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10: What is wrong with this code? ConcurrentHashMap & Atomic operations

Q. What output is expected from the following code? What does it actually output?


import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public final class WhatIsWrong implements Runnable {

	private Map<String, Integer> myMap = null;

	public WhatIsWrong(Map<String, Integer> myMap) {
		this.myMap = myMap;
	}

	public static void main(String[] args) throws InterruptedException {

		long start = System.currentTimeMillis();

		Map<String, Integer> myMap = Collections.synchronizedMap(new HashMap<String, Integer>());
		for (int i = 1; i <= 10; i++) {
			myMap.put("Key-" + i, i); // 10 items in the map
		}

		// process 1 million times with 20 threads
		ExecutorService threadPool = Executors.newFixedThreadPool(20);
		for (int i = 1; i <= 1000000; i++) {
			WhatIsWrong wiw = new WhatIsWrong(myMap);
			threadPool.submit(wiw); //submit 1 million tasks to the thread pool
		}

		threadPool.shutdown();
		threadPool.awaitTermination(60, TimeUnit.SECONDS);
		
		traverseAndPrint(myMap);
		long finish = System.currentTimeMillis();
		System.out.println("Processed in " + (finish - start) + " ms");
	}

	@Override
	public void run() {
		traverseAndUpdate();
	}

	private void traverseAndUpdate() {

		Iterator<String> it = myMap.keySet().iterator();
		while (it.hasNext()) {
		    String key = it.next();
			Integer newValue = this.myMap.get(key) + 1;
			myMap.put(key, newValue);
		}
	}

	private static void traverseAndPrint(Map<String, Integer> myMap) {
		Iterator<String> it = myMap.keySet().iterator();
		while (it.hasNext()) {
			String key = it.next();
			System.out.println(key + " = " + myMap.get(key));
		}

	}
}

import java.util.Collections;

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

public final class WhatIsWrong implements Runnable {

private Map<String, Integer> myMap = null;

public WhatIsWrong(Map<String, Integer> myMap) {

this.myMap = myMap;

}

public static void main(String[] args) throws InterruptedException {

long start = System.currentTimeMillis();

Map<String, Integer> myMap = Collections.synchronizedMap(new HashMap<String, Integer>());

for (int i = 1; i <= 10; i++) {

myMap.put("Key-" + i, i); // 10 items in the map

}

// process 1 million times with 20 threads

ExecutorService threadPool = Executors.newFixedThreadPool(20);

for (int i = 1; i <= 1000000; i++) {

WhatIsWrong wiw = new WhatIsWrong(myMap);

threadPool.submit(wiw); //submit 1 million tasks to the thread pool

}

threadPool.shutdown();

threadPool.awaitTermination(60, TimeUnit.SECONDS);

traverseAndPrint(myMap);

long finish = System.currentTimeMillis();

System.out.println("Processed in " + (finish - start) + " ms");

}

@Override

public void run() {

traverseAndUpdate();

}

private void traverseAndUpdate() {

Iterator<String> it = myMap.keySet().iterator();

while (it.hasNext()) {

String key = it.next();

Integer newValue = this.myMap.get(key) + 1;

myMap.put(key, newValue);

}

private static void traverseAndPrint(Map<String, Integer> myMap) {

Iterator<String> it = myMap.keySet().iterator();

while (it.hasNext()) {

String key = it.next();

System.out.println(key + " = " + myMap.get(key));

}

A. It is expected to output counts from 100,001 to 100,010, but actually the output will be as shown below missing counts. Output with missing counts


Key-1 = 570087
Key-2 = 527793
Key-3 = 518048
Key-4 = 493989
Key-5 = 466364
Key-6 = 578612
Key-7 = 508259
Key-8 = 517953
Key-9 = 480131
Key-10 = 433976
Processed in 1777 ms

Key-1 = 570087

Key-2 = 527793

Key-3 = 518048

Key-4 = 493989

Key-5 = 466364

Key-6 = 578612

Key-7 = 508259

Key-8 = 517953

Key-9 = 480131

Key-10 = 433976

Processed in 1777 ms

Why does the output miss counts?… Read more...

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10+ Domain Driven Design (i.e. DDD) Interview Q&As

Q1. What is Domain Driven Design (i.e. DDD)? What is DDD oriented microservice? A1. Domain-driven design advocates modelling based on the reality of business as relevant to your use cases. In the context of building applications, DDD talks about problems as domains. If you are building a shopping cart application,...

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10+ Key Java Microservices Interview Questions & Answers

Java microservices interview questions and answers covering microservices patterns, best practices, Springboot and Spring cloud . Q1. How will you go about choosing REST vs Messaging for Microservices? A1. Microservices can be invoked both synchronously and asynchronously. Microservices must be organized around business functionalities like Customers, … Read more ›...

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10+ Know your industry Q&As

Insights into what your prospective employers are looking for. What skills are required for you to go places? Do you know your industry well enough to impress your prospective employers?

Prospective employers will be judging you on 4 key aspects

1) Do you have the right technical “know how”?

…

11 Spring MVC Interview Q&As

Q1. Can you describe the Spring Web MVC framework architecture? A1. Spring Web MVC framework provides a Model View Controller architecture to develop loosely coupled 1) web applications 2) RESTful web services. … Read more ›...

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11: Hibernate dependency jars and plugins

Here is a sample maven pom.xml snippets containing the dependency jars.

....
<hibernate.version>4.1.3.Final</hibernate.version>
<hibernate-jpa-2.0-api.version>1.0.1.Final</hibernate-jpa-2.0-api.version>
<hibernate-validator-annotation-processor>4.2.0.Final</hibernate-validator-annotation-processor>
<hibernate-validator.version>4.3.0.Final</hibernate-validator.version>

....

<hibernate.version>4.1.3.Final</hibernate.version>

<hibernate-jpa-2.0-api.version>1.0.1.Final</hibernate-jpa-2.0-api.version>

<hibernate-validator-annotation-processor>4.2.0.Final</hibernate-validator-annotation-processor>

<hibernate-validator.version>4.3.0.Final</hibernate-validator.version>

The above snippet defines the version numbers. The snippet below defines the dependencies.

...
<dependency>
 <groupId>org.hibernate</groupId>
 <artifactId>hibernate-core</artifactId>
</dependency>
<dependency>
 <groupId>org.hibernate</groupId>
 <artifactId>hibernate-entitymanager</artifactId>
</dependency>
<dependency>
 <groupId>org.hibernate.javax.persistence</groupId>
 <artifactId>hibernate-jpa-2.0-api</artifactId>
</dependency>

<dependency>
 <groupId>javax.validation</groupId>
 <artifactId>validation-api</artifactId>
</dependency>

<dependency>
 <groupId>org.hibernate</groupId>
 <artifactId>hibernate-validator</artifactId>
</dependency>
   
<dependency>
 <groupId>org.hibernate.javax.persistence</groupId>
 <artifactId>hibernate-jpa-2.0-api</artifactId>
 <version>${hibernate-jpa-2.0-api.version}</version>
</dependency>

<dependency>
 <groupId>org.hibernate</groupId>
 <artifactId>hibernate-validator-annotation-processor</artifactId>
 <version>${hibernate-validator-annotation-processor}</version>
</dependency>


<!-- bean validation -->
<dependency>
 <groupId>javax.validation</groupId>
 <artifactId>validation-api</artifactId>
 <version>1.0.0.GA</version>
</dependency>

<dependency>
 <groupId>org.hibernate</groupId>
 <artifactId>hibernate-validator</artifactId>
 <version>${hibernate-validator.version}</version>
</dependency>

...

<groupId>org.hibernate</groupId>

<artifactId>hibernate-core</artifactId>

</dependency>

<groupId>org.hibernate</groupId>

<artifactId>hibernate-entitymanager</artifactId>

</dependency>

<groupId>org.hibernate.javax.persistence</groupId>

<artifactId>hibernate-jpa-2.0-api</artifactId>

</dependency>

<groupId>javax.validation</groupId>

<artifactId>validation-api</artifactId>

</dependency>

<groupId>org.hibernate</groupId>

<artifactId>hibernate-validator</artifactId>

</dependency>

<groupId>org.hibernate.javax.persistence</groupId>

<artifactId>hibernate-jpa-2.0-api</artifactId>

<version>${hibernate-jpa-2.0-api.version}</version>

</dependency>

<groupId>org.hibernate</groupId>

<artifactId>hibernate-validator-annotation-processor</artifactId>

<version>${hibernate-validator-annotation-processor}</version>

</dependency>

<groupId>javax.validation</groupId>

<artifactId>validation-api</artifactId>

</dependency>

<groupId>org.hibernate</groupId>

<artifactId>hibernate-validator</artifactId>

<version>${hibernate-validator.version}</version>

</dependency>

If you want to auto-generate your hibernate entities from DDL then.

   <build>
  <resources>
   <resource>
    <directory>${basedir}/src/main/resources</directory>
    <includes>
     <include>**/*</include>
    </includes>
   </resource>
  </resources>

  <plugins>
   <plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>hibernate3-maven-plugin</artifactId>
    <configuration>
     <components>
      <component>
       <name>hbm2ddl</name>
       <implementation>jpaconfiguration</implementation>
      </component>
      <component>
       <name>hbmdoc</name>
      </component>
     </components>
     <componentProperties>
      <persistenceunit>my-data</persistenceunit>
      <outputfilename>schema.ddl</outputfilename>
      <drop>true</drop>
      <create>true</create>
      <export>false</export>
      <format>true</format>
     </componentProperties>
    </configuration>
   </plugin>
  </plugins>
 </build>

<build>

<directory>${basedir}/src/main/resources</directory>

</includes>

</resource>

</resources>

<groupId>org.codehaus.mojo</groupId>

<artifactId>hibernate3-maven-plugin</artifactId>

<implementation>jpaconfiguration</implementation>

</component>

<name>hbmdoc</name>

</component>

</components>

<outputfilename>schema.ddl</outputfilename>

<export>false</export>

</componentProperties>

</configuration>

</plugin>

</plugins>

</build>

Finally, … Read more ›...

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11: JMockit interview Q&As & examples on matching withArgThat(org.hamcrest.Matcher)

This extends Mockit interview Q&As & examples on matching anyX field and withX() methods. JMockit offers a set of utility fields & methods for making argument matching more generic. 1) anyX fields like anyInt, … Read more ›...

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11: Part 4: Java Tree structure interview and coding questions

This is an extension to Java Tree structure interview questions and coding questions — Part 3 where recursion was used for FlattenTree interface. Let’s use iteration here. Step 1: The FlattenTree interface.

package com.mycompany.flatten;

public interface FlattenTree<T>
{
    void flatten(Tree<T> tree);
}

package com.mycompany.flatten;

public interface FlattenTree<T>

{

void flatten(Tree<T> tree);

}

Step 2: The iterative implementation … Read more ›...

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11: Threads performing tasks by talking to each other

Q. Given the following code


public class WorkingOnXandY {

    public static void main(String[] args) {
        SharedObject obj = new SharedObject();

        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 5; i++) {
                    obj.workOnX();
                }
            }
        });

        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 5; i++) {
                    obj.workOnY();
                }
            }
        });

        thread1.start();
        thread2.start();
    }
}

public class WorkingOnXandY {

public static void main(String[] args) {

SharedObject obj = new SharedObject();

Thread thread1 = new Thread(new Runnable() {

@Override

public void run() {

for (int i = 0; i < 5; i++) {

obj.workOnX();

}

});

Thread thread2 = new Thread(new Runnable() {

@Override

public void run() {

for (int i = 0; i < 5; i++) {

obj.workOnY();

}

});

thread1.start();

thread2.start();

}


public class SharedObject {

    private int x = 0;
    private int y = 0;
    private int a = 0;
    private int b = 0;

    public  void workOnX() {
        x = 1; // write to X
        a = x; // read from x
        System.out.println("a=" + a);
    }

    public synchronized void workOnY() {

        y = 1; // write to y
        b = y; // read from y
        System.out.println("b=" + b);
    }
}

public class SharedObject {

private int x = 0;

private int y = 0;

private int a = 0;

private int b = 0;

public void workOnX() {

x = 1; // write to X

a = x; // read from x

System.out.println("a=" + a);

}

public synchronized void workOnY() {

y = 1; // write to y

b = y; // read from y

System.out.println("b=" + b);

}

Output:


a=1
a=1
a=1
a=1
a=1
b=1
b=1
b=1
b=1
b=1

a=1

b=1

Complete the above code so that it prints a=1 and b=1 interleaved.


a=1
b=1
a=1
b=1
a=1
b=1

a=1

b=1

a=1

b=1

a=1

b=1

A. The solution is to have inter thread communication between both threads.

…

12 FAQ JDBC interview Q&As

Q1. What are JDBC Statements? What are different types of statements? How can you create them? A1. A statement object is responsible for sending the SQL statements to the Database. Statement objects are created from the connection object and then executed.

Statement stmt = myConnection.createStatement();
ResultSet rs = stmt.executeQuery(“SELECT id, name FROM myTable where id =1245”); //to read
                            or
stmt.executeUpdate(“INSERT INTO (field1,field2) values (1,3)”); //to insert

Statement stmt = myConnection.createStatement();

ResultSet rs = stmt.executeQuery(“SELECT id, name FROM myTable where id =1245”); //to read

stmt.executeUpdate(“INSERT INTO (field1,field2) values (1,3)”); //to insert

The types of statements are: Statement (regular statement...

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12 Java design patterns interview Q&As

Q1. Why use design patterns? A1. 1) Capture design experience from the past: E.g. Facade and value object patterns evolved from performance problems experienced due to multiple remote calls. 2) Promote reuse without having to reinvent the wheel: E.g. … Read more ›...

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12 things to know about job interviews

1. Interviews are not technical contests

to see who gets the most number of questions right. Nobody knows everything. so, don’t be overwhelmed by the number of Q&As on this site. It is all about effectively & confidently communicating your ability to add value to an organisation.

…

12 UML interview Q&As

Q1. What are the different types of UML diagrams? A1. Use case diagrams, Class diagrams, Package diagrams, Object diagrams, Sequence diagrams, Collaboration diagrams, and State chart diagrams. Q2. What is a use case diagram, and when will you use it? A2. … Read more ›...

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Blog Archives

10: What is wrong with this code? ConcurrentHashMap & Atomic operations

10+ Domain Driven Design (i.e. DDD) Interview Q&As

10+ Key Java Microservices Interview Questions & Answers

10+ Know your industry Q&As

11 Spring MVC Interview Q&As

11: Hibernate dependency jars and plugins

11: JMockit interview Q&As & examples on matching withArgThat(org.hamcrest.Matcher)

11: Part 4: Java Tree structure interview and coding questions

11: Threads performing tasks by talking to each other

12 FAQ JDBC interview Q&As

12 Java design patterns interview Q&As

12 things to know about job interviews

12 UML interview Q&As

300+ Java Interview Questions - FAQs

10+ Companion Techs Interview Q&As

300+ Big Data Interview Questions - FAQs

16+ Java Tech Key Areas Interview Q&As

Your Career Tips to go places

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