Tuesday, 1 May 2018

Immutable Class Example

package com.help4j.core;

import java.util.Date;

/**
 * Immutable class should mark as final so that it can not be extended by another 
class. 
 * Fields should mark as private so direct access is not allowed
 * Fields should mark as final so that value can not be modified once initialized
 **/

public final class MyImmutable {

    /**
     * String class is immutable
     **/

    private final String name;
    
    /**
     * Integer class is immutable
     **/

    private final Integer weight;
    
    /**
     * Date class is mutable
     **/

    private final Date dateOfBirth;
    
    /**
     * Default private constructor will ensure no unplanned construction of class
     * All the final fields are initialized through constructor
     */

    private MyImmutable(String name, Integer weight, Date dateOfBirth){
        this.name = name;
        this.weight = weight;
        this.dateOfBirth = new Date(dateOfBirth.getTime());
    }
    
    /**********************************************
     ***********PROVIDE NO SETTER METHODS *********
     **********************************************/
    
    /**
     * String class is immutable so we can return the instance variable as it is
     **/

    public String getName() {
        return name;
    }
    
    /**
     * Integer class is immutable so we can return the instance variable as it is
     **/

    public Integer getWeight() {
        return weight;
    }
    
    /**
     * Date class is mutable so we need a little care here.
     * We should not return the reference of original instance variable.
     * Instead a new Date object, with content copied to it, should be returned.
     **/

    public Date getDateOfBirth() {
        return new Date(dateOfBirth.getTime());
    }
}

Monday, 1 January 2018

Top Java Interview Questions 2018 for experienced candidates

I have prepared this list from my personal experience of recent interviews. Topics are in the order of recent interview trend. Please follow this post on regular basis as i keep updating it.


Singleton Pattern


This is one of the most commonly asked question in interviews. Interviewer starts this question asking about design patterns. Candidate generally answers singleton and factory patterns. Then interviewer asks to write a singleton class. You should learn how to create a singleton class with private constructor, static getInstance method, double checked locking. You should also learn enum based singleton class. Follow up questions in singleton pattern are as follows:
  • How to make singleton class thread safe?
  • How to prevent deserialization to create new object of singleton class? 
  • How to prevent cloning and reflexion to create new object of singleton class?
  • You can find the singleton pattern implementation here.

HashMap


This is also most commonly asked question in collections. Interviewer start this question asking about collections and what are the data structure/collection you have used in development. Candidate generally answers HashMap, LinkedList, ArrayList, HashSet.  Then interviewer check your knowledge of equals, hashcode and hashing algorithm. You should be aware of HashMap class internal implementation. Follow up questions in HashMap are as follows: 
  • How to use HashMap in multi threading environment ? You should know that HashTable is thread safe. You can make HashMap thread safe by using Collections.synchronizedMap(Map)  
  • What is concurrent hashmap ? How concurrent hashmap is better then thread safe hashmap in multi threading environment ?

LinkedList


LinkedList is something where interviewer can judge your knowledge of collections and algorithm at the same time. 

Collections: What is the difference in ArrayList and LinkedList ? 

Algorithm: How to find middle element of Linked List? How to find a loop in LinkedList ? If you answer it then follow up question may asked:
How to find the starting point of loop in LinkedList ?
How to find the length of the loop in LinkedList ? 

MultiThreading


Interviewer can ask you any of following questions:

1. How to run 5 threads sequentially.

2. Print number 1 to 10 using two threads where thread 1 prints even number and thread 2 prints odd number.
3. Producer consumer implementation using wait notify.
4. What is deadlock ? How to identify deadlock in java application ? How to prevent deadlock situations in application development ?
5. CountDownLatch vs CyclicBarrier ?
6. Future Object, ThreadLocal etc.
7. How to name a thread in executor service ?
8. How to interrupt a thread explicitly ?

String


1. What is immutable object ? How can you write Immutable Class (See example) ? If you know the internal implementation of String class and know how String is immutable, you can answer it easily. Here are few points to remember:-

  • Declare the class as final so it can’t be extended.
  • Initialize all the fields via a constructor. Make all fields private so that direct access is not allowed. Make all fields final so that they’re initialized only once inside the constructor and never modified afterward.
  • Don’t provide setter methods.
  • When exposing methods which return mutable object then you must always return a new object with copied content of mutable object. Immutable variables can be returned safely without extra effort.

2. Difference between String object created using new and literal.
3. Difference between StringBuilder and StringBuffer.
4. Why character array is used over String to store password ? 

Java Theory


1. S.O.L.I.D. (5 class design principle) in Java ?

Single Responsibility Principle - One class should have one and only one responsibility
Open Close Principle - Software components should be open for extension, but closed for modification
Liskov Substitution - Derived types must be completely substitutable for their base types
Interface Segregation - Clients should not be forced to implement unnecessary methods which they will not use
Dependency Inversion - Depend on abstractions, not on concretions

2. Anti patterns in Java. What is god class ?
3. How classes are related to each other through association, aggregation and composition.

Association (bidirectional one to one, one to many, many to one or many to many association, represented by line with arrow in UML) for e.g. Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle.
Aggregation (Has-a relationship, unidirectional association, parent and child can survive individually, represented by line with diamond in UML) for e.g. Car and Wheel. Car can have multiple wheels but wheel can not belong to multiple cars and if we delete the car, wheel need not to be destroyed and used in another car.
Composition (Part-of relationship, unidirectional association, child can not survive without parent, represented by line with filled diamond in UML) for e.g. House and Rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete.

4. Give example of 5-6 design patterns being used in Java classes. 

Factory: java.util.Calendar#getInstance()

Abstract Factory: javax.xml.parsers.DocumentBuilderFactory#newInstance(), javax.xml.transform.TransformerFactory#newInstance(), javax.xml.xpath.XPathFactory#newInstance()
Builder: java.lang.StringBuilder#append(), java.lang.StringBuffer#append()
Decorator: All subclasses of java.io.InputStream, OutputStream, Reader and Writer.
Chain of responsibility: javax.servlet.Filter#doFilter()
Iterator: java.util.Iterator
Observer: JMS message listener
Singleton: java.lang.Runtime#getRuntime()
Adapter: java.util.Arrays#asList(), java.io.InputStreamReader(InputStream) (returns a Reader), java.io.OutputStreamWriter(OutputStream) (returns a Writer)

Java Basics


1. Explain each keyword in main method i.e. public static void main(String[] args).

2. Can you override a static method and private method ?


Java Advance


1. How many ways you can create an object in java ?
        Answer: New Keyword, Cloning, Deserialization and Reflection.
2. Explain Java Memory Model with Heap structure. Explain here three parts of heap i.e. Young, Old and Permanent Generation. Also Explain Minor and Major GC
3. How to do JVM performance tuning ? Explain here parameters 
             a) Heap Memory: -Xms, -Xmx, -Xmn,
             b) Permanent Generation Memory: -XX:PermSize, -XX:MaxPermSize
             c) Garbage Collection i.e. 
                 -XX:+UseSerialGC, 
                 -XX:+UseParellelGC (-XX:ParellelGCThreads=<N>), 
                 -XX:+UseParellelOldGC, 
                 -XX:+UseConcMarkSweepGC (-XX:ParellelCMSThreads=<N>), 
                 -XX:+UseG1GC

4. What is classloader in java ? How to write custom class Loader ? What is linkage error ?

5. Do you know about serialization ? When should we use that ? What is the role of Serialization id ? serializable vs externalizable ?
6. Deep vs Shallow copy in Cloning ?

Collection


These are legacy interview questions which are not asked frequently now a days but they are good to know.
1. Difference between ArrayList and LinkedList.
2. Difference between ArrayList and Vector.

Application


1. Http vs https
2. Rest vs soap

Saturday, 22 July 2017

Singleton Design Pattern


package com.help4j.advance.pattern;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;

/**
 * @author aklahoti
 *
 */
public class Singleton implements Serializable, Cloneable{

 public static void main(String[] args) throws CloneNotSupportedException, FileNotFoundException, IOException, ClassNotFoundException, InstantiationException, IllegalAccessException{
  
  // Test creating an object of Singleton Class
  Singleton obj = Singleton.getInstance();
  System.out.println(obj);
  
  // Test serialzation/deserialization of singleton object. It should not create new object and return same object.
  ObjectOutputStream os = new ObjectOutputStream(new FileOutputStream("resources/singleton.ser"));
  os.writeObject(obj);
  ObjectInputStream is = new ObjectInputStream(new FileInputStream("resources/singleton.ser"));
  Singleton deserObj = (Singleton) is.readObject();
  System.out.println("***Deserialized Singleton Object***");
  System.out.println(deserObj);
  
  // Test cloning of singleton object.  It should not create new object and return same object.
  Singleton cloneObj = (Singleton) obj.clone();
  System.out.println("***Cloned Singleton Object***");
  System.out.println(cloneObj);
  
  // Test reflexion of singleton object. It should throw exception.
  Class<Singleton> classObj = (Class<Singleton>) Class.forName("com.java.test.pattern.Singleton");
  System.out.println(classObj.newInstance());
  
 }
 private static final long serialVersionUID = 1L;

 private static Singleton instance = null;
 
 private static Object DUMMY_OBJECT = new Object();
 
 private Singleton(){
  /*To prevent object creation using reflection*/
  if(instance!=null){
   throw new InstantiationError( "Singleton Object is already created." );
  }
 }
 
 public static Singleton getInstance(){
  /*Double checked locking*/
  if(instance == null){
   synchronized (DUMMY_OBJECT) {
    if(instance == null){
     instance = new Singleton();
    }
   }
  }
  return instance;
 }
 
 public static void print(){
  System.out.println("I am a singleton class.");
 }
 
 /*To prevent object creation using deserialization*/
 private Object readResolve() throws ObjectStreamException{
  return instance;
 }
 
 /*To prevent object creation using cloning*/
 @Override
 protected Object clone() throws CloneNotSupportedException {
  return instance;
 }
}

Sunday, 26 March 2017

Spring Interview Questions

Spring Bean Life Cycle

  1.  Default constructor will be called.
  2.  All properties setter methods will be called.
  3.  If class implements BeanNameAware then setBeanName method will be called.
  4.  If class implements BeanFactoryAware then setBeanFactory method will be called.
  5.  If class implements ApplicationContextAware then setApplicationContext method will be called.
  6.  If class implements BeanPostProcessor then its postProcessBeforeInitialization will be called.
  7.  If class implements InitializingBean then afterPropertiesSet method will be called.
  8.  If class has custom init method defined then it will be called.
  9.  If class implements BeanPostProcessor then its postProcessAfterInitializationwill be called.
  10.  If class implements DisposableBean then destroy method will be called.
  11.  If class has custom destroy method defined then it will be called. 
Spring MVC flow


1. Dipatcher Servlet which is also know as front controller of Spring application handles all the request mapped to it in web.xml and ask for appropriate controller from Handler Mapping

2. Handler Mapping is a configuration for url and controller mapping. It look for controller:

Popular Implentations of HandlerMapping:

BeanNameUrlHandlerMapping - This is a default spring handler mapping. Name of bean considered as URL.


<bean name="/welcome.htm" class="com.help4j.controller.WelcomeController" />

SimpleUrlHandlerMapping - Map with key value pair of URL and controller bean name.

<bean class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
   <property name="mappings">
  <props>
     <prop key="/welcome.htm">welcomeController</prop>
   </props>
   </property>
</bean>

<bean id="welcomeController" class="com.help4j.controller.WelcomeController" />


DefaultAnnotationHandlerMapping - Annotation based @RequestMapping at class and method level.
WelcomeController.java
package com.help4j.controller;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.servlet.ModelAndView;

@Controller
@RequestMapping("/welcome")
public class WelcomeController{

 @RequestMapping(method = RequestMethod.GET)
 public ModelAndView helloWorld(){

  ModelAndView model = new ModelAndView("WelcomePage");
  model.addObject("msg", "hello world");

  return model;
 }
}


3. Controller calls service layer to execute business logic and retrurn ModelAndView, which is wrapper for model object and view name.

Popular Controller Types:

AbstractController
AbstractCommandController
SimpleFormController


4. View Resolver look for appropriate view JSP/HTML based on view name and return to user.

Popular Implentations of ViewResolver:

InternalResourceViewResolver


Wednesday, 9 September 2015

How to use superscript in HighChart labels

This is how you can use superscript in HighChart labels using useHTML property :

(you can run and edit below code here  : http://jsfiddle.net/lahoti32/j0ue1naf/ )

$(function () {
    $('#container').highcharts({
chart: {
            type: 'column'
        },
        credits : {
            enabled : false
        },
        title: {
            text: 'Chart Title<sup>1</sup>',
            useHTML : true,
        },
        xAxis: {
categories: [ 'label1','label2','label3<sup>4</sup>','label4'],
            title:{
                enabled: false
            },
            labels: {
                useHTML : true,
                title: {
                    enabled: false
                }         
            },
        },
        yAxis: {
            title: {
                enabled: false
            },
            labels: {
                useHTML : true,
                formatter:function(){
                    if(this.value != 10){
                        return this.value;
                    }else{
                        return this.value + '<sup> 2</sup>';
                    }
                }
            }
        },
        legend: {
            useHTML : true,
            borderWidth: 0,
            labelFormatter:function(){
                if(this.name != 'legend1'){
                    return this.name;
                }else{
                    return this.name + '<sup> 5</sup>';
                }
            }
        },
        plotOptions: {
            column: {
                dataLabels: {
                    enabled: true,
                    useHTML : true,
                    y:-1,
                    formatter:function(){
                        if(this.y != 0){
                            if(this.y > 8 && this.y < 10){
                                return this.y + '<sup> 3</sup>';
                            }else{
                                return this.y;
                            }

                        }else{
                            return null;
                        }
                    }
                }
            }
        },
        series: [{ 
            data: [{ 
                y: 14.913
            }, { 
                y: 8.281
            }, { 
                y: 3.592
            }, { 
                y: 3.017
            }], 
            showInLegend: false,
        },{
            name: 'legend1',
            color: 'rgb(14,178,89)'      
        },{
            name: 'legend2',
            color: 'rgb(100,100,9)'      
        }]
    });
});

Sunday, 12 July 2015

fibonacci series in java

Fibonacci series implementation in java is frequently asked question in interview at fresher level. Moreover, it is a very famous example to show how to use recursive function in java.


public class Fibonacci {

    public static void main(String[] args){
        fibonacci(15);
    }
    
    /**
     * This method is used to print fibonacci series
     * @param n is total number of elements
     */
    private static void fibonacci(int n){
        for(int i = 0 ; i < n; i ++){
            System.out.print(fib(i)+", ");
        }
    }
    /**
     * This method is used recursively to find fibonacci
     * element at nth index. 
     * @param n is index
     * @return element
     */
    private static int fib(int n){
        if(n < 2)
            return n;
        return fib(n-1) + fib(n-2);
    }
}

Output : 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377 

Tuesday, 9 June 2015

Sorting Algorithm : Bubble Sort

Why it is called bubble sort ?

Bubble Sort is nothing but a comparison algorithm where - 
- At the end of first iteration, largest element in the array get placed at last index
- At the end of second iteration, second largest element in the array get placed at second last index and so on...
This way large elements are moving towards the last indexes and hence small elements are moving towards the starting indexes which is also termed as smaller elements "bubble" to the top of the list that is why it is called bubble sort.

Step-by-step example

Let us take the array of numbers "5 1 4 2 8", and sort the array from lowest number to greatest number using bubble sort. In each step, elements written in bold are being compared. Three passes will be required.
First Pass:
5 1 4 2 8 ) \to ( 1 5 4 2 8 ), Here, algorithm compares the first two elements, and swaps since 5 > 1.
( 1 5 4 2 8 ) \to ( 1 4 5 2 8 ), Swap since 5 > 4
( 1 4 5 2 8 ) \to ( 1 4 2 5 8 ), Swap since 5 > 2
( 1 4 2 5 8 ) \to ( 1 4 2 5 8 ), Now, since these elements are already in order (8 > 5), algorithm does not swap them.
Second Pass:
1 4 2 5 8 ) \to ( 1 4 2 5 8 )
( 1 4 2 5 8 ) \to ( 1 2 4 5 8 ), Swap since 4 > 2
( 1 2 4 5 8 ) \to ( 1 2 4 5 8 )
( 1 2 4 5 8 ) \to ( 1 2 4 5 8 )
Now, the array is already sorted, but the algorithm does not know if it is completed. The algorithm needs one whole pass without any swap to know it is sorted.
Third Pass:
1 2 4 5 8 ) \to ( 1 2 4 5 8 )
( 1 2 4 5 8 ) \to ( 1 2 4 5 8 )
( 1 2 4 5 8 ) \to ( 1 2 4 5 8 )
( 1 2 4 5 8 ) \to ( 1 2 4 5 8 )

Algorithm In Java


package com.test.algorithm.sort;
/*
 *Bubble Sort is nothing but a comparison algorithm where - 
 *- At the end of first iteration, largest element in the array get placed at last index
 *- At the end of second iteration, second largest element in the array get placed at second last index and so on...
 *This way large elements are moving towards the last indexes and hence small elements are moving towards the starting indexes 
 *which is also termed as smaller elements "bubble" to the top of the list that is why it is called bubble sort.
 */

public class BubbleSort {

    public static void main(String[] args){
        int[] array = new int[]{5, 1, 12, -5, 16};
        BubbleSort.sort(array);
    }
    
    private static void sort(int[] array){
        int count = 0;
        for(int i = array.length ; i > 0 ; i --, count++){
            for(int j = 0 ; j < array.length-1 ; j++, count++){
                if(array[j] > array[j+1]){
                    swapNumbers(j, j+1, array);
                }
            }
        }
        printNumbers(array, count);
    }
    
    private static void swapNumbers(int i, int j, int[] array) {
        int temp;
        temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
    
    private static void printNumbers(int[] array, int count) {
        System.out.print("Sorted Array : {");
        for(int i : array){
            System.out.print(i + " ");
        }
        System.out.print("}, n : " + array.length + ", comparisons : " + count);
    }
}

Output : Sorted Array : {-5 1 5 12 16 }, n : 5, comparisons : 25

Time Complexity for worst case is O(n2). Algorithm can be further improved :

private static void sort(int[] array){
        int count = 0;
        for(int i = array.length ; i > 0 ; i --, count++){
            for(int j = 0 ; j < i-1 ; j++, count++){
                if(array[j] > array[j+1]){
                    swapNumbers(j, j+1, array);
                }
            }
        }
        printNumbers(array, count);
    }

Output :Sorted Array : {-5 1 5 12 16 }, n : 5, comparisons : 15