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Java - Interview Questions and Answers

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 Q101. Are constructors inherited? Can a subclass call the parent's class constructor? When?Core Java
Anonymous
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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Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives.
  Q102. Difference between Overloading and Overriding ?Core Java
Anonymous
Ans. Overloading - Similar Signature but different definition , like function overloading.

Overriding - Overriding the Definition of base class in the derived class.

  Sample Code for overloading

  Sample Code for overriding

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  Q103. Difference between Vector and ArrayList ?Core Java
Anonymous
Ans. Vectors are synchronized whereas Array lists are not.

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  Q104. What is Serialization ? Why do we need it ?Core Java
Anonymous
Ans. Storing the state of an object in a file or other medium is called serialization.

Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application.

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 Q105. Difference between object instantiation and construction ?Core Java
Anonymous
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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 Q106. Can we access instance variables within static methods ?Core Java
Anonymous
Ans. Yes.we cannot access them directly but we can access them using object reference.Static methods belong to a class and not objects whereas non static members are tied to an instance. Accessing instance variables without the instance handler would mean an ambiguity regarding which instance the method is referring to and hence its prohibited.

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Very frequently asked in phone and walk in interviews.
  Q107. What are Marker Interfaces ? Name few Java marker interfaces ?Core Java
Anonymous
Ans. These are the interfaces which have no declared methods.Serializable and cloneable are marker interfaces.

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 Q108. Explain java.lang.OutOfMemoryError ?Core Java
Anonymous
Ans. This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.

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 Q109. Difference between Factory and Abstract Factory Design Pattern ?Design
Anonymous
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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Advanced level question. Recently asked in few Indian service companies ( Based on 3 inputs )
 Q110. What are various types of Class loaders used by JVM ?Core Java
Admin
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Ans. Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE

System - Loads classes from system classpath.

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 Q111. Difference between C++ and Java ?Core Java
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Ans. Java does not support pointers.

Java does not support multiple inheritances.

Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method.

Java does not include structures or unions because the traditional data structures are implemented as an object oriented framework.

C++ compiles to machine language , when Java compiles to byte code .

In C++ the programmer needs to worry about freeing the allocated memory , where in Java the Garbage Collector takes care of the the unneeded / unused variables.

Java is platform independent language but c++ is depends upon operating system.

Java uses compiler and interpreter both and in c++ their is only compiler.

C++ supports operator overloading whereas Java doesn't.

Internet support is built-in Java but not in C++. However c++ has support for socket programming which can be used.

Java does not support header file, include library files just like C++ .Java use import to include different Classes and methods.

There is no goto statement in Java.

There is no scope resolution operator :: in Java. It has . using which we can qualify classes with the namespace they came from.

Java is pass by value whereas C++ is both pass by value and pass by reference.

Java Enums are objects instead of int values in C++

C++ programs runs as native executable machine code for the target and hence more near to hardware whereas Java program runs in a virtual machine.

C++ was designed mainly for systems programming, extending the C programming language whereas Java was created initially to support network computing.

C++ allows low-level addressing of data. You can manipulate machine addresses to look at anything you want. Java access is controlled.

C++ has several addressing operators . * & -> where Java has only one: the .

We can create our own package in Java(set of classes) but not in c and c++.

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Frequently asked to fresh graduates.
 Q112. Explain Thread States ?Operating System
Admin
info@buggybread.com
Ans. Runnable - waiting for its turn to be picked for execution by the thread schedular based on thread priorities.

Running - The processor is actively executing the thread code. It runs until it becomes blocked, or voluntarily gives up its turn.

Waiting: A thread is in a blocked state while it waits for some external processing such as file I/O to finish.

Sleeping - Java threads are forcibly put to sleep (suspended) with Thread.sleep. they can resume using Thread.resume method.

Blocked on I/O - Will move to runnable after I/O condition like reading bytes of data etc changes.

Blocked on synchronization - Will move to Runnable when a lock is acquired.

Dead - The thread is finished working.

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 Q113. What are strong, soft, weak and phantom references in Java ?Core Java
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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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 Q114. Difference Between this() and super() ?
Core Java
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Ans. 1.this is a reference to the current object in which this keyword is used whereas super is a reference used to access members specific to the parent Class.

2.this is primarily used for accessing member variables if local variables have same name, for constructor chaining and for passing itself to some method whereas super is primarily used to initialize base class members within derived class constructor.

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Very frequently asked.Usually among first few questions.
  Q115. What is MVC ? Design
Anonymous
Ans. The Model/View/Controller pattern, a strategy for dividing responsibility in a GUI component.

The model is the data for the component.

The view is the visual presentation of the component on the screen.

The controller is responsible for reacting to events by changing the model.

According to the MVC pattern, these responsibilities should be handled by different objects.

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 Q116. What are the disadvantages of using arrays ?Core Java
Admin
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Ans. Arrays are of fixed size and have to reserve memory prior to use. Hence if we don't know size in advance arrays are not recommended to use.

Arrays can store only homogeneous elements.

Arrays store its values in contentious memory location. Not suitable if the content is too large and needs to be distributed in memory.

There is no underlying data structure for arrays and no ready made method support for arrays, for every requriment we need to code explicitly

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 Q117. What are the Disadvantages of using Collection Classes over Arrays ?Core Java
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Ans. Collections can only hold objects, It can't hold primitive data types.

Collections have performance overheads as they deal with objects and offer dynamic memory expansion. This dynamic expansion could be a bigger overhead if the collection class needs consecutive memory location like Vectors.

Collections doesn't allow modification while traversal as it may lead to concurrentModificationException.

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 Q118. What is Generalization and Specialization in terms of casting ?Core Java
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Ans. Generalization or UpCasting is a phenomenon where a sub class is prompted to a super class, and hence becomes more general. Generalization needs widening or up-casting. Specialization or DownCasting is a phenomenon where a super class is narrowed down to a sub class. Specialization needs narrowing or down-casting.

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 Q119. How compiler handles the exceptions in overriding ?Core Java
Admin
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Ans. 1)The overriding methods can throw any runtime Exception , here in the case of runtime exception overriding method (subclass method) should not worry about exception being thrown by superclass method.

2)If superclass method does not throw any exception then while overriding, the subclass method can not throw any new checked exception but it can throw any runtime exception

3) Different exceptions in java follow some hierarchy tree(inheritance). In this case , if superclass method throws any checked exception , then while overriding the method in subclass we can not throw any new checked exception or any checked exception which are higher in hierarchy than the exception thrown in superclass method

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 Q120. Similarity and Difference between static block and static method ?Core Java
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Ans. Both belong to the class as a whole and not to the individual objects. Static methods are explicitly called for execution whereas Static block gets executed when the Class gets loaded by the JVM.

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Very frequently asked in companies using SOA.
  Q121. What are RESTful Web Services ?Rest
Admin
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Ans. REST or Representational State Transfer is a flexible architecture style for creating web services that recommends the following guidelines -

1. http for client server communication,
2. XML / JSON as formatiing language ,
3. Simple URI as address for the services and,
4. stateless communication.

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 Q122. Why are Getter and Setter better than directly changing and retrieving fields ?

or

What is the significance of variable hiding in Java ?

or

Why it's preferred to keep variables private and allow their access through getter and setter methods ?
Core Java
Admin
info@buggybread.com
Ans. 1. Methods can participate in runtime polymorphism whereas member variables cannot.

For example -

Vehicle vehicle = new Car();

car.getObjVariable();// will return variable defined in Car

whereas

car.variable

will return the variable from Car class irrespective of the object it holds.

2. Validations can be performed before setting the variables.

3. If the input format changes or some other value or calculated value needs to be returned , that can be absorbed by making change ( wrapping ) in the setter and getter. By this the call to method from outside or interface to the object will remain the same.

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 Q123. How to configure second level cache in Hibernate ?Hibernate
Admin
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Ans. 1. Configure Provider class in Hibernate configuration file.

2. Add Cache usage tag ( read-only or read-write ) in mapping files ( hbm ).

3. Create an XML file called ehcache.xml and place in classpath which contains time settings and update settings, behavior of cache , lifetime and idletime of Pojos, how many objects are allowed.

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 Q124. What are the contents of Hibernate configuration file ( hibernate.cfg.xml ) ?Hibernate
Admin
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Ans. HBM Files ( Mapping )
DB Connection ( DB Connection String , User Name , Password , Pool Size )
SQL Dialect ( SQL variant to be generated )
Show SQL ( Show / No show SQL on Console )
Auto Commit ( True / False )

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 Q125. Which annotations are used in Hibernate ?Hibernate
admin
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Ans. @Entity
@Table
@Id
@Column
@Temporal
@Basic
@Enumerated
@Access
@Embeddable
@Lob
@AttributeOverride
@Embedded
@GeneratedValue
@ElementCollection
@JoinTable
@JoinColumn
@CollectionId
@GenericGenerator
@OneToOne
@OneToMany
@ManyToOne
@ManyToMany
@NotFound

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 Q126. What things you would care about to improve the performance of Application if its identified that its DB communication that needs to be improved ?Solution
Admin
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Ans. 1. Query Optimization ( Query Rewriting , Prepared Statements )

2. Restructuring Indexes.

3. DB Caching Tuning ( if using ORM )

4. Identifying the problems ( if any ) with the ORM Strategy ( If using ORM )

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 Q127. Can we override compareTo method for Enumerations ?Core Java
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Ans. No. compareTo method is declared final for the Enumerations and hence cannot be overriden. This has been intentionally done so that one cannot temper with the sorting order on the Enumeration which is the order in which Enum constants are declared.

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 Q128. If you are given a choice to implement the code to either Insert a Record or Update if already exist, Which approach will you follow ?

1. Insert into the DB Table. If exception occurs, update the existing record.
2. Check if the record exists and update it if it exists, If not insert a new record.
Solution
admin
info@buggybread.com
Ans. In first case, there would be 2 DB calls in worst case and 1 in best case. In 2nd approach there will be always 2 DB calls.

Decision on the approach should depend on the following considerations -

1. How costly is the call to DB ? Are we using indices , hibernate etc

If calls to DB are costly , 1st approach should be the choice.

2. Exception Book keeping load upon exception.

The benefit of saving 1st call in approach 1 should be bigger than the Book keeping for the exception.

3. Probability of the exception in first apparoach.

If the DB Table is almost empty, it makes sense to follow Approach 1 as majority of the 1st calls will pass through without exception.

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Very Frequently asked.
 Q129. Explain throw, throws , try and catch in Java ?Core Java
Admin
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Ans. throw is used to re throw an exception.

throws is used to declare that the method throws the respective exceptions.

try block is used to identify if the respective block has thrown any exception.

catch is used to catch the exception that has been thrown by the respective try block.

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 Q130. What is a transitive dependency ? Can we override Transitive Dependency version and If Yes, how ?Maven
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Ans. Transitive dependency is the dependencies not defined directly in the current POM but the POM of the dependent projects.

Yes we can override transitive dependency version by specifying the dependency in the current POM.

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Usually asked to experienced developers.
 Q131. What is a cyclic dependency ?Maven
Admin
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Ans. A has dependency of B, B has dependency of C and C has dependency of A,architecture,technical lead

With Maven 2 , came transitive dependency wherein in above scenario, C will acts as a dependency of A as if this dependency has been defined directly in A but the negative side is that if it leads to cyclic dependency , it creates problems.

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 Q132. Which of the following collections stores its elements in insertion Order ?

a. HashMap
b. TreeMap
c. LinkedHashMap
d. LinkedMap
Core Java
Anonymous
Ans. LinkedHashMap

  Sample Code for LinkedHashMap

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 Q133. In what order the elements of a HashSet are retrieved ?

a. Random Order
b. Insertion Order
c. Natural Sorting Order
d. Inverse Natural Sorting Order
Core Java
Anonymous
Ans. Random Order

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 Q134. Which of the following file is called deployment descriptor ?

a. application.xml
b. project.xml
c. web.xml
d. build.xml
Java EE
Anonymous
Ans. web.xml

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 Q135. Which of the following is dependency exclusion ?

a. A doesn't depend on C and then A marks C as excluded.
b. A Depends on B and B depends on C and then A can mark B as excluded.
c. A and B depends on C and then they can mark C as excluded.
d. A depends upon B and B depends upon C then A marks C as excluded.
Maven
Anonymous
Ans. A depends upon B and B depends upon C then A marks C as excluded.

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 Q136. Which of the following is false about Constructors ?

a. Constructor can be overloaded
b. A no argument constructor is provided by the compiler if we declare only constructors with arguments.
c. Constructors shouldn't have any return types , not even void.
d. If super is not explicitly called, still super() is intrinsically added by the compiler.
Core Java
Anonymous
Ans. A no argument constructor is provided by the compiler if we declare only constructors with arguments.

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 Q137. Which of the following doesn't extend Collection interface ?

a. Set
b. List
c. Map
d. Queue
Core Java
Anonymous
Ans. Map

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 Q138. Which of the following Java feature promotes Code Re usability ?

a. Abstraction
b. Encapsulation
c. Inheritance
d. Polymorphism
Core Java
Anonymous
Ans. Inheritance

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 Q139. Which of the following Java feature promotes access protection or Hiding ?

a. Inheritance
b. Encapsulation
c. Abstraction
d. Composition
Core Java
Anonymous
Ans. Encapsulation

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 Q140. Which of the following is not a build tool ?

a. Maven
b. Ant
c. Gradle
d. svn
Tool
Anonymous
Ans. svn

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 Q141. enums are intrinsically ..

a. private
b. public
c. static
d. final
Core Java
Anonymous
Ans. static

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 Q142. Difference between parameters and arguments ?Core Java
Admin
info@buggybread.com
Ans. Parameters are the variables that the method is expected to receive along with the method call. Arguments are the values which are passed on while calling the methods.

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 Q143. In which cases , moving methods to utility class could be useful ?Core Java

Ans. It could be worthy to move a method to util class if the method needs to be shared, doesn't require polymorphic behavior and need not be overridden in special cases.

Don't belong to one group through is-a relationship ( You can share through parent class method )

Don't implement a specific interface ( java 8 default methods )

Doesn't involve complex computing as you will be loosing the benefit of object state with just static method.

Doesn't require polymorphic behavior as static methods don't participate in runtime polymorphism.

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 Q144. 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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This question can be asked in different variants like performace improvement of an application, improving performance for DB communication etc.
 Q145. What are the ways in which execution time of the code can be improved ?Solution2016-05-29 09:42:39

Ans. This depends on entirely upon the code type. For example

1.If its just logic, we can try it to short circuit or put the if / switch case with maximum probability in the beginning.

2. Can use faster data structures, for eg - random retrieval instead of sequence / iterator

3. Working with primitive types or even bytes instead of Objects, even though it may result in marginal improvement.

4. If its service call, then service call with bulk load can help

5. If DB Operation, then with use of Indices , Views or using ORM , cache etc.


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 Q146. Why HashTable has been deprecated ?Core Java2016-07-08 10:01:58

Ans. HashTable has been Deprecated as an alternative ConcurrentHashMap has been provided which 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 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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Frequently asked to fresh graduates.
 Q147. What is ACID ?Database2016-11-04 08:09:44

Ans. ACID stands for Atomicity, Consistency, Isolation, Durability is a set of properties of database transactions.

Atomicity means all or nothing. i.e parts of a transaction shouldn't commit if any one of them fails. Either the whole transaction should succeed or it should be complete rollback.

Consistency means that any transaction should lead database from one stabe state to another.

Isolation means that the execution of transaction results in a system state that would be obtained if transactions were executed serially.

Durability means that when a transaction is committed it forms the permanent state of database.

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 Q148. How do you resolve Maven Dependencies issue while upgrading dependencies versions ?Maven2016-11-11 15:06:45

Ans. Step 1 - Upgrade the required dependency , perform build and check build errors

Step 2.1 - If the error is of missing transitive dependency ( which is rare and means that the previous version has a dependency which is missing in the later version ), I would look for the dependency in google and hence will include it as direct dependency in Pom file.

Step 2.2. If the error is for Duplicate dependencies , and the choice is between transitive and direct dependency, I usually remove the direct dependency.

Step 2.3 - If the error is for Duplicate dependencies , and both are transitive dependencies. I first make a choice ( usually later version ) and then ignore the previous version dependency.

Step 2.4 - If the error is for Duplicate dependencies , and there are more than 2 duplicates, I usually ignore it by specifying within maven-enforcer-plugin config.

Step 3 - Perform a Clean Build.

Step 4 - Check Maven Dependency Tree to make sure that Duplicates have been removed or dependency is there in case of missing dependency.

Step 5 - Perform tests and make sure that there are no runtime problems.

Step 6 - If there are runtime problems ( which very likely occurs if you have different version dependencies , very likely by doing 2.4 , you will have to remove step 2.4 and alternately perform 2.2 or 2.3 )

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 Q149. Difference beween HashSet and TreeSet ?Core Java2016-11-27 19:08:35

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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 Q150. Can you write a "Hello World" program without using any ";" within it?Core Java2016-12-13 08:26:23

Ans. Yes, That is possible

class A {
public static void main(String args[]){
if(System.out.printf("Hello World")==null){}
}
}

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 Q151. Why two types of memory - stack and heap are required by Java ?Core Java2016-12-21 13:08:55

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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 Q152. What are different types of variables a class can have ?Core Java2017-01-07 16:58:43

Ans. 1. Method local variables - These are declared and defined within a method ( instance or static methods ) and their scope is limited to the method itself. They are destructed once the execution of method completes. They are stored in stack memory.

2. Instance variables - These are declared as non static variables as part of the class.They are initialized as part of object creation ( constructor ) and are destructed by java's garbage collection mechanism and hence stored in heap.

3. Static variables - These are declared with the static keyword and are part of the class. They are initialized at the time of class loading and are destructed by java's garbage collection mechanism and hence stored in heap.

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 Q153. Can we have multiple main methods in a single class ?Core Java2017-01-12 11:00:02

Ans. We can overload the main method by specifying different argument types. For example -

2 main methods with different arguments is perfectly legal

public static void main();
public static void main(String[] args);

The following are not legal as compiler will complain of duplicate methods

public static void main(String[] args);
public static void main(String[] args);

Even The following are not legal as we cannot overload on return types

public static String main(String[] args);
public static void main(String[] args);

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 Q154. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java2017-05-07 07:26:29

Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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 Q155. What is the difference between these two method declarations ?

private static void method(String[] arg)

and

private static void method(String... arg)
Core Java2017-05-23 08:41:54

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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 Q156. If you need to consume messages from the queue faster, which approach will you recommend - batching or concurrency ?Design2017-07-24 13:34:47

Ans. Each has it's own advantages

Batching requires less resources but may result in loosing whole batch in case of failure whereas concurrency even though is little more expensive in terms of resources but would result in minimal loss in case of failure.

In case messages are to be consumed in a particular order, batching them in that order and then consuming them makes better sense.

if incoming messages are not continuous , it makes more sense to do concurrency as we need not wait for all messages to form a batch and flush. Though time sensitivity can be added but that would add unnecessary complexity.

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 Q157. When we say final x = 10, is the reference final or the value ?Core Java2017-07-29 13:27:04

Ans. final keyword have meaning only to referenced and not the value. It means that the specified reference cannot be dereferenced. It doesn't control the value assigned to the memory that's being referenced. This is the reason that final object references doesn't mean that the object is immutable but means that the reference cannot be changed to point to new object.

In case of primitive types too, when we assign a reference to another, values are passed and not the object reference, and hence a new placeholder is created in memory with the same value. That is why final to that context means that you cannot change the assigned memory and there is no way we can have that memory place have another value.

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 Q158. What are the benefits of using Wrapper classes over primitive types ?Core Java2017-08-24 21:35:53

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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 Q159. What is the difference between uninitialized values for primitive types and Wrapper classes ? Core Java2017-08-24 21:40:30

Ans. When we create reference for primitive type, it's memory is allocated. So even if we don't assign any value to it, the default value is initialized.

int x; // default value 0 initialized

But for object references, it's different as references hold nothing till an object is assigned.

Object obj; // contains null

So all primitive types when declared , contains their respective default values on the basis of their type whereas all wrapper class References contains null irrespective of their types.

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 Q160. Difference between JVM, JRE and JDK ?Core Java2017-10-31 08:36:22

Ans. JDK(Java Development kit) = Development Kit comprising of JVM , library and development tools for developers

JRE (Java Run time Environment) - Comprise of JVM and set of libraries

JVM(Java Virtual Machine) = Interpreter which reads the .class file line by line.

When we install JDK, JRE also get installed so we can write,compile and excute our code. Used by developer. Without JDK we can only execute the program using JRE.

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 Q161. What is the use of Object class ?Core Java2017-09-02 15:30:43

Ans. 1. Default definition for some of the methods , like equals, hashcode etc that gets carried to all objects even if you don't define anything. But that default definition is kind of like assigning 0 to integer, it just provide a safe state , nothing much for comparison.

2. Places where you have no idea about what object you may receive and just want to perform out of 8 basic methods of object class, something like printing their string representation ( defined by their toString method ), or equality ( using their equals method )

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 Q162. Write a Program to implement stack using an array2017-07-30 14:26:25

Ans. public class Stack{
   static int top = 0;
   static Element[] stack = new Element[10];
   
   static class Element {
      int body;
      
      Element(int value){
         body = value;
      }
   }
   
   public static void main(String[] args){
      push(5);
      System.out.println(top);
      push(7);
      System.out.println(top);
      poll();
      System.out.println(top);
      poll();
      System.out.println(top);   
   }
   
   private static void push(int value){
      Element newElement = new Element(value);
      top++;
   }
   
   private static Element poll(){
      Element topElement = stack[top];
      top--;
      return topElement;
   }
}

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Frequently asked in Tech Mahindra ( Based on 3 inputs )
  Q163. What is the difference between List, Set and Map ?Core Java
Anonymous
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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 Q164. When are static variables loaded in memory ?Core Java
Anonymous
Ans. They are loaded at runtime when the respective Class is loaded.

  Sample Code for static variable

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 Q165. Can we use null as a key for a map collection ?Core Java
Anonymous
Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

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 Q166. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Anonymous
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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 Q167. What is an object's lock and which object's have locks?
Anonymous
Ans. An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

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 Q168. What is JSON ?Json
Anonymous
Ans. JSON is "JavaScript Object Notation", primarily used for client-server or server-server communication. Its a much lighter and readable alternative to XML. JSON is language independent and is easily parse-able in all programming languages.

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 Q169.  What is the role of JSON.stringify ?Json
Anonymous
Ans.  JSON.stringify() turns an object into a JSON text and stores that JSON text in a string. 

So If we stringfy above notation , it will become

{"name":"xyz","gender":"male";"age":30}

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 Q170. Which function is used to convert a JSON text into an object ?Json
Anonymous
Ans. eval

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 Q171. Which data types are supported by JSON ?Json
Anonymous
Ans. Number
String
Boolean
Array
Object
null

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 Q172. What is an API ( Application Programming Interface ) ?
Anonymous
Ans. An API is a kind of technical contract which defines functionality that two parties must provide: a service provider (often called an implementation) and an application. an API simply defines services that a service provider (i.e., the implementation) makes available to applications.

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 Q173. What is URL?
Java EE
Anonymous
Ans. URL is Uniform Resource Locator which is representation of HTTP address.

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 Q174. What is HTTP ?Java EE
Anonymous
Ans. HTTP or Hypertext Transfer Protocol is internet protocol for tranmission of hypertext ( text with meta data ) over internet.

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 Q175. Is tomcat an application or Web server ?Server
Anonymous
Ans. Tomcat is a web server and a Servlet container. It is often used as an application server for web-based applications but does not include the complete suite of capabilities that a Java EE application server would supply.

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 Q176. Is Apache an application or Web server ?
Anonymous
Ans. Web server.

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 Q177. What are the default or implicitly assigned values for data types in java ?
Anonymous
Ans. boolean ---> false
byte ----> 0
short ----> 0
int -----> 0
long ------> 0l
char -----> /u0000
float ------> 0.0f
double ----> 0.0d
any object reference ----> null

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 Q178. Can I import same package/class twice? Will the JVM load the package twice at runtime?Core Java
Anonymous
Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class.

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 Q179. Which access specifier can be used with Class ?
Anonymous
Ans. For top level class we can only use "public" and "default". We can use private with inner class.

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 Q180. What is meant by an "asynchronous event." Give some examples ?Operating System
Anonymous
Ans. An asynchronous event is one that occurs at an unpredictable time outside the control of the program that the CPU is running. It is not "synchronized" with the program.

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 Q181. Give an Example of Annotations ?
Anonymous
Ans. Suppose that a software group traditionally starts the body of every class with comments providing important information:
public class Generation3List extends Generation2List {

// Author: John Doe
// Date: 3/17/2002
// Current revision: 6
// Last modified: 4/12/2004
// By: Jane Doe
// Reviewers: Alice, Bill, Cindy

// class code goes here

}
To add this same metadata with an annotation, you must first define the annotation type. The syntax for doing this is:
@interface ClassPreamble {
String author();
String date();
int currentRevision() default 1;
String lastModified() default "N/A";
String lastModifiedBy() default "N/A";
// Note use of array
String[] reviewers();
}
The annotation type definition looks similar to an interface definition where the keyword interface is preceded by the at sign (@) (@ = AT, as in annotation type). Annotation types are a form of interface, which will be covered in a later lesson. For the moment, you do not need to understand interfaces.
The body of the previous annotation definition contains annotation type element declarations, which look a lot like methods. Note that they can define optional default values.
After the annotation type is defined, you can use annotations of that type, with the values filled in, like this:
@ClassPreamble (
author = "John Doe",
date = "3/17/2002",
currentRevision = 6,
lastModified = "4/12/2004",
lastModifiedBy = "Jane Doe",
// Note array notation
reviewers = {"Alice", "Bob", "Cindy"}
)
public class Generation3List extends Generation2List {

// class code goes here

}

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 Q182. What are meta Annotations ?
Anonymous
Ans. Annotations that apply to other annotations are called meta-annotations.

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 Q183. How to display and set the Class path in Unix ?
Anonymous
Ans. To display the current CLASSPATH variable, use these commands in UNIX (Bourne shell):

% echo $CLASSPATH

To delete the current contents of the CLASSPATH variable,

In UNIX: % unset CLASSPATH; export CLASSPATH

To set the CLASSPATH variable,

In UNIX: % CLASSPATH=/home/george/java/classes; export CLASSPATH

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 Q184. Difference between Abstract and Concrete Class ?Core Java
Anonymous
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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 Q185. Different ways of implementing Threads in Java ?Core Java
Anonymous
Ans. Threads in Java can be implement either by Extending Thread class or implementing runnable interface.

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Frequently asked question for intermediate developers.
  Q186. What is Volatile keyword used for ?Core Java
Anonymous
Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached.

  Sample Code for volatile

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 Q187. What is the use of Transient Keyword ?Core Java
Anonymous
Ans. It in Java is used to indicate that a field should not be serialized.

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 Q188. What is a Final Method ?Core Java
Anonymous
Ans. A Method that cannot be overriden in the sub class.

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Very Frequently asked.Favorite question in Walk in drive for many Indian service companies.
 Q189. What is a Final Class ?
Anonymous
Ans. A Class that cannot be sub classed.

  Sample Code for final class

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Very frequently asked. Usually asked with questions related to String.
  Q190. What is an Immutable Object ?Core Java
Anonymous
Ans. Object that can't be changed after instantiation.

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Very Frequently asked. Usually asked along with String Class related questions.
 Q191. What is an immutable class ?Core Java
Anonymous
Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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 Q192. How to implement an immutable class ?Core Java
Anonymous
Ans. We can make a class immutable by

1. Making all methods and variables as private.

2. Setting variables within constructor.

Public Class ImmutableClass{

private int member;
ImmutableClass(int var){
member=var;
}
}

and then we can initialize the object of the class as

ImmutableClass immutableObject = new ImmutableClass(5);

Now all members being private , you cant change the state of the object.

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 Q193. Does Declaring an object "final" makes it immutable ?Core Java
Anonymous
Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable.

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 Q194. Can we override static methods ? Why ?Core Java
Anonymous
Ans. No.

Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

A static method is not associated with any instance of a class so the concept of overriding for runtime polymorphism using static methods is not applicable.

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 Q195. Can we reduce the visibility of the inherited or overridden method ?Core Java
Anonymous
Ans. No.

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 Q196. Name few Java Exceptions ?Core Java
Anonymous
Ans. IndexOutofBoundException ,
NoClassDefFoundException ,
OutOfMemoryException ,
IllegalArgumentException,
NullPointerException,
IllegalStateException

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Frequently asked to fresh graduates and less experienced.
 Q197. Which of the following is tightly bound ? Inheritance or Composition ?Core Java
Anonymous
Ans. Inheritance.

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 Q198. How can we make sure that a code segment gets executed even in case of uncatched exceptions ?
Anonymous
Ans. By putting it within finally.

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 Q199. What is "super" used for ?
Anonymous
Ans. Used to access members of the base class.

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 Q200. What is "this" keyword used for ?
Anonymous
Ans. Used to represent an instance of the class in which it appears.

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