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Q901. Explain OOps concepts.

Ans. There are four main OOP concepts in Java. These are:

Abstraction. Abstraction means using simple things to represent complexity. We all know how to turn the TV on, but we don?t need to know how it works in order to enjoy it. In Java, abstraction means simple things like objects, classes, and variables represent more complex underlying code and data. This is important because it lets avoid repeating the same work multiple times.

Encapsulation. This is the practice of keeping fields within a class private, then providing access to them via public methods. It?s a protective barrier that keeps the data and code safe within the class itself. This way, we can re-use objects like code components or variables without allowing open access to the data system-wide.

Inheritance. This is a special feature of Object Oriented Programming in Java. It lets programmers create new classes that share some of the attributes of existing classes. This lets us build on previous work without reinventing the wheel.

Polymorphism. This Java OOP concept lets programmers use the same word to mean different things in different contexts. One form of polymorphism in Java is method overloading. That?s when different meanings are implied by the code itself. The other form is method overriding. That?s when the different meanings are implied by the values of the supplied variables. See more on this below.

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Frequently asked in Genpact.

Q902. What is object cloning ?

Ans. Its is the process of creating exact copy of an object being cloned. In Object class one native method called clone() is there which is meant for Shallow Cloning of Object. Shallow cloning means bitwise copy of an object.In case of primitive data type it will create an exact copy of primitive values as well as variables but if the object contains any reference of an object then it will not copy the referenced object rather it will create the copy of reference variable and assigned it to the old object.

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Q903. What are different methods to get the permanent gen space of running application?

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Q904. Create a custom hashmap using synchronization.

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Q905. Explain JVM architecture.

Ans. https://dzone.com/articles/jvm-architecture-explained

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Q906. What is Method overloading ?

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Q907. What is collection framework in Java ?

Ans. The Java collections framework (JCF) is a set of classes and interfaces that implement commonly reusable collection data structures. Although referred to as a framework, it works in a manner of a library. The JCF provides both interfaces that define various collections and classes that implement them.

Sample Code for Map

Sample Code for HashMap

Sample Code for Treemap

Sample Code for set

Sample Code for hashset

Sample Code for treeset

Sample Code for list

Sample Code for arraylist

Sample Code for linkedlist

Sample Code for queue

Sample Code for priorityqueue

Sample Code for concurrenthashmap

Sample Code for vector

Sample Code for stack

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Q908. What is Jar ?

Ans. It is a file format based on the popular ZIP file format and is used for aggregating many files into one.

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Q909. What is the purpose of JDP and JDK ?

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Q910. What is an Interface ?

Ans. The interface is a mechanism to achieve abstraction. Interfaces can have abstract methods and variables. It cannot have a method body.

Sample Code for interface

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Q911. What is composition ?

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Sample Code for composition

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Q912. In the Following code which foo will get called.

foo(Integer i){
}

foo(String s){
}

public static void main(){
passing foo(null);
}

Ans. ambiguity error

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Q913. Can you give a real life example of synchronization

Ans. Intersection with traffic light

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Q914. How do you define a functional interface?

Ans. Create interface with the only one non-overriding abstract method and annotate it with @FunctionalInterface

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Q915. When do you use a Vector?

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Q916. What is Overriding ?

Ans. Overriding refers to the Methods with same name and parameters, such that the later over rides the former method.

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Q917. What is abstraction ?

Ans. The process of hiding the implementation details from the user and providing only functionality.

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Q918. Can we modify a list while iterating it ?

Ans. No,we cannot.I t will give concurrentModificationExceptin error. It can be resolved by using ConcurrentClasses like ConcurrentHashMap,CopyOnWriteArrayList,BlockingQueue etc which are fail-safe and wont give exception.

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Q919. What are the characteristics of the object ?

Ans. 1. State

Instance variable or member elements forms the object state.

2. Behavior

Member functions forms the object behavior

3. Identity

Every object has a unique identity within JVM through a unique Id.

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Q920. How can we make objects if the constructor is private ?

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Q921. Why String is a class in Java ?

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Q922. Write a Program to find number of lines , words and characters in a File.

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

public class FileUtility {

public static void main(String[] args) {
System.out.println(printFile("/home/userme/notes"));
}

private static int printFile(String filePath) {
int wordCount = 0;
try (BufferedReader br = new BufferedReader(new FileReader(filePath))) {
String line = null;
String[] wordsArray = null;

while ((line = br.readLine()) != null) {
wordsArray = line.split(" ");
wordCount = wordsArray.length;
}
} catch (IOException e) {
e.printStackTrace();
}
return wordCount;

}
}

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Q923. Given a String with some Code, Write a Parser to have proper indentation.

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Q924. How do you make instance of an abstract class ?

Ans. By extending it to the derived class and thereby creating instance of derived class.

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Q925. Explain how Mark & Sweep is implemented in garbage collection.

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Q926. How would you stop a Thread in java ?

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Q927. Write code for serialization.

Ans. // Java code for serialization and deserialization
// of a Java object
import java.io.*;

class Demo implements java.io.Serializable
{
   public int a;
   public String b;

   // Default constructor
   public Demo(int a, String b)
   {
      this.a = a;
      this.b = b;
   }

}

class Test
{
   public static void main(String[] args)
   {
      Demo object = new Demo(1, "geeksforgeeks");
      String filename = "file.ser";

      // Serialization
      try
      {
         //Saving of object in a file
         FileOutputStream file = new FileOutputStream(filename);
         ObjectOutputStream out = new ObjectOutputStream(file);

         // Method for serialization of object
         out.writeObject(object);

         out.close();
         file.close();

         System.out.println("Object has been serialized");

      }

      catch(IOException ex)
      {
         System.out.println("IOException is caught");
      }

      Demo object1 = null;

      // Deserialization
      try
      {
         // Reading the object from a file
         FileInputStream file = new FileInputStream(filename);
         ObjectInputStream in = new ObjectInputStream(file);

         // Method for deserialization of object
         object1 = (Demo)in.readObject();

         in.close();
         file.close();

         System.out.println("Object has been deserialized ");
         System.out.println("a = " object1.a);
         System.out.println("b = " object1.b);
      }

      catch(IOException ex)
      {
         System.out.println("IOException is caught");
      }

      catch(ClassNotFoundException ex)
      {
         System.out.println("ClassNotFoundException is caught");
      }

   }
}

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Q928. If compareTo() method returns zero, does it mean that the objects are always equal?

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Q929. Write a method receiving a stream of integers, caching the last 10 min of data and returning a number if it's less than the current input

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Q930. How would you write a socket in Java

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