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Frequently asked interview questions

kCan we serialise the static members
Does not m any sense to make static  members serializable
Staic member belong to the class not the individual objects.
Why use the hashmap keys as immutable objects
Since if we use mutable objects as keys at the time of storage and over the time when we change it may point to entire different suppose for key 3 value 33 is associatedif this key is changed then it may point to diffrent location so to avoid this keys are immutable.
Difference save and persist
Save returns serializable entity
Persist return void
WHAt is executerservice

Executerservice is like a threadpool.
It is a way of submit you thread for is used where asynchronous tasks need to be performed.
Executerservice has submit method.
Which returns future class instance.
Executerservice takes callable instance .
Future returns the result of thread execution

Can we synchronise the constructor

No not possible.since it will be creating lock on object being created but generally the object is not available to other thread till the time it gets created

Can we synchronise the run  method
Yes  we  can  synchronize  run method.

Types of  garbage  collection
Serial gc
Parallel gc=serial works same as parallel but the girl is done by multiple  threads.

What is  difference between notify and notify all

Notify all - will notify all waiting thread
Notify- invokes the first thread which called the wait method on the object

Join()  in multithreading

Join method in multithreading is used to pause execution of all other threads except the one which called the join method.

inner classes are used to hide implementation details. To increase encapsulation.
Some examples of usage of inner classes are iterator inside list. Listener in gui.

ConcurrentHashMap in nutshell

The Java concurrent hash map may be considered as the lodge having many rooms with locking facility.Where as hash map may be considered as the house with one door.

So in case of hash map if you are locking the door. Then you will not be able to access if any of the room of house. Before using hash map every thread locks the hash map making it inaccessible to other thread till the  time it won't complete it's operations. 
Hashmap is like your house

But if concurrent hash map is viewed it may be viewed as multiple separate rooms of Lodge if any room is locked we can still access the other rooms.

Concurrent hashmap used by multiple thread

Creation of ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,float loadFactor, int concurrencyLevel) 

HashMap anatomy:

Initial Capacity: 

You can specify the size of the concurrenthashmap at the time of creation.

 static final int DEFAULT_INITIAL_CAPACITY = 16;

Load Factor: 

Tolerable capacity of concurrenthashmap if it exceeds concurrenthashmap needs to be resized. However resizing operation is relatively slow.

 static final float DEFAULT_LOAD_FACTOR = 0.75f;


Estimated number of concurrently updated threads.

Advantages of concurrenthashmap:

ConcurrentHashMap can be used to make the concurrent operations possible without waiting for other threads job completion

Increased throughput.

For Producer Consumer Example on concurrenthashmap Producer Consumer in ConcurrentHashmap

Disadvantages of concurrency in hashmap

           Methods including size, isEmpty, and containsValue are typically useful only when a map is not undergoing concurrent updates in other threads. Otherwise these methods procure precise results.

When resizing is done?

       When the concurrenthashmap is almost full 75% of its defined capacity.
2.       When collisions are increased so to avoid collisions concurrenthashmap  is resized.

Properties of ConcurrentHashmap


         ConcurrentHashmap does not allow null value.
2.       ConcurrentHashMap does not allow duplicate keys.

public class CHashMap {

            public static void main(String[] args) {
ConcurrentHashMap<String, String> ch=new ConcurrentHashMap<String, String>();
ch.put("a", "a");
ch.put("a", "a");
Enumeration<String> enum1=ch.keys();

            String key=enum1.nextElement();
            System.out.println("Key is ==>"+key+" Value is ===>"+ch.get(key));


Output is: Key is ==>a Value is ===>a

Java Helloworld

Calcuator Demo
package com.oop;
class Calculator
     public Calculator() {
          System.out.println("I am constructor");
     //1.add 2.sub 3.multiplication 4.division
     public int add(int num1,int num2)
          return (num1+num2);
public class CalculatorDemo {

     public static void main(String[] args)
          Calculator calcObj=new Calculator();
          System.out.println("addition is "+calcObj.add(2, 3));

package com.oop;
class Human
     private String address;
    private int age;
     public int getAge() {
          return age;

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

     public String getAddress() {
          return address;
     public void setAddress(String address) {
          this.address = address;
public class HelloWorld {
     //Type2:declaration and oneliner init
     //datatype varname="value"
static String empName="Ashwin More";

//Type1: declaration and then intialization
//dataType variable Name;

//creation of objects
//ClassName objectName=new className();

public static void main(String[] args) {
     System.out.println("HI Java Batch and i am "+empName);
     Human objectHuman=new Human();
     Human objectHuman1=new Human();
//visibily modifier class ClassName