this executor if you and to limit the maximum number of concurrent threads.
  • newScheduledThreadPool(int corePoolSize): creates an executor that can schedule tasks to execute after a given delay, or to execute periodically. Consider using this executor if you want to schedule tasks to execute concurrently.

What is ThreadLocal?

ThreadLocal class provides thread-local variables. It enables you to create variables that can only be read and write by the same thread. If two threads are executing the same code and that code has a reference to a ThreadLocal variable then the two threads can’t see the local variables of each other.

Diffence Volatile vs AtomicInteger?

volatile keyword is used on variables to solve the visibility problem in multi-threaded environment.  AtomicInteger is used if we perform compound operations(incrementing(i++) decrementing(i–)) on variables.

volatile is used on boolean flags, AtomicInteger is used for counters.


Differences between yield, join, & sleep

yield() method pauses the currently executing thread temporarily for giving a chance to the remaining waiting threads of the same priority to execute. If there is no waiting thread or all the waiting threads have a lower priority then the same thread will continue its execution. The yielded thread when it will get the chance for execution is decided by the thread scheduler whose behavior is vendor dependent.

join() If any executing thread t1 calls join() on t2 i.e; t2.join() immediately t1 will enter into waiting state until t2 completes its execution.

sleep() Based on our requirement we can make a thread to be in sleeping state for a specified period of time

Differences between Runnable and Callable

  • Runnable object does not return a result whereas a Callable object returns a result.
  • Runnable object cannot throw a checked exception wheras a Callable object can throw an exception.
  • The Runnable interface has been around since Java 1.0 whereas Callable was only introduced in Java 1.5.
class ThreadA implements Runnable {
public void run() { }

public class ThreadB implements Callable<String> {
public String call() throws Exception {
return "Thread B ran Successfully";

What is Semaphore in concurrency?

Semaphore is used to restrict the entry to a service to a fixed number of threads at a given time. This is generally used on slow services to make it available for fixed number of requests.

Semaphore semaphore = new Semphore(no_of_permits);

In run() method of a thread, we can use semaphore.acquire() before accessing the slow service and semaphore.release() after to ensure fixed number (defined as no_of_permits) of threads are eligible to access it.

Difference between CyclicBarrier and CountDownLatch?

Both CyclicBarrier and CountDownLatch are used in Multi threading scenario where one Thread waits for one or more Thread to complete their job before it continues processing but main difference between two is that, you can not reuse same CountDownLatch instance once count reaches to zero and latch is open, on the other hand, CyclicBarrier can be reused by resetting Barrier, Once barrier is broken.

  • Initialization of countdownlatch is CountDownLatch latch = new CountDownLatch(4);
  • Method used to countdown (generally used inside run method of thread at a specific point) is latch.countDown()
  • Method used to await a specific thread till countdown number completes is latch.await()
  • Phaser can be used either to perform functionality of both CyclicBarrier and CountDownLatch


Rules of method overloading and overriding

In this post we will see the rules which needs to adhered while implementing method overriding and overloading with regards to increasing/decreasing visibility of methods of parent class in child class and throwing Checked Exceptions in child class.

Method Overriding rules

For terminology, original method is known as overridden method and new method is known as overriding method. Below rules must be followed to override a methods in Java :

  • Overriding method cannot throw checked exception which is higher in hierarchy than the checked Exception thrown by overridden method. For example if overridden method throws IOException which is checked Exception, than overriding method can not throw java.lang.Exception because it comes higher in type hierarchy.
    "Exception 'Exception' is not compatible with throws clause in" 
    **** Overriding method can have Runtime Exceptions declared even if Overridden method does not throw any type of Exceptions.
  • Overriding method can not reduce access of overridden method. It means if overridden method is defined as public than overriding method can not be protected or package private. Similarly if original method is protected then overriding method cannot be package-private. You can see what happens if you violate this rule in Java,
     "You cannot reduce visibility of inherited method of a class".
  • Overriding method can increase access of overridden method. This is opposite of earlier rule.
    ****According to this if overridden method is declared as protected then overriding method can be protected or public
  • private, static, final methods can not be overridden.
    "Cannot override the final method from Parent"
  • Return type of overriding method must be same as overridden method. Changing return type of method in child class will throw compile time error
    "return type is incompatible with parent class method"

Method Overloading rules

Here is the list of rules which must be followed to overload a method:

  • First rule to overload a method is to change method signature. method signature is made of number of arguments, type of arguments and order of arguments if they are of different types.  One can change any of these or combinations of them to overload a method in Java.
  • Return type of method is not part of method signature, hence changing the return type alone will not overload a method in Java.  In fact, it will result in compile time error.

String.join() Example – Java 8

Java 8 has String.join() method where first parameter is separator and then you can pass either multiple strings or some instance of Iterable having instances of strings as second parameter. Here is the sample program:

package in.mallikatalksjava.java8;
import java.time.ZoneId;

public class StringJoinDemo {
   public static void main(String[] args) {
	String joined = String.join("", "mallik", "talks", "java",".in");
	String directory = String.join("/", "C:", "java", "programs");

	String ids = String.join(", ", ZoneId.getAvailableZoneIds());

Asia/Aden, America/Cuiaba, Etc/GMT+9, Etc/GMT+8, Africa/Nairobi, America/Marigot, Asia/Aqtau ....etc.

How to convert a JKS Keystore to a PKCS12 (.p12) format

To convert a JKS (.jks) keystore to a PKCS12 (.p12) keystore, run the following command:

Note: This command is supported on JDK / JRE keytool versions 1.6 and greater.

keytool -importkeystore -srckeystore <jks_file_name.jks> -destkeystore <pk12_file_name.p12> -srcstoretype JKS -deststoretype PKCS12 -deststorepass <password>

To verify the content of .p12 (e.g. pk12_file_name.p12), run the following command:
keytool -list -v -keystore <“pk12_file_name.p12”> -storetype <password>

Create Web service using wsgen in command line

Generally bottom up approach, where the service implementation is done first and wsdl will be generated next, is not a suggested way of implementing a web service. In some cases where the service implementation class is already exists in the application and wants to expose it as web service then it is acceptable to go with bottom up approach.

In this post, I would like to show how the web service can be developed in bottom up approach using the wsgen command.

The wsgen tool reads an existing web service implementation class and generates the required JAX–WS portable artifacts for web service development and deployment. The wsgen tool can be used for bottoms-up approach, where you are starting from a service endpoint implementation rather than a wsdl. The generated JAX-WS artifacts can be used by both service and client implementations.

Below is the sample service class created – HellowWorldService.java

package in.malliktalksjava.service;

import javax.jws.WebMethod;
import javax.jws.WebService;

public class HellowWorldService{

public void printHelloMessage(){
System.out.println(“Printing hello message”);

public String getThankyouMessage(){
System.out.println(“Printing thank you message”);

return null;


Compile the java classes using below command:

javac -d . in/malliktalksjava/service/*.java

Once the compilation is successful, use the below wsgen command to generate the artifacts:

wsgen -verbose -keep -cp . in.malliktalksjava.service.HellowWorldService

Once click on the enter button, below are the artifacts generated in jaxws folder:


If it is required to generate the wsdl file along with the artifacts, we need to use the below command:

wsgen -verbose -keep -cp . -wsdl in.malliktalksjava.service.HellowWorldService

Below are the list of options available in wsgen command:



Differences between wsimport and wsgen


  • The wsimport tool reads a WSDL and generates all the required artifacts for web service development, deployment, and invocation.
  • It supports the top-down approach to developing JAX-WS Web services, where you are starting from a wsdl.
  • wsimport tool generated JAX-WS portable artifacts include Service Endpoint Interface (SEI), Service, Exception class mapped from wsdl:fault (if any), JAXB generated value types (mapped java classes from schema types) etc. These artifacts can be packaged in a WAR file with the WSDL and schema documents along with the endpoint implementation to be deployed.

Ex Command :

wsimport http://localhost:8090/MyJaxWSService?wsdl

The wsgen tool reads an existing web service implementation class and generates the required JAX–WS portable artifacts for web service development and deployment. The wsgen tool can be used for bottoms-up approach, where you are starting from a service endpoint implementation rather than a wsdl.

Ex Command :

wsgen -verbose -keep -cp . in.malliktalksjava.service.HellowWorldService
  • wsgen and wsimport generate request and response wrapper bean classes and the JAXB classes. However, wsgen generates the JAXB classes and put them in a jaxws folder and wsimport does not put those classes in any folder instead they will be placed in the current directory.
  • JAX-WS artifacts generated can be used by both service implementation and client implementation.
  • Using wsgen you can also generate the wsdl based on webservice implementation class.