Category Archives: Java

Spring MVC interview Questions

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Q1.What is the Difference between Filters and Interceptors?
Filter: – A filter as the name suggests is a Java class executed by the servlet container for each incoming HTTP request and for each http response. This way, is possible to manage HTTP incoming requests before they reach the resource, such as a JSP page, a servlet or a simple static page; in the same way, it’s possible to manage HTTP outbound response after resource execution.

  1. A filter dynamically intercepts requests and responses to transform or use the information contained in the requests or responses.
  2. Filters typically do not themselves create a response, but instead provide universal functions that can be “attached” to any type of servlet or JSP page.
  3. They provide the ability to encapsulate recurring tasks in reusable units. Organized developers are constantly on the lookout for ways to modularize their code.
  4. Modular code is more manageable and documentable, is easier to debug, and if done well, can be reused in another setting.

Interceptor: – Spring Interceptors are similar to Servlet Filters but they act in Spring Context so are many powerful to manage HTTP Request and Response but they can implement more sophisticated behavior because can access to all Spring context. Interceptors are used in conjunction with Java EE managed classes to allow developers to invoke interceptor methods in conjunction with method invocations or lifecycle events on an associated target class. Common uses of interceptors are logging, auditing, or profiling.

  1. Interceptor can be defined within a target class as an interceptor method, or in an associated class called an interceptor class.
  2. Interceptor classes contain methods that are invoked in conjunction with the methods or lifecycle events of the target class.
  3. Interceptor classes and methods are defined using metadata annotations, or in the deployment descriptor of the application containing the interceptors and target classes.
  4. Interceptor classes may be targets of dependency injection. Dependency injection occurs when the interceptor class instance is created, using the naming context of the associated target class, and before any @PostConstruct callbacks are invoked.

Refer Here

Q2.Spring interceptor vs servlet filter?

  1. Using Interceptor we can inject other beans in the interceptor
  2. Can use more advanced mapping patterns (ant-style)
  3. You have the target handler object (controller) available, as well as the result ModelAndView
  4. It is a bean, so you can use AOP with it (althoug that would be rare)

Q3.How to avoid multiple submission of Form to Server?

  1. Use JavaScript to disable the button a few ms after click. This will avoid multiple submits being caused by impatient users clicking multiple times on the button.
  2. Send a redirect after submit, this is known as Post-Redirect-Get (PRG) pattern. This will avoid multiple submits being caused by users pressing F5 on the result page and ignoring the browser warning that the data will be resend, or navigating back and forth by browser back/forward buttons and ignoring the same warning.
  3. Generate an unique token when the page is requested and put in both the session scope and as hidden field of the form. During processing, check if the token is there and then remove it immediately from the session and continue processing. If the token is not there, then block processing. This will avoid the aforementioned kinds of problems.

Q4.What is POST-REDIRECT-GET Pattern?
The client gets a page with a form.The form POSTs to the server.The server performs the action, and then redirects to another page.The client follows the redirect.
For example, say we have this structure of the website as below

  1. /posts (shows a list of posts and a link to “add post”) and / (view a particular post)
  2. /create (if requested with the GET method, returns a form posting to itself; if it’s a POST request, creates the post and redirects to the / endpoint)

For retrieving posts /posts/ might be implemented like this:

  1. Find the post with that ID in the database.
  2. Render a template with the content of that post.

For creating posts /posts/create might be implemented like this:

  1. If the request is a GET request for the Insert posts page Show an empty form with the target set to itself and the method set to POST.
  2. If the request is a POST request
    • Validate the fields.
    • If there are invalid fields, show the form again with errors indicated.
  3. Otherwise, if all fields are valid
    • Add the post to the database.
    • Redirect to /posts/ (where is returned from the call to the database)

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Filters vs Interceptors

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Filters

  1. A filter as the name suggests is a Java class executed by the servlet container for each incoming http request and for each http response. This way, is possible to manage HTTP incoming requests before them reach the resource, such as a JSP page, a servlet or a simple static page; in the same way is possible to manage HTTP outbound response after resource execution.
  2. The filter runs in the web container so its definition will also be contained in the web.xml file
  3. filer include three main methods:
    • init: executed to initialize filter using init-param element in filter definition
    • doFilter: executed for all HTTP incoming request that satisfy “url-pattern”
    • release resources used by the filter

web.xml

<filter>
    <filter-name>CORSFilter</filter-name>
    <filter-class>com.listfeeds.components.CORSFilter</filter-class>
    <init-param>
        <param-name>fake-param</param-name>
        <param-value>fake-param-value</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>CORSFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

TestFilter.java

package com.listfeeds.filters;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletResponse;
import org.springframework.stereotype.Component;

public class TestFilter implements Filter {

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {

        HttpServletResponse response = (HttpServletResponse) res;
        response.setHeader("Access-Control-Allow-Origin", "*");
        response.setHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE");
        response.setHeader("Access-Control-Max-Age", "3600");
        response.setHeader("Access-Control-Allow-Headers", "x-requested-with");
        chain.doFilter(req, res);
    }

    public void init(FilterConfig filterConfig) {}

    public void destroy() {}

}

Filters can perform many different types of functions.

  1. Authentication-Blocking requests based on user identity.
  2. Logging and auditing-Tracking users of a web application.
  3. Image conversion-Scaling maps
  4. Data compression-Making downloads smaller
  5. Localization-Targeting the request and response to a particular locale

Request Filters can:

  1. perform security checks
  2. reformat request headers or bodies
  3. audit or log requests

Response Filters can:

  1. Compress the response stream
  2. Append or alter the response stream
  3. Create a different response altogether

Interceptors

  1. Spring Interceptors are similar to Servlet Filters but they acts in Spring Context so are many powerful to manage HTTP Request and Response but they can implement more sophisticated behavior because can access to all Spring context.
  2. Developers can invoke interceptor methods in conjunction with method invocations or lifecycle events on an associated target class. Common uses of interceptors are logging, auditing, or profiling.
  3. Spring interceptor execute in Spring context so they have be defined in rest-servlet.xml file:
  4. The interceptor include three main methods:
    • preHandle: executed before the execution of the target resource
    • afterCompletion: executed after the execution of the target resource (after rendering the view)
    • posttHandle: Intercept the execution of a handler

rest-servlet.xml

<mvc:interceptors>
    <bean class="com.listfeeds.interceptors.LogContextInterceptor" />
    <bean class="com.listfeeds.interceptors.TimedInterceptor" />
</mvc:interceptors>

LogContextInterceptor.java

public class LogContextInterceptor extends HandlerInterceptorAdapter 
{
 private static final Logger log = LoggerFactory.getLogger(LogContextInterceptor.class);

 @Override
 public void afterCompletion(
  HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
 throws Exception 
 {
  HandlerMethod methodHandler = (HandlerMethod) handler;
  log.debug("END EXECUTION method {} request: {}", methodHandler.getMethod().getName(), request.getRequestURI());
  }
 

 @Override
 public boolean preHandle(HttpServletRequest request,
  HttpServletResponse response, Object handler) throws Exception 
  {

  } catch (IllegalArgumentException e) 
  {
   log.warn("Prehandle", e);
   return true;
  } finally 
  {
   HandlerMethod methodHandler = (HandlerMethod) handler;
   log.debug("START EXECUTION method {} request: {}", methodHandler.getMethod().getName(), request.getRequestURI());
  }
  return true;
}

For authentication of web pages you would use a servlet filter which acts at weblayer. For security stuff in your business layer or logging/bugtracing (a.k.a. independent of the web layer) you would use an Interceptor.

Java Memory Management

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Q1.What is the Difference between Stack and Heap?
Stack vs Heap

Q2.Does Wrapper Classes are immutable Similar to String?
Yes, Wrapper classes are immutable similar to String.

Q3.Does Wrapper Classes would be cached Similar to String Pool for Strings?
Yes.Java has Integer pool for small integers between -128 to 127 so it will behave same for Integer also similar to String Constant pool
java.lang.Boolean store two inbuilt instances TRUE and FALSE, and return their reference if new keyword is not used.
java.lang.Character has a cache for chars between unicodes 0 and 127 (ascii-7 / us-ascii).
java.lang.Long has a cache for long between -128 to +127.
java.lang.String has a whole new concept of string pool.

Q4.How String will behave in memory management incase of String Literal or String Object?

Q5.See the Below Code

class D {
    public static void main(String args[]) {
        Integer b1=127;
        Integer b2=127;
        Integer b3=128;
        Integer b4=128;
        System.out.println(b1==b2);
        System.out.println(b3==b4);
    }
}

true 
false

Why it is so?
If the value p being boxed is true, false, a byte, a char in the range \u0000 to \u007f, or an int or short number between -128 and 127, then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

Q6.In which Memory would the following would be created?

int a = 0; 
Integer b = 0;

It Depends whether a and b variables are local variables or fields (static or instance) of an object.

If they are local variables:
a is on the stack.
b is on the stack (a reference) and it refers to an object in the heap.

If they are fields of an instance or class:
a is on the heap (as part of the instance or the class).
b is on the heap (as above) and it refers to an object in the heap.

Q7.Why the value of i didnt change after modify being called?

class Demo 
{ 
    public static void main(String[] args) 
    { 
        Integer i = new Integer(12); 
        System.out.println(i); 
        modify(i); 
        System.out.println(i); 
    } 
  
    private static void modify(Integer i) 
    { 
        i = i + 1; 
    } 
}

Output

12
12
12
12

The reason again traces back to the Immutability of wrapper class.

i = i + 1;

It does the following:

  1. Unbox i to an int value
  2. Add 1 to that value
  3. Box the result into another Integer object
  4. Assign the resulting Integer to i (thus changing what object i references)

Since object references are passed by value, the action taken in the modified method does not change i that was used as an argument in the call to modify. Thus the main routine still prints 12 after the method returns.

Q8.How the array is stored in the memory?

arr[0] = new String("abc");
arr[1] = new List();

Stack has a single pointer to a location in the heap that contains the array itself. The array itself is just an array of pointers which also point to locations in the heap that contain the objects you reference.

Q9.What is Contagious memory block?
Array are “contiguous”. That means the elements are laid out end-to-end, with no discontinuities and no padding between them (there may be padding inside each element, but not between elements). So an array of 5 4-byte elements looks like this (1 underscore character per byte, the | symbols don’t represent memory).Arrays and ArrayList uses Contagious memory whereas LinkedList uses Non Contagious memory.

Contiguous

Non-Contiguous

What is Type inference

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Type inference is a feature of Java which provides ability to compiler to look at each method invocation and corresponding declaration to determine the type of arguments.
Java provides improved version of type inference in Java 8.
Here, we are creating arraylist by mentioning integer type explicitly at both side. The following approach is used earlier versions of Java.

List<Integer> list = new ArrayList<Integer>();

In the following declaration, we are mentioning type of arraylist at one side. This approach was introduce in Java 7. Here, you can left second side as blank diamond and compiler will infer type of it by type of reference variable.

List<Integer> list2 = new ArrayList<>();

Improved Type Inference
In Java 8, you can call specialized method without explicitly mentioning of type of arguments.

showList(new ArrayList<>());

Example
You can use type inference with generic classes and methods.

import java.util.ArrayList;
import java.util.List;
public class TypeInferenceExample {
 public static void showList(List < Integer > list) {
  if (!list.isEmpty()) {
   list.forEach(System.out::println);
  } else System.out.println("list is empty");
 }

 public static void main(String[] args) {

  // An old approach(prior to Java 7) to create a list  
  List < Integer > list1 = new ArrayList < Integer > ();
  list1.add(11);
  showList(list1);

  // Java 7    
  List < Integer > list2 = new ArrayList < > (); // You can left it blank, compiler can infer type  
  list2.add(12);
  showList(list2);

  // Compiler infers type of ArrayList, in Java 8  
  showList(new ArrayList < > ());
 }
}

Output

11
12
list is empty

Type inference for Custom Classes

class GenericClass <X> {
 X name;
 public void setName(X name) {
  this.name = name;
 }
 public X getName() {
  return name;
 }
 public String genericMethod(GenericClass < String > x) {
  x.setName("John");
  returnx.name;
 }
}

public class TypeInferenceExample {
 public static void main(String[] args) {
  GenericClass < String > genericClass = new GenericClass < String > ();
  genericClass.setName("Peter");
  System.out.println(genericClass.getName());

  GenericClass < String > genericClass2 = new GenericClass < > ();
  genericClass2.setName("peter");
  System.out.println(genericClass2.getName());

  // New improved type inference  
  System.out.println(genericClass2.genericMethod(new GenericClass < > ()));
 }
}

Output

Peter
peter
John

Lambda Expression vs Anonymous Inner Class

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Lambdas implement a functional interface.Anonymous Inner Classes can extend a class or implement an interface with any number of methods.
Variables – Lambdas can only access final or effectively final.
State – Anonymous inner classes can use instance variables and thus can have state, lambdas cannot.
Scope – Lambdas can’t define a variable with the same name as a variable in enclosing scope.
Compilation – Anonymous compiles to a class, while lambda is an invokedynamic instruction.

Syntax
Lambda expressions looks neat as compared to Anonymous Inner Class (AIC)

public static void main(String[] args) {
    Runnable r = new Runnable() {
        @Override
        public void run() {
            System.out.println("in run");
        }
    };

    Thread t = new Thread(r);
    t.start(); 
}

//syntax of lambda expression 
public static void main(String[] args) {
    Runnable r = ()->{System.out.println("in run");};
    Thread t = new Thread(r);
    t.start();
}

Scope
An anonymous inner class is a class, which means that it has scope for variable defined inside the inner class.

Whereas,lambda expression is not a scope of its own, but is part of the enclosing scope.

Similar rule applies for super and this keyword when using inside anonymous inner class and lambda expression. In case of anonymous inner class this keyword refers to local scope and super keyword refers to the anonymous class’s super class. While in case of lambda expression this keyword refers to the object of the enclosing type and super will refer to the enclosing class’s super class.

//AIC
    public static void main(String[] args) {
        final int cnt = 0; 
        Runnable r = new Runnable() {
            @Override
            public void run() {
                int cnt = 5;    
                System.out.println("in run" + cnt);
            }
        };

        Thread t = new Thread(r);
        t.start();
    }

//Lambda
    public static void main(String[] args) {
        final int cnt = 0; 
        Runnable r = ()->{
            int cnt = 5; //compilation error
            System.out.println("in run"+cnt);};
        Thread t = new Thread(r);
        t.start();
    }

Performance
At runtime anonymous inner classes require class loading, memory allocation, and object initialization and invocation of a non-static method while lambda expression is a compile-time activity and don’t incur extra cost during runtime. So the performance of lambda expression is better as compare to anonymous inner classes.

Thread Code Examples Basics

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Reading a File extending Thread API

  1. ReadFile.java has a run() method which implements the reading the file code within the try with resources block
  2. In the main method start method is called over the ReadFile class instance
  3. In thread we have coded is asynchrobnous(order of execution cannot be guaranteed) which we can see from the output below

TestThread.java

package com.mugil.test;

import com.mugil.runnables.ReadFile;

public class TestThread {
	public static void main(String[] args) {
		ReadFile objReadFileThread1 = new ReadFile();
		ReadFile objReadFileThread2 = new ReadFile();
		ReadFile objReadFileThread3 = new ReadFile();
				
		objReadFileThread1.start();
		objReadFileThread2.start();
		objReadFileThread3.start();
	}
}

ReadFile.java

package com.mugil.runnables;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class ReadFile extends Thread {

 public void run() {

  try (BufferedReader reader = new BufferedReader(new FileReader(new File("E:\\JavaProjects\\JavaThreads\\src\\Sample.txt")))) {
   String line = null;

   while ((line = reader.readLine()) != null) {
    System.out.println(Thread.currentThread().getName() + " reading line " + line);
   }

  } catch (IOException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }

 }
}

Output

Thread-2 reading line Line1
Thread-0 reading line Line1
Thread-0 reading line Line2
Thread-0 reading line Line3
Thread-1 reading line Line1
Thread-1 reading line Line2
Thread-2 reading line Line2
Thread-1 reading line Line3
Thread-1 reading line Line4
Thread-1 reading line Line5
Thread-0 reading line Line4
Thread-0 reading line Line5
Thread-2 reading line Line3
Thread-2 reading line Line4
Thread-2 reading line Line5

Reading a File implementing Runnable API

  1. Now in the below code the runnable API is implemented rather than extending like Thread
  2. The run() is called over instance of ReadFile rather than start() method
  3. Calling run() method will start the execution of thread in the present running thread rather than creating new Thread for execution which can been seen in output main reading line rather than Thread-N reading line

TestThread.java

package com.mugil.test;

import com.mugil.runnables.ReadFile;

public class TestThread {
	public static void main(String[] args) {
		ReadFile objReadFileThread1 = new ReadFile();
		ReadFile objReadFileThread2 = new ReadFile();
		ReadFile objReadFileThread3 = new ReadFile();
				
		objReadFileThread1.run();
		objReadFileThread2.run();
		objReadFileThread3.run();
	}
}

ReadFile.java

public class ReadFile implements Runnable {

 public void run() {

  try (BufferedReader reader = new BufferedReader(new FileReader(new File("E:\\JavaProjects\\JavaThreads\\src\\Sample.txt")))) {
   String line = null;

   while ((line = reader.readLine()) != null) {
    System.out.println(Thread.currentThread().getName() + " reading line " + line);
   }

  } catch (IOException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
}

Output

main reading line Line1
main reading line Line2
main reading line Line3
main reading line Line4
main reading line Line5
main reading line Line1
main reading line Line2
main reading line Line3
main reading line Line4
main reading line Line5
main reading line Line1
main reading line Line2
main reading line Line3
main reading line Line4
main reading line Line5

Methods to Manage thread are available on Thread class not in Runnable. So we can pass the runnable instance as parameter like one below
TestThread.java

.
.
.
Thread objThread = new Thread(runObj);
objThread.start();
.
.

Different ways of Executing a process

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In a real world analogy. Let’s say you have to get done 2 very important tasks in one day:

  • Get a passport
  • Get a presentation done

Now, the problem is that task-1 requires you to go to an extremely bureaucratic government office that makes you wait for 4 hours in a line to get your passport. Meanwhile, task-2 is required by your office, and it is a critical task. Both must be finished on a specific day.

Case 1: Sequential Execution
Ordinarily, you will drive to passport office for 2 hours, wait in the line for 4 hours, get the task done, drive back two hours, go home, stay awake 5 more hours and get presentation done.

Case 2: Concurrent Execution
But you’re smart. You plan ahead. You carry a laptop with you, and while waiting in the line, you start working on your presentation. This way, once you get back at home, you just need to work 1 extra hour instead of 5.

In this case, both tasks are done by you, just in pieces. You interrupted the passport task while waiting in the line and worked on presentation. When your number was called, you interrupted presentation task and switched to passport task. The saving in time was essentially possible due to interruptability of both the tasks.

Concurrency, IMO, can be understood as the “isolation” property in ACID. Two database transactions are considered isolated if sub-transactions can be performed in each and any interleaved way and the final result is same as if the two tasks were done sequentially. Remember, that for both the passport and presentation tasks, you are the sole executioner.

Case 3: Parallel Execution
Now, since you are such a smart fella, you’re obviously a higher-up, and you have got an assistant. So, before you leave to start the passport task, you call him and tell him to prepare first draft of the presentation. You spend your entire day and finish passport task, come back and see your mails, and you find the presentation draft. He has done a pretty solid job and with some edits in 2 more hours, you finalize it.

Now since, your assistant is just as smart as you, he was able to work on it independently, without needing to constantly ask you for clarifications. Thus, due to the independentability of the tasks, they were performed at the same time by two different executioners.

Still with me? Alright…

Case 4: Concurrent But Not Parallel
Remember your passport task, where you have to wait in the line? Since it is your passport, your assistant cannot wait in line for you. Thus, the passport task has interruptability (you can stop it while waiting in the line, and resume it later when your number is called), but no independentability (your assistant cannot wait in your stead).

Case 5: Parallel But Not Concurrent
Suppose the government office has a security check to enter the premises. Here, you must remove all electronic devices and submit them to the officers, and they only return your devices after you complete your task.

In this, case, the passport task is neither independentable nor interruptible. Even if you are waiting in the line, you cannot work on something else because you do not have necessary equipment.

Similarly, say the presentation is so highly mathematical in nature that you require 100% concentration for at least 5 hours. You cannot do it while waiting in line for passport task, even if you have your laptop with you.

In this case, the presentation task is independentable (either you or your assistant can put in 5 hours of focused effort), but not interruptible.

Case 6: Concurrent and Parallel Execution
Now, say that in addition to assigning your assistant to the presentation, you also carry a laptop with you to passport task. While waiting in the line, you see that your assistant has created the first 10 slides in a shared deck. You send comments on his work with some corrections. Later, when you arrive back home, instead of 2 hours to finalize the draft, you just need 15 minutes.

This was possible because presentation task has independentability (either one of you can do it) and interruptability (you can stop it and resume it later). So you concurrently executed both tasks, and executed the presentation task in parallel.

Let’s say that, in addition to being overly bureaucratic, the government office is corrupt. Thus, you can show your identification, enter it, start waiting in line for your number to be called, bribe a guard and someone else to hold your position in the line, sneak out, come back before your number is called, and resume waiting yourself.

In this case, you can perform both the passport and presentation tasks concurrently and in parallel. You can sneak out, and your position is held by your assistant. Both of you can then work on the presentation, etc.

Back to Computer Science
In computing world, here are example scenarios typical of each of these cases:

Case 1: Interrupt processing.
Case 2: When there is only one processor, but all executing tasks have wait times due to I/O.
Case 3: Often seen when we are talking about map-reduce or hadoop clusters.
Case 4: I think Case 4 is rare. It’s uncommon for a task to be concurrent but not parallel. But it could happen. For example, suppose your task requires access to a special computational chip which can be accessed through only processor-1. Thus, even if processor-2 is free and processor-1 is performing some other task, the special computation task cannot proceed on processor-2.
Case 5: also rare, but not quite as rare as Case 4. A non-concurrent code can be a critical region protected by mutexes. Once it is started, it must execute to completion. However, two different critical regions can progress simultaneously on two different processors.
Case 6: IMO, most discussions about parallel or concurrent programming are basically talking about Case 6. This is a mix and match of both parallel and concurrent executions.

1 server , 1 job queue (with 5 jobs) -> no concurrency, no parallelism (Only one job is being serviced to completion, the next job in the queue has to wait till the serviced job is done and there is no other server to service it)

1 server, 2 or more different queues (with 5 jobs per queue) -> concurrency (since server is sharing time with all the 1st jobs in queues, equally or weighted) , still no parallelism since at any instant, there is one and only job being serviced.

2 or more servers , one Queue -> parallelism ( 2 jobs done at the same instant) but no concurrency ( server is not sharing time, the 3rd job has to wait till one of the server completes.)

2 or more servers, 2 or more different queues -> concurrency and parallelism

In other words, concurrency is sharing time to complete a job, it MAY take up the same time to complete its job but at least it gets started early. Important thing is , jobs can be sliced into smaller jobs, which allows interleaving.Parallelism is achieved with just more CPUs , servers, people etc that run in parallel.If the resources are shared, pure parallelism cannot be achieved, but this is where concurrency would have it’s best practical use, taking up another job that doesn’t need that resource.

Thread Interview Questions

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What is Thread in Java?
Thread is an independent path of execution.

Difference between Thread and Process
The thread is a subset of Process, in other words, one process can contain multiple threads. Two process runs on different memory space, but all threads share same memory space.

How do you implement Thread in Java?
By extending java.lang.Thread class
By implementing java.lang.Runnable interface.

Which way of implementing Thread is better? Extending Thread class or implementing Runnable method?
Implementing Runnable is better because in Java we can only extend one class so if we extend Thread class we can not extend any other class while by implementing Runnable interface we still have that option open with us

What is the difference between start() and run() method of Thread class?
start() method is used to start newly created thread, while start() internally calls run() method

When you invoke run() as normal method, its called in the same thread, no new thread is started

Q1.Difference between Concurrency and Parallelism
Concurrency is when two or more tasks can start, run, and complete in overlapping time periods. It doesn’t necessarily mean they’ll ever both be running at the same instant. For example, multitasking on a single-core machine.In Concurrency Interruptability exists

Parallelism is when tasks literally run at the same time, e.g., on a multicore processor.In Parallelism Independabality exists.

Concurrency                 Concurrency + parallelism
(Single-Core CPU)           (Multi-Core CPU)
 ___                         ___ ___
|th1|                       |th1|th2|
|   |                       |   |___|
|___|___                    |   |___
    |th2|                   |___|th2|
 ___|___|                    ___|___|
|th1|                       |th1|
|___|___                    |   |___
    |th2|                   |   |th2|

In both cases we have concurrency from the mere fact that we have more than one thread running.If we ran this program on a computer with a single CPU core, the OS would be switching between the two threads, allowing one thread to run at a time.If we ran this program on a computer with a multi-core CPU then we would be able to run the two threads in parallel – side by side at the exact same time.

Q2.What is the difference between process and threads
Process:

  1. An executing instance of a program is called a process.
  2. Some operating systems use the term ‘task‘ to refer to a program that is being executed.
  3. A process is always stored in the main memory also termed as the primary memory or random access memory.
  4. Therefore, a process is termed as an active entity. It disappears if the machine is rebooted.
  5. Several process may be associated with a same program.
  6. On a multiprocessor system, multiple processes can be executed in parallel.
  7. On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time yielding an illusion of concurrency.
  8. Example: Executing multiple instances of the ‘Calculator’ program. Each of the instances are termed as a process.

Thread:

  1. A thread is a subset of the process.
  2. It is termed as a ‘lightweight process’, since it is similar to a real process but executes within the context of a process and shares the same resources allotted to the process by the kernel.
  3. Usually, a process has only one thread of control – one set of machine instructions executing at a time.
  4. A process may also be made up of multiple threads of execution that execute instructions concurrently.
  5. Multiple threads of control can exploit the true parallelism possible on multiprocessor systems.
  6. On a uni-processor system, a thread scheduling algorithm is applied and the processor is scheduled to run each thread one at a time.
  7. All the threads running within a process share the same address space, file descriptors, stack and other process related attributes.
  8. Since the threads of a process share the same memory, synchronizing the access to the shared data within the process gains unprecedented importance.

Q3.Why should I prefer Thread over process?
Inter-thread communication (sharing data etc.) is significantly simpler to program than inter-process communication.
Context switches between threads are faster than between processes. That is, it’s quicker for the OS to stop one thread and start running another than do the same with two processes.

Example:
Applications with GUIs typically use one thread for the GUI and others for background computation. The spellchecker in MS Office, for example, is a separate thread from the one running the Office user interface. In such applications, using multiple processes instead would result in slower performance and code that’s tough to write and maintain.

It entirely depends on the design perspective whether to go for a thread or process. When I want to set of logically co-related operations to be carried out parallel. For example, if you run a Notepad++ there will be one thread running in the foreground as an editor and other thread running in background auto saving the document at regular intervals so no one would design a process to do that autosaving task separately.

Q4.What is the difference between Asynchronous vs synchronous execution?
synchronous – When you execute something synchronously, you wait for it to finish before moving on to another task. You are in a queue to get a movie ticket. You cannot get one until everybody in front of you gets one, and the same applies to the people queued behind you.

asynchronous -When you execute something asynchronously, you can move on to another task before it finishes. i.e. You are in a restaurant with many other people. You order your food. Other people can also order their food, they don’t have to wait for your food to be cooked and served to you before they can order. In the kitchen, restaurant workers are continuously cooking, serving, and taking orders. People will get their food served as soon as it is cooked.

Synchronous (one thread):

Single Thread  |--------A--------||--------B--------|

Synchronous (Multi-Threaded):

Thread A |--------A--------|
Thread B                   |--------B--------|
Thread C                                     |--------C--------|

ASynchronous (One thread):

           A-Start ------------------------------------------ A-End   
               | B-Start -----------------------------------------|--- B-End   
	       |    |      C-Start ------------------- C-End      |      |   
	       |    |       |                           |         |      |
  	       V    V       V                           V         V      V      
Single thread->|--A-|---B---|--C-|-A-|-C-|--A--|-B-|--C---|---A---|--B-->|

Asynchronous (Multi-Threaded):

Thread A ->     |----A-----|
Thread B ----->     |-----B-----------| 
Thread C --------->     |-------C----------|

Q5.What is Difference between thread, process and Tasks?
Process
A process is an instance of a computer program that is being executed.a process may be made up of multiple threads of execution that execute instructions concurrently.Process-based multitasking enables you to run the Java compiler at the same time that you are using a text editor. In employing multiple processes with a single CPU,context switching between various memory context is used. Each process has a complete set of its own variables.

Process
A thread is a basic unit of CPU utilization, consisting of a program counter, a stack, and a set of registers. A thread of execution results from a fork of a computer program into two or more concurrently running tasks. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources.

Tasks
A task is a set of program instructions that are loaded in memory.

Q6.What is Interprocess Communication?
Interprocess communication (IPC) is a set of programming interfaces that allow a programmer to coordinate activities among different program processes that can run concurrently in an operating system. This allows a program to handle many user requests at the same time. Since even a single user request may result in multiple processes running in the operating system on the user’s behalf, the processes need to communicate with each other. The IPC interfaces make this possible. Each IPC method has its own advantages and limitations so it is not unusual for a single program to use all of the IPC methods.

Java interprocess communication is based at the lowest level on turning state, requests, etc into sequences of bytes that can be sent as messages or as a stream to another Java process. You can do this work yourself, or you can use a variety of “middleware” technologies of various levels of complexity to abstract away the implementation details. Technologies that may be used include, Java object serialization, XML, JSON, RMI, CORBA, SOAP / “web services”, message queing, and so on.

Q7.Interprocess Communication vs Inter-Thread Communication?
The fundamental difference is that threads live in the same address spaces, but processes live in the different address spaces. This means that inter-thread communication is about passing references to objects and changing shared objects, but processes is about passing serialized copies of objects.In practice, Java interthread communication can be implemented as plain Java method calls on a shared object with appropriate synchronization thrown in.

Inter-Thread Communication = threads inside the same JVM talking to each other.Threads inside the same JVM can use pipelining through lock-free queues to talk to each other with nanosecond latency.

Inter-Process Communication (IPC) = threads inside the same machine but running in different JVMs talking to each other.Threads in different JVMs can use off-heap shared memory (usually acquired through the same memory-mapped file) to talk to each other with nanosecond latency.

Q8.What is Starvation?
Starvation describes a situation where a thread is unable to gain regular access to shared resources and is unable to make progress. This happens when shared resources are made unavailable for long periods by “greedy” threads. For example, suppose an object provides a synchronized method that often takes a long time to return. If one thread invokes this method frequently, other threads that also need frequent synchronized access to the same object will often be blocked.

Q9.What is Livelock?
A thread often acts in response to the action of another thread. If the other thread’s action is also a response to the action of another thread, then livelock may result. As with deadlock, livelocked threads are unable to make further progress. However, the threads are not blocked — they are simply too busy responding to each other to resume work. This is comparable to two people attempting to pass each other in a corridor: Alphonse moves to his left to let Gaston pass, while Gaston moves to his right to let Alphonse pass. Seeing that they are still blocking each other, Alphone moves to his right, while Gaston moves to his left. They’re still blocking each other, so…

Q10.Preemptive vs Non-Preemptive Scheduling
Scheduling is order of executuion of threads. JVM would simply use the underlying threading mechanism provided by the OS.
Non-preemptive Scheduling: The current process releases the CPU either by terminating or by switching to the waiting state. (Used in MS Windows family)

Advantages are Decreases turnaround time and Does not require special HW (e.g., timer)
Disadvantages are Limited choice of scheduling algorithm

Preemptive Scheduling: The current process needs to involuntarily release the CPU when a more important process is inserted into the ready queue or once an allocated CPU time has elapsed. (Used in Unix and Unix-like systems).This is determined by priority assigned to thread.Despite priority JVM may decide to execute thread of lower priority inorder to avoid starvation.

Advantages are No limitation on the choice of scheduling algorithm
Disadvantages are Additional overheads (e.g., more frequent context switching, HW timer, coordinated access to data, etc.)

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Custom Functional Interface definition using Lamdas

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Supplier Accounts.java

@FunctionalInterface
public interface Accounts{
  abstract String showAccountType(); 
}

AccountImpl.java

public class AccountImpl {
 public static void main(String[] args) {
  //Implementation of Custom Supplier Method for Accounts Interface 
  Accounts squareRoot = () -> "Hi there";
  System.out.println(squareRoot.showAccountType());
 }
}

The above code could be expanded as below using Anonymous Inner Class

public class AccountImpl {
 public static void main(String[] args) {
  Accounts squareRoot = new Accounts() {
   @Override
   public String showAccountType() {
    return "Hi there";
   }
  };
  System.out.println(squareRoot.showAccountType());
 }
}
Consumer Accounts.java

@FunctionalInterface
public interface Accounts {
 abstract void showAccountType(String strAccType);
}

AccountImpl.java

public class AccountImpl {
 public static void main(String[] args) {
  //Implementation of Custom Consumer Method for Accounts Interface
  Accounts squareRoot = (strAccType) -> System.out.println(strAccType);
  squareRoot.showAccountType("Savings");
 }
}

The above code could be expanded as below using Anonymous Inner Class

public class AccountImpl {
 public static void main(String[] args) {
  Accounts squareRoot = new Accounts() {
   @Override
   public void showAccountType(String strAccType) {
    System.out.println(strAccType);
   }
  };
 }
}
Predicate Accounts.java

@FunctionalInterface
public interface Accounts{
  abstract boolean showAccountType(String accountType);
}

AccountImpl.java

public class AccountImpl {
 public static void main(String[] args) {
  //Implementation of Custom Predicate Method for Accounts Interface
  Accounts squareRoot = (accountType) -> {
   if ("Savings" == accountType)
    return true;
   else
    return false;
  };

  if (squareRoot.showAccountType("Savings"))
   System.out.println("Savings");
  else
   System.out.println("Invalid Account");
 }
}

The above code could be expanded as below using Anonymous Inner Class

public class AccountImpl {
 public static void main(String[] args) {
  Accounts squareRoot = new Accounts() {
   @Override
   public boolean showAccountType(String accountType) {
    if ("Savings" == accountType)
     return true;
    else
     return false;
   }
  };

  if (squareRoot.showAccountType("Savings"))
   System.out.println("Savings");
  else
   System.out.println("Invalid Account");   
 }
}
Function Accounts.java

@FunctionalInterface
public interface Accounts  
{
 abstract String showAccountType(String accountType, String returnAccType);
}

AccountImpl.java

public class AccountImpl {
 public static void main(String[] args) {
  //Implementation of Custom Function Method for Accounts Interface
  Accounts squareRoot = (accountType, returnType) -> {
   if (accountType == "Savings")
    return "Credit";
   else
    return "Debit";
  };

  System.out.println(squareRoot.showAccountType("Savings", null));
 }
}

The above code could be expanded as below using Anonymous Inner Class

public class AccountImpl {
 public static void main(String[] args) {
  Accounts squareRoot = new Accounts() {
   @Override
   public String showAccountType(String accountType, String returnAccType) {

    if (accountType == "Savings")
     return "Credit";
    else
     return "Debit";
   }
  };

  System.out.println(squareRoot.showAccountType("Savings", null));
 }
}
Urnary Operator Accounts.java

@FunctionalInterface
public interface Accounts{
 abstract String showAccountType(String accountType);
}

AccountImpl.java

public class AccountImpl {
 public static void main(String[] args) {
  //Implementation of Custom Operator Method for Accounts Interface
  Accounts squareRoot = (accountType) -> {
   return "AccountType is " + accountType;
  };
  squareRoot.showAccountType("Savings");
 }
}

The above code could be expanded as below using Anonymous Inner Class

public class AccountImpl {
 public static void main(String[] args) {
  Accounts squareRoot = new Accounts() {
   @Override
   public String showAccountType(String accountType) {
    return "AccountType is " + accountType;
   }
  };

  System.out.println(squareRoot.showAccountType("Savings"));
 }
}

What is Optional in Java 8

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  1. Optional is a wrapper class which makes a field optional which means it may or may not have values.
  2. ptional as a single-value container that either contains a value or doesn’t (it is then said to be “empty”)
  3. The advantage compared to null references is that the Optional class forces you to think about the case when the value is not present. As a consequence, you can prevent unintended null pointer exceptions.
  4. The intention of the Optional class is not to replace every single null reference. Instead, its purpose is to help design more-comprehensible APIs so that by just reading the signature of a method, you can tell whether you can expect an optional value. This forces you to actively unwrap an Optional to deal with the absence of a value.

Lets take the below code

String version = computer.getSoundcard().getUSB().getVersion();

In the above piece of java code if any of the 3 values other the Version is NULL will throw a null pointer exception. To prevent this lets add a null check

String version = "UNKNOWN";
if(computer != null){
  Soundcard soundcard = computer.getSoundcard();
  if(soundcard != null){
    USB usb = soundcard.getUSB();
    if(usb != null){
      version = usb.getVersion();
    }
  }
}

Now the above code has become Clumsy with less readability and lot of boilerplate code has been added.
In languages like Groovy these conditions could be handles like one below

String version = computer?.getSoundcard()?.getUSB()?.getVersion();
(or)
String version = 
    computer?.getSoundcard()?.getUSB()?.getVersion() ?: "UNKNOWN";

Now lets replace the above code with new Optional in Java 8

public class Computer {
  private Optional<Soundcard> soundcard;  
  public Optional<Soundcard> getSoundcard() { ... }
  ...
}

public class Soundcard {
  private Optional<USB> usb;
  public Optional<USB> getUSB() { ... }

}

public class USB{
  public String getVersion(){ ... }
}

The advantage compared to null references is that the Optional class forces you to think about the case when the value is not present. As a consequence, you can prevent unintended null pointer exceptions.

What is the Point of Optional when the same could be done using NULL Check?
If you are doing NULL check the traditional way there would be no much difference. However, the difference is felt when you are carrying out chaining operations in streams and the datatypes returned are optional.The difference may not be significant in this case but as the chain of objects increases e.g. person.getAddress.getCity().getStreet().getBlock(),

Methods in Optional
get()
If a value is present in this Optional, returns the value, otherwise throws NoSuchElementException

void ifPresent(Consumer consumer)
If a value is present, it invokes the specified consumer with the value, otherwise does nothing.

boolean isPresent()
Returns true if there is a value present, otherwise false.

static Optional ofNullable(T value)
Returns an Optional describing the specified value, if non-null, otherwise returns an empty Optional.

T orElse(T other)
Returns the value if present, otherwise returns other.

T orElseGet(Supplier other)
Returns the value if present, otherwise invokes other and returns the result of that invocation.

orElseThrow(Supplier exceptionSupplier)
Returns the contained value, if present, otherwise throws an exception to be created by the provided supplier.

Lets take a simple example where Optional returns Empty or Value based on some Condition

package com.example.demo;
import java.util.Optional;
public class Test {

 public static void main(String[] args) {
  //IfPresent
  Optional < String > strOpt = getName(" Piggy");
  System.out.print("First Call -");
  strOpt.ifPresent(System.out::println);


  Optional < String > strOpt2 = getName("");
  System.out.print("Second Call -");
  strOpt2.ifPresent(System.out::println);

  System.out.println();

  //IsPresent and get
  Optional < String > strOpt3 = getNewName(" Biggy");
  System.out.print("Third Call -");

  if (strOpt3.isPresent())
   System.out.println(strOpt3.get());


  //orElse
  Optional < String > strOpt4 = getNewName(null);
  System.out.print("Fourth Call -");
  System.out.println(strOpt4.orElse(" Hippi"));


 }

 public static Optional < String > getName(String strName) {
  if (strName.length() > 0)
   return Optional.of(strName);
  else
   return Optional.empty();
 }

 public static Optional < String > getNewName(String strName) {
  //Optional strNewName = (strName!=null)?Optional.of(strName):Optional.empty();
  return Optional.ofNullable(strName);
 }
}

Output

First Call - Piggy
Second Call -
Third Call - Biggy
Fourth Call - Hippi