Interview Questions and Answers for 'AT' | Search Interview Question

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Q91. Which of the following Java feature promotes access protection or Hiding ?

a. Inheritance
b. Encapsulation
c. Abstraction
d. Composition

Core Java

Ans. Encapsulation

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Q92. Why Java is not considered pure OOP's language ?

Core Java

Ans. There are 2 reasons for it.

1. Usage of Primitive types - Though Java provides classes for the primitive data types but as the usage of primitives is permissible, its considered unpure OOP's language.

2. Usage of Static members - Static members belong to the class and not objects and hence not considered fit for pure OOP's programming.

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Q93. Why HashTable has been deprecated ?

Core Java

Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.'

Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so.

As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable.

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Q94. Difference beween HashSet and TreeSet ?

Core Java

Ans. 1. HashSet doesnt maintain its elements in any specific order and is all random whereas TreeSet maintains elements in natural order 9 order defined by the equals method of TreeSet element type )

2. TreeSet doesnt allow null elements whereas HashMap does.

3. As TreeSet orders elements and is hence insertion is comparatively slower.

4. HashSet performs basic operations like add(), remove(), contains(), size() etc. in a constant size time. A TreeSet performs these operations at the order of log(n) time.

5. HashMap in Java internally backs a HashSet. A NavigableMap backs a TreeSet internally.

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Q95. 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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Q96. 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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Q97. 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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Q98. What is a cookie ?

Java EE

Ans. A cookie is a small piece of text stored on a user's computer by the browser for a specific domain. Commonly used for authentication, storing site preferences, and server session identification.

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Q99. 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 at HCL Technologies ( Based on 3 feedback )

Q100. 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.

Q101. 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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Q102. 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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Q103. 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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Usually asked only to fresh graduates.

Q104. 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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Q105. 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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Q106. 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.

Q107. 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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Q108. 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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Very frequently asked in phone and walk in interviews.

Q109. What are Marker Interfaces ? Name few Java marker interfaces ?

Core Java

Ans. An interface without any method declaration is called as marker interface. there are 3 in-built interfaces in JVM i.e. serializable, clonable, remote

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Frequently asked to fresh graduates.

Q110. Difference between Process and Thread ?

Operating System

Ans. Process is a program in execution whereas thread is a separate path of execution in a program.

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Q111. Can we declare interface methods as private ?

Ans. With Java 9 we can declare interface methods as private

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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations.

Q112. Difference between StringBuffer and StringBuilder ?

Core Java

Ans. StringBuffer is synchronized whereas StringBuilder is not.

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Q113. Difference between Factory and Abstract Factory Design Pattern ?

Design

Ans. Factory Pattern deals with creation of objects delegated to a separate factory class whereas Abstract Factory patterns works around a super-factory which creates other factories.

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Q114. Difference between Factory and Builder Design Pattern ?

Design

Ans. Builder pattern is the extension of Factory pattern wherein the Builder class builds a complex object in multiple steps.

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Q115. Difference between Builder and Composite ?

Ans. Builder is a creational Design Pattern whereas Composite is a structural design pattern. Composite creates Parent - Child relations between your objects while Builder is used to create group of objects of predefined types.

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Q116. Difference between Factory and Strategy Design Pattern ?

Design

Ans. Factory is a creational design pattern whereas Strategy is behavioral design pattern. Factory revolves around the creation of object at runtime whereas Strategy or Policy revolves around the decision at runtime.

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Q117. How can we create objects if we make the constructor private ?

Core Java

Ans. We can do so through a static public member method or static block.

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Q118. What is reflection ?

Ans. It is the process of examining / modifying the behaviour of an object at runtime.

Sample Code for reflection

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Q119. When do you get ClassCastException?

or

What is ClassCastException ?

Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance.

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Q120. What are strong, soft, weak and phantom references in Java ?

Core Java

Ans. Garbage Collector wont remove a strong reference.

A soft reference will only get removed if memory is low.

A weak reference will get removed on the next garbage collection cycle.

A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to be notified that an object is about to be collected.

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