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Java enum interview questions & answers

Covers the power of enumand 3 design patterns that can be applied with the help of enums. Q1. Why is it a best practice to use enums over static final int or String constants? A1. 1) static final allows invalid values. … Read more ›...

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Java ExecutorService with strategy design pattern to execute concurrently & sequentially

Java executor framework for running multi-threaded applications concurrently, sequentially, and with a strategy design pattern to switch between sequential and concurrent execution. 1. Running concurrently

package com.writtentest13;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 
 * Tasks are executed concurrently in a thread pool
 */
public class ThreadExecutorMainCon  {

 private ExecutorService concurrentThreadExecutor;

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

  // concurrent executor
  main.concurrentThreadExecutor = Executors.newCachedThreadPool();// or
                  // newFixedThreadPool(3)
  List<Callable<Boolean>> tasks = new ArrayList<Callable<Boolean>>();

  // create dummy tasks
  for (int i = 1; i <= 5; i++) {
   tasks.add(main.createTask(i));
  }

  // submit the tasks to the concurrentThreadExecutor
  try {
      main.concurrentThreadExecutor.invokeAll(tasks);

  } catch (InterruptedException e) {
       e.printStackTrace();
  }

  System.out.println("Completed .........");

 }

 private Callable<Boolean> createTask(final int i) {

  Callable<Boolean> task = new Callable<Boolean>() {

   @Override
   public Boolean call() throws Exception {
      System.out.println("Performing task " + i + " on thread - " + Thread.currentThread().getName());
      return true;
   }

  };

  return task;

 }
}

package com.writtentest13;

import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.Callable;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

/**

* Tasks are executed concurrently in a thread pool

public class ThreadExecutorMainCon {

private ExecutorService concurrentThreadExecutor;

public static void main(String[] args) {

ThreadExecutorMainCon main = new ThreadExecutorMainCon();

// concurrent executor

main.concurrentThreadExecutor = Executors.newCachedThreadPool();// or

// newFixedThreadPool(3)

List<Callable<Boolean>> tasks = new ArrayList<Callable<Boolean>>();

// create dummy tasks

for (int i = 1; i <= 5; i++) {

tasks.add(main.createTask(i));

}

// submit the tasks to the concurrentThreadExecutor

try {

main.concurrentThreadExecutor.invokeAll(tasks);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println("Completed .........");

}

private Callable<Boolean> createTask(final int i) {

Callable<Boolean> task = new Callable<Boolean>() {

@Override

public Boolean call() throws Exception {

System.out.println("Performing task " + i + " on thread - " + Thread.currentThread().getName());

return true;

}

};

return task;

}

The output will be

Performing task 1 on thread - pool-1-thread-1
Performing task 5 on thread - pool-1-thread-4
Performing task 4 on thread - pool-1-thread-1
Performing task 3 on thread - pool-1-thread-3
Performing task 2 on thread - pool-1-thread-2
Completed .........

Performing task 1 on thread - pool-1-thread-1

Performing task 5 on thread - pool-1-thread-4

Performing task 4 on thread - pool-1-thread-1

Performing task 3 on thread - pool-1-thread-3

Performing task 2 on thread - pool-1-thread-2

Completed .........

You can see more than 1 thread process 5 tasks concurrently. … Read more ›...

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Java Generics in no time “? extends” & “? super” explained with a diagram

Generics in Java can be be a bit tricky to get your head around. Hope the explanation below enhances your understanding of generics. This complements 5 Java generics interview Q&As with examples. Raw List, List <Object>, and List<?> A raw List: is a heterogeneous mixture or a mixed bag that...

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Java identifiers

Q1. Which of these are legal Identifiers in Java? a) $Ident1 b) _Ident1 c) -Ident1 d) 2Ident1 e) private f) private1 g) null h) Ident-1 i) Ident$1 j) \u00A3Ident1 k) \u00A5Ident1 A1. … Read more ›...

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Java initializers, constructors, regular methods and static factory methods – when to use them with examples.

Q1. What are “static initializers” or “static blocks with no function names” in Java? A1. When a class is loaded, all blocks that are declared static and don’t have function name (i.e. static initializers) are executed even before the constructors are executed. … Read more ›...

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Java serialization, cloning, and casting interview Q&A

Q1. Which Java interface must be implemented by a class whose instances are transported via a Web service? a. Accessible b. BeanInfo c. Remote d. Serializable A1. Answer is “d”. Q2. What is serialization? A2. … Read more ›...

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Learning to write functional programming with Java 8 with examples

Scenario 1: The Operation interface with the annotation @FunctionalInterface. This annotation ensures that you can only have a single abstract method. You can have additional default and static method implementations. Step 1: Define the interface. … Read more ›...

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Mocks, stubs, domain, and anemic objects interview Q&A

Q1. What are mock objects? A1. Mock objects are used in unit testing to ensure that your tests don’t fail due to volatility of the data changes. There are mocking frameworks like EasyMock, Mockito, and PowerMock. … Read more ›...

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Multithreading interview Q&As on differences between X and Y

Q1. What is difference between ‘Executor.submit()’ and ‘Executor.execute()’ method ? A1. The difference is that execute() doesn’t return a “Future” object, so you can’t wait for the completion of the Runnable, and get any exception it throws. The submit(…) method accepts Callable<V> and returns an instance of Future<V>, … Read...

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Non-trival Java 8 tutorial to appreciate the power of functional programming (FP)

Prerequisite: Understanding of Java 8 streams.

Q. You have a list of key value pairs (“student name”, “subject”) as shown below.

Input

("Sam Peter", "English"),
("Sam Peter", "Science"),
("John English", "Science"),
("Neil Graham", "Maths"),
("Neil Graham", "Arts"),
("Neil Graham", "Music")

("Sam Peter", "English"),

("Sam Peter", "Science"),

("John English", "Science"),

("Neil Graham", "Maths"),

("Neil Graham", "Arts"),

("Neil Graham", "Music")

The required task is to list all the subjects for each student as shown below.

Result

"Samp Pter"    --> [English, Science]
"John English" --> [Science]
"Neil Graham"  --> [Maths, Arts, Music]

"Samp Pter" --> [English, Science]

"John English" --> [Science]

"Neil Graham" --> [Maths, Arts, Music]

…

Object equals Vs == and pass by reference Vs value

Q1. What is the difference between “==” and equals(..) method when comparing 2 objects?
A1.It is important to understand the difference between identity (i.e. ==) comparison, which is a shallow comparison that compares only the object references, and the equals( ) comparison,

…

Processing large files efficiently – BigData & Hadoop – part 3

Q1. What are the different data sizes, and what technologies can be used to process them? A1. In general, data sizes can be classified as shown below. 1) Small size data is < 10 GB in multiple files. … Read more ›...

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Blog Archives

Java enum interview questions & answers

Java ExecutorService with strategy design pattern to execute concurrently & sequentially

Java Generics in no time “? extends” & “? super” explained with a diagram

Java identifiers

Java initializers, constructors, regular methods and static factory methods – when to use them with examples.

Java serialization, cloning, and casting interview Q&A

Learning to write functional programming with Java 8 with examples

Mocks, stubs, domain, and anemic objects interview Q&A

More Java annotations interview questions & answers

Multithreading interview Q&As on differences between X and Y

Non-trival Java 8 tutorial to appreciate the power of functional programming (FP)

Object equals Vs == and pass by reference Vs value

Processing large files efficiently – BigData & Hadoop – part 3

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