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 Q131. Why two types of memory - stack and heap are required by Java ?Core Java
Ans. Because of the life cycle requirement for different type of values in java.

variables initialized and used in functions needs to be destructed with the execution of function and hence kept in stack. Same is applicable for the object references initialized within the method. If objects would have been created in stack, they wouldnt have been passed around across methods and hence they are created on heap.

So anything that is required beyond the scope of a method or function is kept on heap which is usually garbage collected by Java.

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   Like         Discuss         Correct / Improve     memory management  heap  stack
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 Q132. Which elements of a class are ignored during serialization ?Core Java
Ans. 1. Objects are serialized and not classes and hence Static variables are ignored.

2. Transient is an explicit declaration to ignore the variable during serialization and hence transient instance variables are ignored too.

3. Base class instance variables if the base class hasn't been declared serializable.

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   Like         Discuss         Correct / Improve     serialization     Asked in 1 Companies      intermediate        frequent
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 Q133. What is the difference between these two method declarations ?

private static void method(String[] arg)

and

private static void method(String... arg)		Core Java
Ans. First expects the argument as a string array whereas second expects variable number of string arguments or a string array.

So we can call both by providing string array as an argument but second can be called with 0 to n string arguments which cannot be done for first.

for example - We can call second method with any of following

method();
method("Hello");
method("Hello","World");
method(new String[4]);


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   Like         Discuss         Correct / Improve     var args  methods  method declarations  functions

 Q134. What is the difference between inner class and sub class ?Core Java
Ans. Inner Class is a class that is nested within another class whereas sub class is a class that extends or inherit another class.

Inner class can only be accessed using reference of outer class ( Class name , if inner class is static or object reference, if inner class is non static ) whereas Sub class is accessed directly.

In terms of memory, inner class is stored just like another class having it's own body whereas sub class carries the body of parent class as well as it's own fields in memory.

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   Like         Discuss         Correct / Improve     inner classes  nested classes  inner class vs sub class  nested class vs sub class      Basic

 Q135. What is the difference between Graph's Breadth first and Depth First algorithm ?Algorithm
Ans. In Breadth first algorithm, all the adjacent nodes of the starting node is visited first and then the same rule is followed while moving inwards whereas

In Depth first algorithm, all the nodes of a single traversal path are visited first till a cycle or an end is found.

For example , given the following entries of adjacent nodes

1,2
1,3
1,6
2,4
2,5
3,6

The Breadth first path would be

1,2,3,6,4,5

and Depth first path would be

1,2,4,5,3,6

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   Like         Discuss         Correct / Improve     graph traversal  breadth first vs depth first        frequent

 Q136. Is this a valid initialization ? Explain.

Collection<Collection> collection = new LinkedList<LinkedList>();		Core Java
Ans. No. It will result in a type mismatch error.

Collection<Collection> collection = new LinkedList<Collection>();

is a valid initialization as collection being reference of "Collection" class can hold object of derived Class "LinkedList" due to runtime Polymorphism. Runtime polymorphism is not applicable to type arguments.

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   Like         Discuss         Correct / Improve     generics  type argument

 Q137. What are the benefits of using Wrapper classes over primitive types ?Core Java
Ans. With Collection classes , we cannot use primitive types. Moreover for any class using generic types, we cannot use primitive types.

They add more functionality by means of additional methods.

As their reference can be null , they offer consistent check for uninitialized state.

They facilitate caching and reuse by means of constant Pools.

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   Like         Discuss         Correct / Improve     wrapper classes  benefits of wrapper classes over primitives

 Q138. If arrays cannot be resized , Why is this code valid

String[] strArray = new String[2];
strArray = new String[5];		Core Java
Ans. We are not resizing the first array here but assigning the reference strArray to a new Array with size 5.

So after line 2, We have 2 arrays in memory, one with size 2 and other with size 5 with strArray referring to second array with size 5.

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 Q139. What are the different S3 storage Tiers ?Amazon Web Services (AWS)
Ans. S3 Standard
S3 IA - Infrequently Accessed
S3 One Zone IA - One Zone only , Infrequently Accessed
S3 Glacier - achieved

S3 Standard is most expensive , Glacier is least expensive

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   Like         Discuss         Correct / Improve     aws s3  aws storage  amazon cloud storage  amazon s3

 Q140. What are the factors on which customers are charged for using S3 ?Amazon Web Services (AWS)
Ans. Storage Tier ( IA , Standard , Glacier etc )
Storage ( Volume of Data )
No of Requests
Meta Data
Data Transfer Speed acceleration through cache and Edge Location

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 Q141. What are the factors on which price classes are based on in case of EC2, S3, CloudFront and EBS ?Amazon Web Services (AWS)
Ans. EC2 - Time and Reservation , Instance Type ( Memory , Computation power )
S3 - Accessibility Frequency , Data Transfer , Storage Space
CloudFront - Geography, No of Locations
EBS - IOPS, Access Frequency

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 Q142. What are the different Route53 routing policies ?Amazon Web Services (AWS)
Ans. Simple
Weighted
Latency
Failover
GeoLocation
Multivalue

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   Like         Discuss         Correct / Improve     aws route53     Asked in 1 Companies

 Q143. What is the relationship between VPC and subnet ? Can you tell with an example ?Networking
Ans. A VPC is devided into multiple subnets. Subnet in a particular VPC could be public or private.

For Example - A particular account may have multiple vpc's for each of the business type. For each Business or VPC , One can have different subnets to cater to each department so that access across departments could be restricted. But still there could be identities that have control over different subnets. Moreover we could have multiple subnets for each department so as to have distinguished public and private networks having different types of applications ( public facing or internal consumption )

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   Like         Discuss         Correct / Improve     aws vpc  aws subnet  subnet  vpc  Amazon Web Services (AWS)

  Q144. What are the different types of inner classes ?Core Java
Ans. Simple Inner Class,
Local Inner Class,
Anonymous Inner Class,
Static Nested Inner Class.

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Frequently asked in Indian Service Companies. Tech Mahindra ( Based on 3 inputs ) and Infosys ( 3 inputs )
  Q145. What is the difference between List, Set and Map ?

or

What are the different Java Collections Interfaces ?		Core Java
Ans. List - Members are stored in sequence in memory and can be accessed through index.
Set - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates.
Map - Contains Key , Value pairs.

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  Q146. Does Java support Multiple Inheritance ?Core Java
Ans. Java doesn't support multiple inheritance. Interfaces does't facilitate inheritance and hence implementation of multiple interfaces doesn't make multiple inheritance.

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Frequently asked at HCL Technologies ( Based on 3 feedback )
  Q147. Difference between Checked and Unchecked exceptions ?Core Java
Ans. Checked exceptions are the exceptions for which compiler throws an errors if they are not checked whereas unchecked exceptions are caught during run time only and hence can't be checked.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q148. What is a Final Variable ?Core Java
Ans. Final variable is a variable constant that cannot be changed after initialization.

  Sample Code for final variable

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 Q149. When are static variables loaded in memory ?Core Java
Ans. They are loaded at runtime when the respective Class is loaded.

  Sample Code for static variable

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 Q150. Can we serialize static variables ?Core Java
Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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 Q151. Can we use null as a key for a map collection ?Core Java
Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

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Usually asked only to fresh graduates.
 Q152. What is the difference between time slicing and preemptive scheduling ?Operating System
Ans. In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher priority task comes into existence. In time slicing, the task continues its execution for a predefined period of time and reenters the pool of ready tasks.

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 Q153. What one should take care of, while serializing the object?
Ans. One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializable Exception.

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 Q154. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Ans. Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

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 Q155. Are constructors inherited? Can a subclass call the parent's class constructor? When?Core Java
Ans. You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of one of it's superclasses. One of the main reasons is because you probably don't want to override the superclasses constructor, which would be possible if they were inherited. By giving the developer the ability to override a superclasses constructor you would erode the encapsulation abilities of the language.

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 Q156. Difference between Abstract and Concrete Class ?Core Java
Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

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  Q157. Difference between Vector and ArrayList ?Core Java
Ans. Vectors are synchronized whereas Array lists are not.

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Very Frequently asked. Usually asked along with String Class related questions.
  Q158. What is an immutable class ?Core Java
Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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 Q159. Difference between object instantiation and construction ?Core Java
Ans. Though It's often confused with each other, Object Creation ( Instantiation ) and Initialization ( Construction ) are different things in Java. Construction follows object creation.

Object Creation is the process to create the object in memory and returning its handler. Java provides New keyword for object creation.

Initialization is the process of setting the initial / default values to the members. Constructor is used for this purpose. If we don't provide any constructor, Java provides one default implementation to set the default values according to the member data types.

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Frequently asked to fresh graduates and less experienced.
 Q160. Which of the following is tightly bound ? Inheritance or Composition ?Core Java
Ans. Inheritance.

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