Java Interview Questions

Front 1

What is the difference between JDK, JRE and JVM?

Back 1

JVM - Java virtual machine - provides the runtime environment for java bytecode execution

JRE - Java Runtime Environment - implementation of the JVM

JDK - Java development kit. has JRE and development tools

Front 2

How many types of memory areas are allocated by JVM?

Back 2

1.Class

2. Heap

3. Stack

4. Program Counter Register

5. Native method stack

Front 3

What is JIT Compiler

Back 3

Just-in-time: used to improve performance. Compiles similar bytecode together so less time is needed for compilation

Front 4

What is platform?

Back 4

A platform is the hardware or software environment on which the program runs

Front 5

What is the main difference between Java platform and other platforms?

Back 5

Java platform is a software=based platform that runs on top of other hardware-based platforms.

made up of : runtime environment and API

Front 6

What gives Java its write once and run anywhere nature?

Back 6

The bytecode. It is not platform specific and can be fed to any platform

Front 7

What is classloader?

Back 7

The classloader is a subsystem of JVM that is used to load classes and interfaces.

Front 8

Is Empty .java file a valid source file name?

Back 8

Yes it would run by just writing the class. ex java A

Front 9

Is delete, next, main, exit or null keywords in Java

Back 9

no

Front 10

If I don't provide any arguments on the command line, then the string array of Main method will be empty or null?

Back 10

Empty but not null

Front 11

What if I write static public void instead of public stativ void

Back 11

Will run properly

Front 12

What is the default value of the local variables?

Back 12

local variables are not initialized to any default value, neither primitives nor object references

Front 13

What is the difference between OOP and OBP

Back 13

OBP follows all of the features of OOP except Inheritance (ex. JavaScript)

Front 14

What will the initial value of an object reference which is defined as an instance variable?

Back 14

null

Front 15

What is a constructor?

Back 15

Constructor is like a method that is used to initialize the state of an object

Front 16

What is the purpose of default constructor?

Back 16

to provide the default values to the objects

Front 17

Does constructor return any value?

Back 17

yes, the current instance

Front 18

is the constructor inherited?

Back 18

no

Front 19

Can you make a constructor final?

Back 19

no

Front 20

What is a static variable

Back 20

used to refer to the common property of all objects, that is not unique to each object

Front 21

What is a static method

Back 21

method that belongs to the class rather than the object of a class. can be invoked without the need for creating an instance of the class

Front 22

Why is the main method static?

Back 22

because the object is not required to class static methods.

Front 23

What is a static block?

Back 23

It is used to initialize the static data member. It is executed before main method at the time of classloading

Front 24

Can we execute a program without a main method

Back 24

yes, through the static block

Front 25

What if the static modifier is removed from the signature of the main method

Back 25

Program would compile, but at runtime throw an error nosuchmethod error

Front 26

What is the difference between static (class) method and instance method

Back 26

static - object is not required to call the method

instance - object is required to call the method

Front 27

What is 'this'?

Back 27

this refers to the current object

Front 28

What is inheritance?

Back 28

Inheritance is a mechanism in which one object acquires all the properties and behaviors of another object of another class. It represents an IS-A relationship

Front 29

Which class is the superclass for every class?

Back 29

Object

Front 30

Why is multiple inheritance not supported in Java?

Back 30

To reduce complexity and to simplify the language

Front 31

What is composition?

Back 31

Holding the reference of the other class within some other class

Front 32

What is the difference between aggregation and composition

Back 32

Aggregation represents a weak relationship, whereas composition represents a strong relationship

Front 33

Why does Java not support pointers

Back 33

Pointers refer to the memory address. These are not used in Java for security reasons and because of complexity

Front 34

What is super in java?

Back 34

Keyword which refers to the immediate parent

Front 35

Can you use this and super both in a constructor?

Back 35

no, it can only be this or super

Front 36

What is object cloning

Back 36

creating the exact copy of an object

Front 37

What is method overloading

Back 37

Having a class with multiple methods by the same name, but different parameters. Used to avoid unneeded nulls in methods

Front 38

Why is method overloading not possibly by changing the return type in java?

Back 38

It creates ambiguity

Front 39

Can we overload the main method

Back 39

yes

Front 40

What is method overriding

Back 40

When a subclass provides a specific implementation of a method of the parent class

Front 41

Can we override static methods?

Back 41

No

Front 42

Why can we not override static methods?

Back 42

Because the static method is the part of the class that is bound. whereas instance method is bought with the object.

Front 43

Can we override overloaded methods?

Back 43

Yes

Front 44

Difference between Overloading and overriding

Back 44

overloading - multiple methods which accept different parameters

overriding - when a subclass creates a new method with the same name of the parent method and same parameters

Front 45

Can you have virtual functions in Java

Back 45

all functions are virtual by default

Front 46

What is the covariant return type?

Back 46

It is a return type which returns the type of a subclass

Front 47

What is the final variable?

Back 47

It is a variable which cannot be changed and is constant

Front 48

What is a final method

Back 48

cannot be overriden

Front 49

what is a final class

Back 49

cannot be inherited

Front 50

What is a blank final variable?

Back 50

A final variable which is not initialized at the time of declaration

Front 51

Can we initialize a blank final variable?

Back 51

yes only in the constructor if it is non-static. if it is a static blank final it can be initialized only in the static block

Front 52

Can you declare the main method final

Back 52

yes, public static final void main

Front 53

What is runtime polymorphism?

Back 53

Runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time

Front 54

Can you achieve runtime polymorphism by data members?

Back 54

no

Front 55

What is the difference between static binding and dynamic binding

Back 55

static binding type of object is determined at compile time. dynamic binding is determined at runtime

Front 56

What is abstraction?

Back 56

Process of hiding the implementation details and showing only the functionality to the user

Front 57

What is the difference between abstraction and encapsulation?

Back 57

Abstraction hides the implementation details whereas encapsulation wraps code and data into a single unit

Front 58

What is an abstract class?

Back 58

A class that is declared as abstract. it needs to be extended and for its methods to be implemented. Contains all the common attributes

Front 59

Can there be any abstract method without abstract class?

Back 59

If a method is abstract, the class must be abstract

Front 60

Can you use abstract and final both with a method?

Back 60

no, because an abstract method must be overridden. And a final cannot be

Front 61

Is it possible to instantiate the abstract class?

Back 61

No

Front 62

What is interface?

Back 62

Interface is a blueprint of a class that has static constants and abstract methods.

Front 63

Can you declare an interface method static

Back 63

no, because methods of an interface are abstract by default. static and abstract cannot be used together

Front 64

Can an interface be final

Back 64

no because it needs to be implemented by another class

Front 65

What is market interface

Back 65

An interface that has no data member and methods

Front 66

Difference between abstract class and interface

Back 66

abstract - can have method body, instance variables, constructor, static methods, and be extended once

interface - only abstract methods, cannot have instance variables, no constructor, no static methods, can be implemented multiple times

Front 67

Can we define private and protected in an interface

Back 67

no, because interfaces are implicitly public

Front 68

When can an object reference be cast to an interface reference?

Back 68

An object reference can be cast to an interface reference when the object implements the referenced interface

Front 69

What is a package?

Back 69

A package is a group of similar type of classes, interfaces, and sub-packages

Front 70

Do I need to import java.lang package

Back 70

no. it is default

Front 71

Can i import the same package/class twice? Will the JVM load the package twice at runtime?

Back 71

You can import the same twice. The JVM will interanally load the class only once no matter how many times it is imported

Front 72

What is a static import

Back 72

We can access the static members of a class directly

Front 73

What is exception handling?

Back 73

Exception handling is a mechanism to handle runtime errors. it is used to handle checked exceptions

Front 74

What is the difference between checked exception and unchecked exception?

Back 74

checked - classes that extend throwable (SQLException, IOExceptions) - they are checked in compile-time

unchecked - classes that exend runtimeexception - exceptions that cannot be stopped, such as nullpointer

Front 75

What is the base class for Error and Exception?

Back 75

Throwable

Front 76

Is it necessary that each try block must be followed by a catch?

Back 76

It is not necessary. it should be followed by a catch or finally

Front 77

What is a finally block?

Back 77

Finally block is a block that is always executed

Front 78

Can finally block be used without catch?

Back 78

Yes, by try block

Front 79

Is there any case when finally will not be executed?

Back 79

finally will not be executed if the program exists (system.exit or fatal error)

Front 80

difference between throw and throws

Back 80

throw - used to explicitly throw an exception, checked exceptions can not be propagated with throw only, throw is followed by an instance

throws - used to declare an exception, checked exceptions can be propagated with throws, throws is followed by class

Front 81

Can an exception be rethrown

Back 81

yes

Front 82

Can subclass overriding method declare an exception if the parent class method doesnt thrown an exception?

Back 82

yes, but only unchecked exceptions not checked

Front 83

What is exception propagation?

Back 83

Forwarding the exception object to the invoking method is known as exception propagation

Front 84

What is the meaning of immutable in terms of String?

Back 84

unmodifiable or unchangeable. Once string object has been created, its value cant be changed

Front 85

Why string objects are immutable in java?

Back 85

Because java uses the concept of string literal.

Front 86

How many ways can we create the string object?

Back 86

String literal or by the new keyword

Front 87

How many objects will be created in the following code? String s1 = "one"; String s2="two", String s3="three"

Back 87

One object

Front 88

Why does java use the concept of string literal?

Back 88

To make Java more memory efficient. Because no new objects are created if it exists already in a string constant pool

Front 89

How many objects will be created int he following code? String s= new String("Two");

Back 89

two objects, one in the string constant pool and one in non-pool(heap)

Front 90

What is the basic difference between string and stringbuffer object?

Back 90

String is an immutable object, StringBuffer is a mutable object

Front 91

What is the difference between StringBuffer and StringBuilder?

Back 91

StringBuffer is synchronized whereas StringBuilder is not synchronized

Front 92

How can we create an immutable class in java?

Back 92

We can create immutable classes as the String class by defining the class final

Front 93

What is the purpose of toString() method in java?

Back 93

it returns the string representation of any object. It is mostly used for error logging

Front 94

What is a nested class?

Back 94

A class which is declared inside another class. There are 4 types of nested class members: inner class, local inner class, anonymous inner class and static nested class

Front 95

Is there any difference between nested and inner classes?

Back 95

inner class - non-static nested class, they are part of nested classes

Front 96

Can we access the non-final local variable inside the local inner class?

Back 96

No, local variables must be constant if you want to access it in a local inner class

Front 97

What is a nested interface

Back 97

Any interface declared inside of an interface or class

Front 98

Can a class have an interface?

Back 98

yes, a nested interface

Front 99

Can an interface have a class?

Back 99

Yes, they are static

Front 100

What is garbage collection?

Back 100

process of reclaiming the runtime unused objects. It is performed for memory management

Front 101

What is gc()

Back 101

a daemon thread. gc() method is defined in system class that is used to send requests to JVM to perform garbage collection

Front 102

What is the purpose of finalize() method?

Back 102

invoked just before the object is garbage collected. Used to perform cleanup processing

Front 103

Can an unreferenced object be referenced again?

Back 103

yes

Front 104

Waht kind of thread is the garbage collector thread?

Back 104

daemon

Front 105

What is the difference between final, finally, finalize

Back 105

final - keyword, method var or class which cannot be changed

finally - used in exception handling. block which is always executed

finalize - used in garbage collection. It is invoked jsut before the object is garbage collected.

Front 106

What is the purpose of the runtime class?

Back 106

to provide access to the java runtime environment

Front 107

How will you invoke any external process in java?

Back 107

By runtime.getRuntime().exec(?) method

Front 108

What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

Back 108

The reader/writer class hierarchy is character-oriented. the inputstream/outputstream class hierarchy is byte oriented

Front 109

What is an I/O filter?

Back 109

An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it passe from one stream to another

Front 110

What is serialization?

Back 110

process of writing the state of an object into a byte stream. mainly used to travel object's state on the network

Front 111

What is deserialization?

Back 111

Deserialization is the process of reconstructing the object from the serialized state. it is the reverse of serialization

Front 112

What is the transient keyword

Back 112

If you define any data member as transient, it will not be serialized

Front 113

What is externalizable?

Back 113

Externalizable interface is used to write the state of an object into a byte stream in compressed format. it is not a marker interface

Front 114

What is the difference between serializable and externalizable itnerface

Back 114

Serializable is a marker interface but Externalizable is not a marker interface.

Front 115

How do I conver a numeric IP address like 192.18.9.39 into a hostname like java.sun.com

Back 115

By InetAddres.getByName("192.1.3.4").getHostName()

Front 116

What is reflection?

Back 116

Reflection is the process of examining or modifying the runtime behaviour of a class at runtime

Front 117

Can you access the private method from outside the class?

Back 117

Yes, by changing the runtime behavior of a class if the class is not secured

Front 118

What are wrapper classes?

Back 118

Wrapper classes are classes tha tallow primitive type sot be access as objects

Front 119

What is a native method?

Back 119

A native method is a method that is implemented in a language other than Java

Front 120

What is the purpose of the System class?

Back 120

The purpose of the System class is to provide access to system resources

Front 121

What comes to mind when someone mentions a shallow copy in java?

Back 121

Object cloning

Front 122

What is a singleton class

Back 122

means that at any given time only one instance of the class is present

Front 123

What is Java Bean?

Back 123

A reusable software component written in JAva programming

Front 124

What is multithreading?

Back 124

multi threading is a process of executing multiple threads simultaneously. Its main advantage is: threads share the same address space, thread is lightweight, cost of communication between process is low

Front 125

What is a thread?

Back 125

A thread is a lightweight sub process. It is a separate path of execution. It is called separate path of execution because each thread runs in a separate stack frame.

Front 126

What is the difference between preemptive scheduling and time slicing?

Back 126

preemptive scheduling - the highest priority task executed until it enters the waiting or dead states or a higher priority task comes into existence

time slicing - task executes for a predefined slice of time and then reenters the pool of ready tasks. then scheduler determines which task should execute next

Front 127

What does join() method do?

Back 127

join() method waits for a thread to die. in other words, it causes the currently running threads to stop executing until the thread it joins will comple its task

Front 128

what is the difference between wait and sleep method?

Back 128

wait() - defined in object class, releases the lock

sleep() - defined in thread class, doesn't release the lock

Front 129

Is it possible to start a thread twice?

Back 129

No, if it does it throws an exception

Front 130

Can we call the run() method isntead of start()

Back 130

yes, but it will not work as a thread rather it will work as a normal object so there will not be context-switching between threads

Front 131

What about the daemon threads

Back 131

daemon threads are the low priority threads that provide the background support to the user threads

Front 132

Can we make the user thread as a daemon thread if thread is started

Back 132

no, it will throw an illegal thread state exception

Front 133

What is shutdownhook?

Back 133

a thread invoked simply before JVM shuts down

Front 134

When should we interrupt a thread?

Back 134

If we want to break out the sleep or wait state of a thread

Front 135

What is synchronization?

Back 135

capability of controlling the access of multiple threads to any shared resource. used to: prevent thread interference, to prevent consistency problems

Front 136

What is the purpose of synchronized block?

Back 136

synchronized block is used to lock an object for any shared resource. Scope of synchronized block is smaller than the method

Front 137

Can java objects be locked down for exlcusive use by a given thread?

Back 137

Yes, you can lock an object by putting it in a synchronized block

Front 138

What is static synchronizatiion

Back 138

If you make any static method as synchronized, the lock will be on the class not on the object

Front 139

What is the difference between notify and notifyAll

Back 139

notify() - used to unblock one waiting thread

notifyAll() - used to unblock all the threads in a waiting state

Front 140

What is a deadlock?

Back 140

a situation when two threads are waiting for each other to release a resource

Front 141

What is the difference between an ArrayList and Vector

Back 141

ArrayList - not synchronized, not a legacy class, increases its size by 50% of the array size

Vector - synchronized, legacy class, increases its size by double the array size

Front 142

What is the difference between an ArrayList and a LinkedList

Back 142

ArrayList - uses dynamic array, not efficient for manipulation before of shifting, better to store and fetch data

LinkedList - doubly linked lists, efficient for manipulation, better to manipulate data

Front 143

Iterator vs ListIterator

Back 143

Iterator - traverses the elements in forward direction only, can be used in List, Set and Queue

ListIterator - traverses the elements in backward and forward fashion, can be used in List only

Front 144

Iterator vs Enumaration

Back 144

Iterator - can traverse legacy and non-legacy elements, fail-fast, slower than enumeration

Enumeration - can traverse only legacy elements, not fail-fast, faster than iteration

Front 145

List vs Set

Back 145

List - contain duplicate elements

Set - only unique elements

Front 146

HashSet vs Treeset

Back 146

HashSet - no order

treeSet - ascending order

Front 147

Set vs Map

Back 147

Set - contains values only

Map - contains key and values both

Front 148

HashSet vs HashMap

Back 148

HashSet contains only values whereas HashMap contains entry(key,value). HashSet can be iterated by HashMap needs to convert into set to be iterated

Front 149

HashMap vs HashTable

Back 149

HashMap - not synchronized, contain one null key and multiple null values

Hashtable - synchronized, cannot contain any null key or value

Front 150

Collection vs Collections

Back 150

Collection - an interface - provides normal functionality of data structure to List, Set and Queue

Collections - class - is to sort and synchronize collection elements

Front 151

Comparable vs Comparator

Back 151

Comparable - provides only one sort sequence, provides one method name compareTo()

Comparator - provides multiple sort of sequences, method compare()

Front 152

What is the advantage of Properties file?

Back 152

If you change the value in the properties file, you don't need to recompile the java class

Front 153

What does the hashCode() method do?

Back 153

returns a hash code value.

Front 154

Why do we override the equals() method?

Back 154

It needs to be override if we want to check the objects based on property

Front 155

What is the advantage of generic collection?

Back 155

If we use generic class, we don't need typecasting. It is typesafe and checked at compile time

Front 156

What is hash-collision in Hashtable and how is it handled

Back 156

two different keys with the same has value. two different entries will be kept in a single hash bucket to avoid the collisison

Front 157

What is the dictionary class?

Back 157

provides the capability to store key-value pairs

Front 158

What is the default size of load factor in hashing based collection?

Back 158

.75

Front 159

What is JDBC?

Back 159

Java API that is used to connect and execute query to the database.

Front 160

What is JDBC Driver?

Back 160

JDBC driver is a software component that enables java application to interact with the database.

Front 161

What are the steps to connect to the database in java?

Back 161

1. Registering the driver class

2. Creating the connection

3. Creating the statement

4. Executing queries

5. Closing the connection

Front 162

What are the JDBC API components?

Back 162

Interfaces: connection, statement, preparedstatement, resultsset, resultssetmetadata, databasemetadata, callablestatement

Classes:DriverManager, Blob, Clob, Types, SQLException

Front 163

What are the JDBC statements?

Back 163

Statement, PreparedStatement, CallableStatement

Front 164

Statement vs preparedstatement

Back 164

Statement - query is compiled each time

PreparedStatement - query is compiled only once

Front 165

How can we execute stored procedures and functions?

Back 165

By using callable statement interface

Front 166

What is the role of JDBC DriverManager class?

Back 166

DriverManager class manages the registered drivers. it can be used to register and unregister drivers

Front 167

What does the JDBC Connection interface do?

Back 167

Connection interface maintains a session with the database

Front 168

What does the JDBC ResultSet intrace do?

Back 168

The ResultSet object represents a row of a table. It can be used to change the cursor pointer and get the information from the database

Front 169

What does the JDBC DatabaseMetaData interface do?

Back 169

Returns the information of the database such as username, driver name, driver version, ,number of tables, number of views

Front 170

Which interface is responsible for transaction management in JDBC?

Back 170

Connection interface provides methods for transaction management such as commit(), rollback() etc

Front 171

What is batch processing and how do you perform batch processing in JDBC?

Back 171

We can execute multiple queries

Front 172

How can we store and retrieve images from the database?

Back 172

By using PreparedStatement interface