100+ Java
Interview Questions And Answers
Basic Java Interview Questions
Q1.
Explain JDK, JRE and JVM?
JDK vs JRE vs JVM |
||
JDK |
JRE |
JVM |
It
stands for Java Development Kit. |
It
stands for Java Runtime Environment. |
It
stands for Java Virtual Machine. |
It is
the tool necessary to compile, document and package Java programs. |
JRE
refers to a runtime environment in which Java bytecode can be executed. |
It is
an abstract machine. It is a specification that provides a run-time
environment in which Java bytecode can be executed. |
It
contains JRE + development tools. |
It’s an
implementation of the JVM which physically exists. |
JVM
follows three notations: Specification, Implementation, and Runtime
Instance. |
Q2. Explain public static void main(String args[])
in Java.
main() in Java is the entry point for any Java program. It is always
written as public static void main(String[] args).
·
public: Public is an access
modifier, which is used to specify who can access this method. Public
means that this Method will be accessible by any Class.
·
static: It is a keyword in java which
identifies it is class-based. main() is made static in Java so that it can be
accessed without creating the instance of a Class. In case, main is not made
static then the compiler will throw an error as main() is called by
the JVM before any objects are made and only static methods can be directly
invoked via the class.
·
void: It is the return type of the
method. Void defines the method which will not return any value.
·
main: It is the name of the method which
is searched by JVM as a starting point for an application with a particular
signature only. It is the method where the main execution occurs.
·
String args[]: It is the parameter passed to the
main method.
Q3. Why Java is platform independent?
Java is called platform independent because of its
byte codes which can run on any system irrespective of its underlying operating
system.
Q4. Why Java is not 100% Object-oriented?
Java is not 100% Object-oriented because it makes use of eight primitive data
types such as boolean, byte, char, int, float, double, long, short which are
not objects.
Q5.
What are wrapper classes in Java?
Wrapper classes
convert the Java primitives into the reference types (objects). Every primitive
data type has a class dedicated to it. These are known as wrapper classes
because they “wrap” the primitive data type into an object of that class. Refer
to the below image which displays different primitive type, wrapper class and
constructor argument.
Q6. What are constructors in Java?
In Java, constructor refers to a block of code
which is used to initialize an object. It must have the same name as that of
the class. Also, it has no return type and it is automatically called when an
object is created.
There are two types of constructors:
1.
Default Constructor: In Java, a
default constructor is the one which does not take any inputs. In other words,
default constructors are the no argument constructors which will be created by
default in case you no other constructor is defined by the user. Its main
purpose is to initialize the instance variables with the default values. Also,
it is majorly used for object creation.
2.
Parameterized Constructor: The
parameterized constructor in Java, is the constructor which is capable of
initializing the instance variables with the provided values. In other words,
the constructors which take the arguments are called parameterized
constructors.
Q7.
What is singleton class in Java and how can we make a class singleton?
Singleton class is a class whose only one instance can be created at any
given time, in one JVM. A class can be made singleton by making its constructor
private.
Q8.
What is the difference between Array list and vector in Java?
ArrayList |
Vector |
Array
List is not synchronized. |
Vector
is synchronized. |
Array
List is fast as it’s non-synchronized. |
Vector
is slow as it is thread safe. |
If an
element is inserted into the Array List, it increases its Array size by 50%. |
Vector
defaults to doubling size of its array. |
Array
List does not define the increment size. |
Vector
defines the increment size. |
Array
List can only use Iterator for traversing an Array List. |
Vector
can use both Enumeration and Iterator for traversing. |
Q9. What is the difference between equals() and ==
in Java?
Equals() method is defined in Object class in Java and used for checking
equality of two objects defined by business logic.
“==” or equality operator in Java is a binary operator provided by Java
programming language and used to compare primitives and objects. public
boolean equals(Object o) is the method provided by the Object class.
The default implementation uses == operator to compare two objects. For
example: method can be overridden like String class. equals() method is used to
compare the values of two objects.
Q10.
What are the differences between Heap and Stack Memory in Java?
The major difference between Heap and Stack memory are:
Features |
Stack |
Heap |
Memory |
Stack
memory is used only by one thread of execution. |
Heap
memory is used by all the parts of the application. |
Access |
Stack
memory can’t be accessed by other threads. |
Objects stored
in the heap are globally accessible. |
Memory
Management |
Follows
LIFO manner to free memory. |
Memory
management is based on the generation associated with each object. |
Lifetime |
Exists
until the end of execution of the thread. |
Heap
memory lives from the start till the end of application execution. |
Usage |
Stack
memory only contains local primitive and reference variables to objects in
heap space. |
Whenever
an object is created, it’s always stored in the Heap space. |
Q11. What is a package in Java? List down
various advantages of packages.
Packages in Java, are the collection of related
classes and interfaces which are bundled together. By using packages,
developers can easily modularize the code and optimize its reuse. Also, the
code within the packages can be imported by other classes and reused. Below I
have listed down a few of its advantages:
·
Packages help in avoiding name clashes
·
They provide easier access control on the code
·
Packages can also contain hidden classes which are not visible to the
outer classes and only used within the package
·
Creates a proper hierarchical structure which makes it easier to locate
the related classes
Q12. Why pointers are not used in Java?
Java doesn’t use pointers because they are unsafe
and increases the complexity of the program. Since, Java is known for its
simplicity of code, adding the concept of pointers will be contradicting.
Moreover, since JVM is responsible for implicit memory allocation, thus in
order to avoid direct access to memory by the user, pointers are
discouraged in Java.
Q13. What
is JIT compiler in Java?
JIT stands for Just-In-Time compiler in Java. It is
a program that helps in converting the Java bytecode into instructions that are
sent directly to the processor. By default, the JIT compiler is enabled in Java
and is activated whenever a Java method is invoked. The JIT compiler then
compiles the bytecode of the invoked method into native machine code, compiling
it “just in time” to execute. Once the method has been compiled, the JVM
summons the compiled code of that method directly rather than interpreting it.
This is why it is often responsible for the performance optimization of Java
applications at the run time.
Q14.
What are access modifiers in Java?
In Java, access modifiers are special keywords
which are used to restrict the access of a class, constructor, data member and
method in another class. Java supports four types of access modifiers:
1.
Default
2.
Private
3.
Protected
4.
Public
Modifier |
Default |
Private |
Protected |
Public |
Same
class |
YES |
YES |
YES |
YES |
Same
Package subclass |
YES |
NO |
YES |
YES |
Same
Package non-subclass |
YES |
NO |
YES |
YES |
Different
package subclass |
NO |
NO |
YES |
YES |
Different
package non-subclass |
NO |
NO |
NO |
YES |
Q15. Define a Java Class.
A class in Java is a blueprint which includes all
your data. A class contains fields (variables) and methods to describe
the behavior of an object. Let’s have a look at the syntax of a class.
1 2 3 |
class Abc { member variables // class body methods} |
Q16.
What is an object in Java and how is it created?
An object is a real-world entity that has a state
and behavior. An object has three characteristics:
1.
State
2.
Behavior
3.
Identity
An object is created using the ‘new’ keyword. For example:
ClassName obj = new ClassName();
Q17. What
is Object Oriented Programming?
Object-oriented programming or popularly known as
OOPs is a programming model or approach where the programs are organized around
objects rather than logic and functions. In other words, OOP mainly focuses on
the objects that are required to be manipulated instead of logic. This approach
is ideal for the programs large and complex codes and needs to be actively
updated or maintained.
Q18. What
are the main concepts of OOPs in Java?
Object-Oriented Programming or OOPs is a
programming style that is associated with concepts like:
1.
Inheritance: Inheritance is a
process where one class acquires the properties of another.
2.
Encapsulation: Encapsulation in
Java is a mechanism of wrapping up the data and code together as a single unit.
3.
Abstraction: Abstraction is the
methodology of hiding the implementation details from the user and only
providing the functionality to the users.
4.
Polymorphism: Polymorphism is the
ability of a variable, function or object to take multiple forms.
Q19. What
is the difference between a local variable and an instance variable?
In Java, a local variable is
typically used inside a method, constructor, or a block and
has only local scope. Thus, this variable can be used only within the scope of
a block. The best benefit of having a local variable is that other methods in
the class won’t be even aware of that variable.
Example
1 2 3 4 |
if(x > 100) { String test = "Edureka"; } |
Whereas, an instance variable in
Java, is a variable which is bounded to its object itself. These variables are
declared within a class, but outside a method. Every object of that
class will create it’s own copy of the variable while using it. Thus, any
changes made to the variable won’t reflect in any other instances of that class
and will be bound to that particular instance only.
1 2 3 4 |
class Test{ public String EmpName; public int empAge; } |
Q20. Differentiate
between the constructors and methods in Java?
Methods |
Constructors |
1. Used to represent the
behavior of an object |
1. Used to initialize the state
of an object |
2. Must have a return type |
2. Do not have any return type |
3. Needs to be invoked
explicitly |
3. Is invoked implicitly |
4. No default method is
provided by the compiler |
4. A default constructor is
provided by the compiler if the class has none |
5. Method name may or may not
be same as class name |
5. Constructor name must always
be the same as the class name |
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Q21. What is final keyword in Java?
final is a special
keyword in Java that is used as a non-access modifier. A final variable can be
used in different contexts such as:
·
final variable
When the final keyword is used with a variable then
its value can’t be changed once assigned. In case the no value has been
assigned to the final variable then using only the class constructor a value
can be assigned to it.
·
final method
When a method is declared final then it can’t be
overridden by the inheriting class.
·
final class
When a class is declared as final in Java, it can’t
be extended by any subclass class but it can extend other class.
Q22. What is the difference between break and
continue statements?
break |
continue |
1. Can be used in switch and
loop (for, while, do while) statements |
1. Can be only used with loop
statements |
2. It causes
the switch or loop statements to terminate the moment it is
executed |
2. It doesn’t terminate
the loop but causes the loop to jump to the next iteration |
3. It terminates the
innermost enclosing loop or switch immediately |
3. A continue within
a loop nested with a switch will cause the next loop iteration to
execute |
Example break:
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for (int i = 0; i < 5; i++) { if (i == 3) { break; } System.out.println(i); } |
Example continue:
1 2 3 4 5 6 7 8 |
for (int i = 0; i < 5; i++) { if(i == 2) { continue; } System.out.println(i); } |
Q23.What
is an infinite loop in Java? Explain with an example.
An infinite loop is an instruction sequence in Java that loops endlessly
when a functional exit isn’t met. This type of loop can be the result of a
programming error or may also be a deliberate action based on the application
behavior. An infinite loop will terminate automatically once the application
exits.
For example:
1 2 3 4 5 6 7 8 |
public class InfiniteForLoopDemo { public static void main(String[] arg) { for(;;) System.out.println("Welcome to Edureka!"); // To terminate this program press ctrl + c in the console. } } |
Q24.
What is the difference between this() and super() in Java?
In Java, super() and this(), both are special
keywords that are used to call the constructor.
this() |
super() |
1. this() represents the
current instance of a class |
1. super() represents the
current instance of a parent/base class |
2. Used to call the default
constructor of the same class |
2. Used to call the default
constructor of the parent/base class |
3. Used to access methods of
the current class |
3. Used to access methods of
the base class |
4. Used for pointing the
current class instance |
4. Used for pointing the
superclass instance |
5. Must be the first line of a
block |
5. Must be the first line of a
block |
Q25.
What is Java String Pool?
Java String pool refers to a collection of Strings
which are stored in heap memory. In this, whenever a new object is created,
String pool first checks whether the object is already present in the pool or
not. If it is present, then the same reference is returned to the variable else
new object will be created in the String pool and the respective reference will
be returned.
Q26. Differentiate
between static and non-static methods in Java.
Static Method |
Non-Static Method |
1. The
static keyword must be used before the method name |
1. No
need to use the static keyword before the method name |
2. It is called using the class
(className.methodName) |
2. It
is can be called like any general method |
3. They
can’t access any non-static instance variables or methods |
3. It
can access any static method and any static variable without creating an
instance of the class |
Q27. What
is constructor chaining in Java?
In Java, constructor chaining is the process of
calling one constructor from another with respect to the current object.
Constructor chaining is possible only through legacy where a subclass
constructor is responsible for invoking the superclass’ constructor first.
There could be any number of classes in the constructor chain. Constructor
chaining can be achieved in two ways:
1.
Within the same class using this()
2.
From base class using super()
Q28. Difference between String,
StringBuilder, and StringBuffer.
Factor |
String |
StringBuilder |
StringBuffer |
Storage
Area |
Constant
String Pool |
Heap
Area |
Heap
Area |
Mutability |
Immutable |
Mutable |
Mutable |
Thread
Safety |
Yes |
No |
Yes |
Performance |
Fast |
More
efficient |
Less
efficient |
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Q29. What is a classloader in Java?
The Java ClassLoader is a subset of JVM (Java Virtual
Machine) that is responsible for loading the class files. Whenever a Java
program is executed it is first loaded by the classloader. Java provides three
built-in classloaders:
1.
Bootstrap ClassLoader
2.
Extension ClassLoader
3.
System/Application ClassLoader
Q30.
Why Java Strings are immutable in nature?
In Java, string objects are immutable in
nature which simply means once the String object is created its state cannot be
modified. Whenever you try to update the value of that object instead of
updating the values of that particular object, Java creates a new string
object. Java String objects are immutable as String objects are generally
cached in the String pool. Since String literals are usually shared between
multiple clients, action from one client might affect the rest. It enhances
security, caching, synchronization, and performance of the application.
Q31.
What is the difference between an array and an array list?
Array |
ArrayList |
Cannot
contain values of different data types |
Can
contain values of different data types. |
Size
must be defined at the time of declaration |
Size
can be dynamically changed |
Need to
specify the index in order to add data |
No need
to specify the index |
Arrays
are not type parameterized |
Arraylists
are type |
Arrays
can contain primitive data types as well as objects |
Arraylists
can contain only objects, no primitive data types are allowed |
Q32. What is a Map in Java?
In Java, Map is an interface of Util package which
maps unique keys to values. The Map interface is not a subset of the main
Collection interface and thus it behaves little different from the other
collection types. Below are a few of the characteristics of Map
interface:
1.
Map doesn’t contain duplicate keys.
2.
Each key can map at max one value.
Q33. What is collection class in Java? List down
its methods and interfaces.
In Java, the collection is a framework that acts as
an architecture for storing and manipulating a group of objects. Using
Collections you can perform various tasks like searching, sorting, insertion,
manipulation, deletion, etc. Java collection framework includes the following:
·
Interfaces
·
Classes
·
Methods
The below image shows the complete hierarchy of the
Java Collection.
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OOPS Java Interview Questions
Q1. What is Polymorphism?
Polymorphism is briefly described as “one
interface, many implementations”. Polymorphism is a characteristic of being
able to assign a different meaning or usage to something in different contexts
– specifically, to allow an entity such as a variable, a function, or an object
to have more than one form. There are two types of polymorphism:
1.
Compile time polymorphism
2.
Run time polymorphism
Compile time polymorphism is method overloading whereas Runtime time
polymorphism is done using inheritance and interface.
Q2. What is runtime polymorphism or dynamic method
dispatch?
In Java, 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. In this process, an overridden method is
called through the reference variable of a superclass. Let’s take
a look at the example below to understand it better.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 |
class Car { void run() { System.out.println(“car is running”); } } class Audi extends Car { void run() { System.out.prinltn(“Audi is running safely with
100km”); } public static void main(String args[]) { Car b= new Audi(); //upcasting b.run(); } } |
Q3. What
is abstraction in Java?
Abstraction refers to the quality of dealing with
ideas rather than events. It basically deals with hiding the details and
showing the essential things to the user. Thus you can say that abstraction in
Java is the process of hiding the implementation details from the user and
revealing only the functionality to them. Abstraction can be achieved in two
ways:
1.
Abstract Classes (0-100% of abstraction can be
achieved)
2.
Interfaces (100% of abstraction can be
achieved)
Q4.
What do you mean by an interface in Java?
An interface in Java is a blueprint of a class or
you can say it is a collection of abstract methods and static constants. In an
interface, each method is public and abstract but it does not contain any
constructor. Thus, interface basically is a group of related methods with empty
bodies. Example:
public interface Animal {
public void eat();
public void sleep();
public void run();
}
Q5. What is the difference between abstract
classes and interfaces?
Abstract Class |
Interfaces |
An
abstract class can provide complete, default code and/or just the details
that have to be overridden |
An
interface cannot provide any code at all, just the signature |
In the
case of an abstract class, a class may extend only one abstract class |
A Class
may implement several interfaces |
An
abstract class can have non-abstract methods |
All
methods of an Interface are abstract |
An
abstract class can have instance variables |
An
Interface cannot have instance variables |
An
abstract class can have any visibility: public, private, protected |
An
Interface visibility must be public (or) none |
If we
add a new method to an abstract class then we have the option of providing
default implementation and therefore all the existing code might work
properly |
If we
add a new method to an Interface then we have to track down all the
implementations of the interface and define implementation for the new method |
An
abstract class can contain constructors |
An
Interface cannot contain constructors |
Abstract
classes are fast |
Interfaces
are slow as it requires extra indirection to find the corresponding method in
the actual class |
Q6. What is inheritance in Java?
Inheritance in Java is the concept where the
properties of one class can be inherited by the other. It helps to reuse the
code and establish a relationship between different classes. Inheritance is
performed between two types of classes:
1.
Parent class (Super or Base class)
2.
Child class (Subclass or Derived class)
A class which inherits the properties is known as
Child Class whereas a class whose properties are inherited is known as Parent
class.
Q7.
What are the different types of inheritance in Java?
Java supports four types of inheritance which are:
1.
Single Inheritance: In single
inheritance, one class inherits the properties of another i.e there will be
only one parent as well as one child class.
2.
Multilevel Inheritance: When a class is
derived from a class which is also derived from another class, i.e.
a class having more than one parent class but at different levels, such
type of inheritance is called Multilevel Inheritance.
3.
Hierarchical Inheritance: When a class
has more than one child classes (subclasses) or in other words, more than one
child classes have the same parent class, then such kind of inheritance is
known as hierarchical.
4.
Hybrid Inheritance: Hybrid
inheritance is a combination of two or more types of
inheritance.
Q8. What is method overloading and method
overriding?
Method Overloading :
·
In Method Overloading, Methods of the same class shares the same name
but each method must have a different number of parameters or parameters having
different types and order.
·
Method Overloading is to “add” or “extend” more to the method’s
behavior.
·
It is a compile-time polymorphism.
·
The methods must have a different signature.
·
It may or may not need inheritance in Method Overloading.
Let’s take a look at the example below to understand
it better.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 |
class Adder { Static int add(int a, int b) { return a+b; } Static double add( double a, double b) { return a+b; } public static void main(String args[]) { System.out.println(Adder.add(11,11)); System.out.println(Adder.add(12.3,12.6)); }} |
Method
Overriding:
·
In Method Overriding, the subclass has the same method with the same
name and exactly the same number and type of parameters and same return type as
a superclass.
·
Method Overriding is to “Change” existing behavior of the method.
·
It is a run time polymorphism.
·
The methods must have the same signature.
·
It always requires inheritance in Method Overriding.
Let’s take a look at the example below to
understand it better.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
class Car { void run(){ System.out.println(“car is running”); } Class Audi extends Car{ void run() { System.out.prinltn("Audi is running safely with 100km"); } public static void main( String args[]) { Car b=new Audi(); b.run(); } } |
Q9. Can you override a private or static method in
Java?
You cannot override a private or static method in
Java. If you create a similar method with the same return type and same method
arguments in child class then it will hide the superclass method; this is known
as method hiding. Similarly, you cannot override a private method in subclass
because it’s not accessible there. What you can do is create another private
method with the same name in the child class. Let’s take a look at the example
below to understand it better.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 |
class Base { private static void display() { System.out.println("Static or class method from Base"); } public void print() { System.out.println("Non-static or instance method from
Base"); } class Derived extends Base { private static void display() { System.out.println("Static or class method from Derived"); } public void print() { System.out.println("Non-static or instance method from
Derived"); } public class test { public static void main(String args[]) { Base obj= new Derived(); obj1.display(); obj1.print(); } } |
Q10. What is multiple inheritance? Is it supported
by Java?
If a child class
inherits the property from multiple classes is known as multiple inheritance.
Java does not allow to extend multiple classes.
The problem with multiple inheritance is that if
multiple parent classes have the same method name, then at runtime it becomes
difficult for the compiler to decide which method to execute from the child
class.
Therefore, Java doesn’t support multiple inheritance.
The problem is commonly referred to as Diamond Problem.
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Q11. What is encapsulation in Java?
Encapsulation is a mechanism where you bind your
data(variables) and code(methods) together as a single unit. Here, the data is
hidden from the outer world and can be accessed only via current class methods.
This helps in protecting the data from any unnecessary modification. We
can achieve encapsulation in Java by:
·
Declaring the variables of a class as private.
·
Providing public setter and getter methods to modify and view the values
of the variables.
Q12. What is an association?
Association is a relationship where all object have
their own lifecycle and there is no owner. Let’s take the example of Teacher
and Student. Multiple students can associate with a single teacher and a single
student can associate with multiple teachers but there is no ownership between
the objects and both have their own lifecycle. These relationships can be one
to one, one to many, many to one and many to many.
Q13. What do you mean by aggregation?
An aggregation is a specialized form of Association
where all object has their own lifecycle but there is ownership and child
object can not belong to another parent object. Let’s take an example of
Department and teacher. A single teacher can not belong to multiple
departments, but if we delete the department teacher object will not
destroy.
Q14. What is composition in Java?
Composition is again a specialized form of
Aggregation and we can call this as a “death” relationship. It is a strong type
of Aggregation. Child object does not have their lifecycle and if parent object
deletes all child object will also be deleted. Let’s take again an example of a
relationship between House and rooms. House can contain multiple rooms there is
no independent life of room and any room can not belongs to two different
houses if we delete the house room will automatically delete.
Q15. What is a marker interface?
A Marker interface can be defined as the interface
having no data member and member functions. In simpler terms, an empty
interface is called the Marker interface. The most common examples of Marker
interface in Java are Serializable, Cloneable etc. The marker interface can be
declared as follows.
1 2 |
public interface Serializable{ } |
Q16. What is object cloning in Java?
Object cloning in Java is the process of creating
an exact copy of an object. It basically means the ability to create an object
with a similar state as the original object. To achieve this, Java provides a
method clone() to make use of this functionality. This method
creates a new instance of the class of the current object and then initializes
all its fields with the exact same contents of corresponding fields. To object
clone(), the marker interface java.lang.Cloneable must be
implemented to avoid any runtime exceptions. One thing you must note is Object
clone() is a protected method, thus you need to override it.
Q17.
What is a copy constructor in Java?
Copy constructor is a member function that is used
to initialize an object using another object of the same class. Though there is
no need for copy constructor in Java since all objects are passed by reference.
Moreover, Java does not even support automatic pass-by-value.
Q18. What is a constructor overloading in Java?
In Java, constructor overloading is a
technique of adding any number of constructors to a class each having
a different parameter list. The compiler uses the number of parameters and
their types in the list to differentiate the overloaded constructors.
1 2 3 4 5 6 7 8 9 10 11 12 |
class Demo { int i; public Demo(int a) { i=k; } public Demo(int a, int b) { //body } } |
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Servlets – Java Interview Questions
Q1. What is a servlet?
·
Java Servlet is server-side technologies to extend the capability of web
servers by providing support for dynamic response and data persistence.
·
The javax.servlet and javax.servlet.http packages provide interfaces and
classes for writing our own servlets.
·
All servlets must implement the javax.servlet.Servlet interface, which
defines servlet lifecycle methods. When implementing a generic service, we can
extend the GenericServlet class provided with the Java Servlet API. The
HttpServlet class provides methods, such as doGet() and doPost(), for handling
HTTP-specific services.
·
Most of the times, web applications are accessed using HTTP protocol and
thats why we mostly extend HttpServlet class. Servlet API hierarchy is shown in
below image.
Q2. What are the differences between Get and Post
methods?
Get |
Post |
Limited amount of data can be
sent because data is sent in header. |
Large amount of data can be
sent because data is sent in body. |
Not Secured because data
is exposed in URL bar. |
Secured because data is
not exposed in URL bar. |
Can be bookmarked |
Cannot be bookmarked |
Idempotent |
Non-Idempotent |
It is more efficient and
used than Post |
It is less efficient and
used |
Q3. What is Request Dispatcher?
RequestDispatcher interface is used to forward the
request to another resource that can be HTML, JSP or another servlet in same
application. We can also use this to include the content of another resource to
the response.
There are two methods defined in this interface:
1.void forward()
2.void include()
Q4. What are the differences between forward()
method and sendRedirect() methods?
forward() method |
SendRedirect() method |
forward() sends the same
request to another resource. |
sendRedirect() method sends new
request always because it uses the URL bar of the browser. |
forward() method works at
server side. |
sendRedirect() method
works at client side. |
forward() method works
within the server only. |
sendRedirect() method works within
and outside the server. |
Q5. What is the life-cycle of a servlet?
There are 5 stages in the lifecycle of a servlet:
1.
Servlet is loaded
2.
Servlet is instantiated
3.
Servlet is initialized
4.
Service the request
5.
Servlet is destroyed
Q6. How does cookies work in Servlets?
·
Cookies are text data sent by server to the client and it gets saved at
the client local machine.
·
Servlet API provides cookies support through javax.servlet.http.Cookie
class that implements Serializable and Cloneable interfaces.
·
HttpServletRequest getCookies() method is provided to get the array of
Cookies from request, since there is no point of adding Cookie to request,
there are no methods to set or add cookie to request.
·
Similarly HttpServletResponse addCookie(Cookie c) method is provided to
attach cookie in response header, there are no getter methods for cookie.
Q7. What are the differences between ServletContext
vs ServletConfig?
The difference between ServletContext and
ServletConfig in Servlets JSP is in below tabular format.
ServletConfig |
ServletContext |
Servlet config object represent
single servlet |
It represent whole web
application running on particular JVM and common for all the servlet |
Its like local parameter
associated with particular servlet |
Its like global parameter
associated with whole application |
It’s a name value pair defined
inside the servlet section of web.xml file so it has servlet wide scope |
ServletContext has application
wide scope so define outside of servlet tag in web.xml file. |
getServletConfig() method is
used to get the config object |
getServletContext() method is
used to get the context object. |
for example shopping cart of a
user is a specific to particular user so here we can use servlet config |
To get the MIME type of a file
or application session related information is stored using servlet context
object. |
Q8. What are the different methods of
session management in servlets?
Session is a conversational state between client
and server and it can consists of multiple request and response between client
and server. Since HTTP and Web Server both are stateless, the only way to
maintain a session is when some unique information about the session (session
id) is passed between server and client in every request and response.
Some of the common ways of session management in
servlets are:
1.
User Authentication
2.
HTML Hidden Field
3.
Cookies
4.
URL Rewriting
5.
Session Management API
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JDBC – Java Interview Questions
1. What is JDBC Driver?
JDBC Driver is a software component that enables
java application to interact with the database. There are 4 types of JDBC
drivers:
1.
JDBC-ODBC bridge driver
2.
Native-API driver (partially java driver)
3.
Network Protocol driver (fully java driver)
4.
Thin driver (fully java driver)
2. What are the steps to connect to a database in
java?
·
Registering the driver class
·
Creating connection
·
Creating statement
·
Executing queries
·
Closing connection
3. What are the JDBC API components?
The java.sql package contains interfaces and
classes for JDBC API.
Interfaces:
·
Connection
·
Statement
·
PreparedStatement
·
ResultSet
·
ResultSetMetaData
·
DatabaseMetaData
·
CallableStatement etc.
Classes:
·
DriverManager
·
Blob
·
Clob
·
Types
·
SQLException etc.
4. What is the role of JDBC DriverManager class?
The DriverManager class manages
the registered drivers. It can be used to register and unregister drivers. It
provides factory method that returns the instance of Connection.
5. What is JDBC Connection interface?
The Connection interface maintains a
session with the database. It can be used for transaction management. It
provides factory methods that returns the instance of Statement,
PreparedStatement, CallableStatement and DatabaseMetaData.
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6. What is the purpose of JDBC ResultSet
interface?
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.
7. What is JDBC ResultSetMetaData interface?
The ResultSetMetaData interface returns the
information of table such as total number of columns, column name, column type
etc.
8. What is JDBC DatabaseMetaData interface?
The DatabaseMetaData interface returns the
information of the database such as username, driver name, driver version,
number of tables, number of views etc.
9. What do you mean by batch processing in JDBC?
Batch processing helps you to group related SQL
statements into a batch and execute them instead of executing a single query.
By using batch processing technique in JDBC, you can execute multiple queries
which makes the performance faster.
10. What is the difference between execute,
executeQuery, executeUpdate?
Statement execute(String query) is
used to execute any SQL query and it returns TRUE if the result is an ResultSet
such as running Select queries. The output is FALSE when there is no ResultSet
object such as running Insert or Update queries. We can use getResultSet() to
get the ResultSet and getUpdateCount() method to retrieve the
update count.
Statement executeQuery(String query) is
used to execute Select queries and returns the ResultSet. ResultSet returned is
never null even if there are no records matching the query. When executing select
queries we should use executeQuery method so that if someone tries to execute
insert/update statement it will throw java.sql.SQLException with message
“executeQuery method can not be used for update”.
Statement executeUpdate(String query)
is used to execute Insert/Update/Delete (DML) statements or DDL statements that
returns nothing. The output is int and equals to the row count for SQL Data
Manipulation Language (DML) statements. For DDL statements, the output is 0.
You should use execute() method only when you are
not sure about the type of statement else use executeQuery or executeUpdate
method.
Q11. What do you understand by JDBC Statements?
JDBC statements are basically the statements which
are used to send SQL commands to the database and retrieve data back from the
database. Various methods like execute(), executeUpdate(), executeQuery, etc.
are provided by JDBC to interact with the database.
JDBC supports 3 types of statements:
1.
Statement: Used for general purpose access to
the database and executes a static SQL query at runtime.
2.
PreparedStatement: Used to provide
input parameters to the query during execution.
3.
CallableStatement: Used to access the
database stored procedures and helps in accepting runtime parameters.
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Spring Framework – Java Interview Questions
Q1. What is Spring?
Wikipedia defines the Spring framework as “an
application framework and inversion of control container for the Java platform.
The framework’s core features can be used by any Java application, but there
are extensions for building web applications on top of the Java EE platform.”
Spring is essentially a lightweight, integrated framework that can be used for
developing enterprise applications in java.
Q2. Name the different modules of the Spring
framework.
Some of the important Spring Framework modules are:
·
Spring Context – for dependency injection.
·
Spring AOP – for aspect oriented programming.
·
Spring DAO – for database operations using DAO pattern
·
Spring JDBC – for JDBC and DataSource support.
·
Spring ORM – for ORM tools support such as Hibernate
·
Spring Web Module – for creating web applications.
·
Spring MVC – Model-View-Controller implementation for creating web
applications, web services etc.
Q3. List some of the important annotations in
annotation-based Spring configuration.
The important annotations are:
·
@Required
·
@Autowired
·
@Qualifier
·
@Resource
·
@PostConstruct
·
@PreDestroy
Q4. Explain Bean in Spring and List the different
Scopes of Spring bean.
Beans are objects that form the backbone of a
Spring application. They are managed by the Spring IoC container. In other
words, a bean is an object that is instantiated, assembled, and managed by a
Spring IoC container.
There are five Scopes defined in Spring beans.
·
Singleton: Only one instance of the bean will
be created for each container. This is the default scope for the spring beans.
While using this scope, make sure spring bean doesn’t have shared instance
variables otherwise it might lead to data inconsistency issues because it’s not
thread-safe.
·
Prototype: A new instance will be created
every time the bean is requested.
·
Request: This is same as prototype scope,
however it’s meant to be used for web applications. A new instance of the bean
will be created for each HTTP request.
·
Session: A new bean will be created for each
HTTP session by the container.
·
Global-session: This is used to create global
session beans for Portlet applications.
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Q5. Explain the role of DispatcherServlet and
ContextLoaderListener.
DispatcherServlet is basically
the front controller in the Spring MVC application as it loads the spring bean
configuration file and initializes all the beans that have been configured. If
annotations are enabled, it also scans the packages to configure any bean
annotated with @Component, @Controller, @Repository or @Service annotations.
ContextLoaderListener, on the other
hand, is the listener to start up and shut down the WebApplicationContext in
Spring root. Some of its important functions includes tying up the lifecycle of
Application Context to the lifecycle of the ServletContext and automating the
creation of ApplicationContext.
Q6. What are the differences between constructor
injection and setter injection?
No. |
Constructor Injection |
Setter Injection |
1) |
No Partial Injection |
Partial Injection |
2) |
Doesn’t override the
setter property |
Overrides the constructor
property if both are defined. |
3) |
Creates a new instance if any
modification occurs |
Doesn’t create a new instance
if you change the property value |
4) |
Better for too many
properties |
Better for a few
properties. |
Q7.
What is autowiring in Spring? What are the autowiring modes?
Autowiring enables the programmer to inject the
bean automatically. We don’t need to write explicit injection logic. Let’s
see the code to inject bean using dependency injection.
1.
<bean
id=“emp” class=“com.javatpoint.Employee” autowire=“byName” />
The autowiring modes are given below:
No. |
Mode |
Description |
1) |
no |
this is the default mode,
it means autowiring is not enabled. |
2) |
byName |
Injects the bean based on
the property name. It uses setter method. |
3) |
byType |
Injects the bean based on
the property type. It uses setter method. |
4) |
constructor |
It injects the bean using
constructor |
Q8. How to handle exceptions in Spring MVC
Framework?
Spring MVC Framework provides the following ways to
help us achieving robust exception handling.
Controller Based:
We can define exception handler methods in our
controller classes. All we need is to annotate these methods with
@ExceptionHandler annotation.
Global Exception Handler:
Exception Handling is a cross-cutting concern and
Spring provides @ControllerAdvice annotation that we can use with any class to
define our global exception handler.
HandlerExceptionResolver implementation:
For generic exceptions, most of the times we serve
static pages. Spring Framework provides HandlerExceptionResolver interface that
we can implement to create global exception handler. The reason behind this
additional way to define global exception handler is that Spring framework also
provides default implementation classes that we can define in our spring bean
configuration file to get spring framework exception handling benefits.
Q9.
What are some of the important Spring annotations which you have used?
Some of the Spring annotations that I have used in
my project are:
@Controller – for
controller classes in Spring MVC project.
@RequestMapping – for
configuring URI mapping in controller handler methods. This is a very important
annotation, so you should go through Spring MVC RequestMapping Annotation
Examples
@ResponseBody – for sending
Object as response, usually for sending XML or JSON data as response.
@PathVariable – for mapping
dynamic values from the URI to handler method arguments.
@Autowired – for autowiring
dependencies in spring beans.
@Qualifier – with
@Autowired annotation to avoid confusion when multiple instances of bean type
is present.
@Service – for service
classes.
@Scope – for
configuring the scope of the spring bean.
@Configuration, @ComponentScan and
@Bean – for java based configurations.
AspectJ annotations for configuring aspects and
advices , @Aspect, @Before, @After, @Around, @Pointcut, etc.
Q10. How to integrate Spring and Hibernate
Frameworks?
We can use Spring ORM module to integrate Spring
and Hibernate frameworks if you are using Hibernate 3+ where SessionFactory
provides current session, then you should avoid
using HibernateTemplate or HibernateDaoSupport classes and
better to use DAO pattern with dependency injection for the integration.
Also, Spring ORM provides support for using Spring
declarative transaction management, so you should utilize that rather than
going for hibernate boiler-plate code for transaction management.
Q11.
Name the types of transaction management that Spring supports.
Two types of transaction management are supported
by Spring. They are:
1.
Programmatic transaction management: In this, the
transaction is managed with the help of programming. It provides you extreme
flexibility, but it is very difficult to maintain.
2.
Declarative transaction management: In this,
transaction management is separated from the business code. Only annotations or
XML based configurations are used to manage the transactions.
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Hibernate – Java Interview Questions
1. What is Hibernate Framework?
Object-relational mapping or ORM is the programming
technique to map application domain model objects to the relational database
tables. Hibernate is Java-based ORM tool that provides a framework for mapping
application domain objects to the relational database tables and vice versa.
Hibernate provides a reference implementation of
Java Persistence API, that makes it a great choice as ORM tool with benefits of
loose coupling. We can use the Hibernate persistence API for CRUD operations.
Hibernate framework provide option to map plain old java objects to traditional
database tables with the use of JPA annotations as well as XML based
configuration.
Similarly, hibernate configurations are flexible
and can be done from XML configuration file as well as programmatically.
2. What are the important benefits of using
Hibernate Framework?
Some of the important benefits of using hibernate
framework are:
1.
Hibernate eliminates all the boiler-plate code that comes with JDBC and
takes care of managing resources, so we can focus on business logic.
2.
Hibernate framework provides support for XML as well as JPA annotations,
that makes our code implementation independent.
3.
Hibernate provides a powerful query language (HQL) that is similar to
SQL. However, HQL is fully object-oriented and understands concepts like
inheritance, polymorphism, and association.
4.
Hibernate is an open source project from Red Hat Community and used
worldwide. This makes it a better choice than others because learning curve is
small and there are tons of online documentation and help is easily available
in forums.
5.
Hibernate is easy to integrate with other Java EE frameworks, it’s so
popular that Spring Framework provides built-in support for integrating
hibernate with Spring applications.
6.
Hibernate supports lazy initialization using proxy objects and perform
actual database queries only when it’s required.
7.
Hibernate cache helps us in getting better performance.
8.
For database vendor specific feature, hibernate is suitable because we
can also execute native sql queries.
Overall hibernate is the best choice in current
market for ORM tool, it contains all the features that you will ever need in an
ORM tool.
3. Explain Hibernate architecture.
Hibernate has a layered architecture which helps the user to operate
without having to know the underlying APIs. Hibernate makes use of the database
and configuration data to provide persistence services (and persistent objects)
to the application. It includes many objects such as persistent object,
session factory, transaction factory, connection factory, session, transaction
etc.
The Hibernate architecture is categorized in four layers.
·
Java application layer
·
Hibernate framework layer
·
Backhand API layer
·
Database layer
4. What are the differences between get and load
methods?
The differences between get() and load() methods
are given below.
No. |
get() |
load() |
1) |
Returns null if
object is not found. |
Throws ObjectNotFoundException if
an object is not found. |
2) |
get() method
always hit the database. |
load() method doesn’t
hit the database. |
3) |
It returns a real
object, not a proxy. |
It returns a proxy
object. |
4) |
It should be used if you
are not sure about the existence of instance. |
It should be used if you
are sure that the instance exists. |
5. What are the advantages of Hibernate over JDBC?
Some of the important advantages of Hibernate
framework over JDBC are:
1.
Hibernate removes a lot of boiler-plate code that comes with JDBC API,
the code looks cleaner and readable.
2.
Hibernate supports inheritance, associations, and collections. These
features are not present with JDBC API.
3.
Hibernate implicitly provides transaction management, in fact, most of
the queries can’t be executed outside transaction. In JDBC API, we need to
write code for transaction management using commit and rollback.
4.
JDBC API throws SQLException that is a checked exception, so
we need to write a lot of try-catch block code. Most of the times it’s
redundant in every JDBC call and used for transaction management. Hibernate
wraps JDBC exceptions and
throw JDBCException or HibernateException un-checked
exception, so we don’t need to write code to handle it. Hibernate built-in
transaction management removes the usage of try-catch blocks.
5.
Hibernate Query Language (HQL) is more object-oriented and close to Java
programming language. For JDBC, we need to write native SQL queries.
6.
Hibernate supports caching that is better for performance, JDBC queries
are not cached hence performance is low.
7.
Hibernate provides option through which we can create database tables
too, for JDBC tables must exist in the database.
8.
Hibernate configuration helps us in using JDBC like connection as well
as JNDI DataSource for the connection pool. This is a very important feature in
enterprise application and completely missing in JDBC API.
9.
Hibernate supports JPA annotations, so the code is independent of the
implementation and easily replaceable with other ORM tools. JDBC code is very
tightly coupled with the application.
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JSP – Java Interview Questions
1. What are the life-cycle methods for a jsp?
Methods |
Description |
public void jspInit() |
It is invoked only once, same
as init method of servlet. |
public void
_jspService(ServletRequest request,ServletResponse)throws
ServletException,IOException |
It is invoked at each request,
same as service() method of servlet. |
public void jspDestroy() |
It is invoked only once, same
as destroy() method of servlet. |
2. What are the JSP implicit objects?
JSP provides 9 implicit objects by default. They
are as follows:
Object |
Type |
1) out |
JspWriter |
2) request |
HttpServletRequest |
3) response |
HttpServletResponse |
4) config |
ServletConfig |
5) session |
HttpSession |
6) application |
ServletContext |
7) pageContext |
PageContext |
8) page |
Object |
9) exception |
Throwable |
3. What are the differences between include directive
and include action?
include directive |
include action |
The include directive includes
the content at page translation time. |
The include action includes the
content at request time. |
The include directive includes
the original content of the page so page size increases at runtime. |
The include action doesn’t
include the original content rather invokes the include() method of Vendor
provided class. |
It’s better for static
pages. |
It’s better for dynamic
pages. |
4. How to disable caching on back button of the
browser?
<%
response.setHeader(“Cache-Control”,”no-store”);
response.setHeader(“Pragma”,”no-cache”);
response.setHeader (“Expires”, “0”);
//prevents caching at the proxy server
%>
5. What are the different tags provided in JSTL?
There are 5 type of JSTL tags.
1.
core tags
2.
sql tags
3.
xml tags
4.
internationalization tags
5.
functions tags
6. How to disable session in JSP?
1.
<%@ page session=“false” %>
7. How to delete a Cookie in a JSP?
The following code explains how to delete a Cookie
in a JSP :
1 2 3 4 5 6 7 8 9 10 11 |
Cookie mycook = new Cookie("name1","value1"); response.addCookie(mycook1); Cookie killmycook = new Cookie("mycook1","value1"); killmycook . set MaxAge ( 0 ); killmycook . set Path ("/"); killmycook . addCookie ( killmycook 1 ); |
8. Explain the jspDestroy() method.
jspDestry() method is invoked from javax.servlet.jsp.JspPage interface
whenever a JSP page is about to be destroyed. Servlets destroy methods can be
easily overridden to perform cleanup, like when closing a database connection.
9. How is JSP better than Servlet technology?
JSP is a technology on the server’s side to make
content generation simple. They are document-centric, whereas servlets are
programs. A Java server page can contain fragments of Java program, which
execute and instantiate Java classes. However, they occur inside an HTML
template file. It provides the framework for the development of a Web
Application.
10. Why should we not configure JSP standard tags
in web.xml?
We don’t need to
configure JSP standard tags in web.xml because when container loads the web
application and find TLD files, it automatically configures them to be used
directly in the application JSP pages. We just need to include it in the JSP
page using taglib directive.
11.
How will you use JSP EL in order to get the HTTP method name?
Using pageContext JSP EL implicit object you can get the request object
reference and make use of the dot operator to retrieve the HTTP method name in
the JSP page. The JSP EL code for this purpose will look
like ${pageContext.request.method}.
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Exception and Thread Java Interview Questions
Q1. What is the difference between Error and
Exception?
An error is an irrecoverable condition occurring at
runtime. Such as OutOfMemory error. These JVM errors you cannot repair them at
runtime. Though error can be caught in the catch block but the execution of
application will come to a halt and is not recoverable.
While exceptions are conditions that occur because
of bad input or human error etc. e.g. FileNotFoundException will be thrown if
the specified file does not exist. Or a NullPointerException will take place if
you try using a null reference. In most of the cases it is possible to recover
from an exception (probably by giving the user feedback for entering proper
values etc.
Q2. How can you handle Java exceptions?
There are five keywords used to handle exceptions
in Java:
1.
try
2.
catch
3.
finally
4.
throw
5.
throws
Q3. What are the differences between Checked
Exception and Unchecked Exception?
Checked Exception
·
The classes that extend Throwable class except RuntimeException and
Error are known as checked exceptions.
·
Checked exceptions are checked at compile-time.
·
Example: IOException, SQLException etc.
Unchecked Exception
·
The classes that extend RuntimeException are known as unchecked
exceptions.
·
Unchecked exceptions are not checked at compile-time.
·
Example: ArithmeticException, NullPointerException etc.
Q4. What purpose do the keywords final, finally,
and finalize fulfill?
Final:
Final is used to apply restrictions on class,
method, and variable. A final class can’t be inherited, final method can’t be
overridden and final variable value can’t be changed. Let’s take a look at the
example below to understand it better.
1 2 3 4 5 6 |
class FinalVarExample { public static void main( String args[]) { final int a=10; // Final variable a=50;
//Error as value can't be changed } |
Finally
Finally is used to place important code, it will be
executed whether the exception is handled or not. Let’s take a look at the
example below to understand it better.
1 2 3 4 5 6 7 8 9 10 11 12 |
class FinallyExample { public static void main(String args[]){ try { int x=100; } catch(Exception e) { System.out.println(e); } finally { System.out.println("finally block is executing");} }} } |
Finalize
Finalize is used to perform clean up processing
just before the object is garbage collected. Let’s take a look at the example
below to understand it better.
1 2 3 4 5 6 7 8 9 10 11 12 13 |
class FinalizeExample { public void finalize() { System.out.println("Finalize is called"); } public static void main(String args[]) { FinalizeExample f1=new FinalizeExample(); FinalizeExample f2=new FinalizeExample(); f1= NULL; f2=NULL; System.gc(); } } |
Q5. What
are the differences between throw and throws?
throw keyword |
throws keyword |
Throw is used to explicitly
throw an exception. |
Throws is used to declare an
exception. |
Checked exceptions can not be
propagated with throw only. |
Checked exception can be
propagated with throws. |
Throw is followed by an
instance. |
Throws is followed by class. |
Throw is used within the
method. |
Throws is used with the method
signature. |
You cannot throw multiple
exception |
You can declare multiple
exception e.g. public void method()throws IOException,SQLException. |
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Q6. What is exception hierarchy in java?
The hierarchy is as follows:
Throwable is a parent class of all Exception
classes. There are two types of Exceptions: Checked exceptions and
UncheckedExceptions or RunTimeExceptions. Both type of exceptions extends
Exception class whereas errors are further classified into Virtual Machine
error and Assertion error.
Q7. How to create a custom Exception?
To create you own exception extend the Exception
class or any of its subclasses.
·
class New1Exception extends Exception { }
// this will create Checked Exception
·
class NewException extends IOException { }
// this will create Checked exception
·
class NewException extends NullPonterExcpetion { } // this will
create UnChecked exception
Q8. What are the important methods of Java
Exception Class?
Exception and all of it’s subclasses doesn’t
provide any specific methods and all of the methods are defined in the base
class Throwable.
1.
String getMessage() – This method
returns the message String of Throwable and the message can be provided while
creating the exception through it’s constructor.
2.
String getLocalizedMessage() – This method is
provided so that subclasses can override it to provide locale specific message
to the calling program. Throwable class implementation of this method simply
use getMessage() method to return the exception message.
3.
Synchronized Throwable getCause() – This method
returns the cause of the exception or null id the cause is unknown.
4.
String toString() – This method returns the
information about Throwable in String format, the returned String contains the
name of Throwable class and localized message.
5.
void printStackTrace() – This method
prints the stack trace information to the standard error stream, this method is
overloaded and we can pass PrintStream or PrintWriter as an argument to write
the stack trace information to the file or stream.
Q9. What are the differences between processes and
threads?
|
Process |
Thread |
Definition |
An executing instance of a
program is called a process. |
A thread is a subset of the
process. |
Communication |
Processes must use
inter-process communication to communicate with sibling processes. |
Threads can directly
communicate with other threads of its process. |
Control |
Processes can only exercise
control over child processes. |
Threads can exercise
considerable control over threads of the same process. |
Changes |
Any change in the parent
process does not affect child processes. |
Any change in the main thread
may affect the behavior of the other threads of the process. |
Memory |
Run in separate memory spaces. |
Run in shared memory spaces. |
Controlled
by |
Process is controlled by the
operating system. |
Threads are controlled by
programmer in a program. |
Dependence |
Processes are independent. |
Threads are dependent. |
Q10. What is a finally block? Is there a case
when finally will not execute?
Finally block is a block which always executes a
set of statements. It is always associated with a try block regardless of any
exception that occurs or not.
Yes, finally will not be executed if the program exits either by calling
System.exit() or by causing a fatal error that causes the process to abort.
Q11. What is synchronization?
Synchronization refers to multi-threading. A
synchronized block of code can be executed by only one thread at a time. As
Java supports execution of multiple threads, two or more threads may access the
same fields or objects. Synchronization is a process which keeps all concurrent
threads in execution to be in sync. Synchronization avoids memory consistency
errors caused due to inconsistent view of shared memory. When a method is
declared as synchronized the thread holds the monitor for that method’s object.
If another thread is executing the synchronized method the thread is blocked
until that thread releases the monitor.
Q12. Can
we write multiple catch blocks under single try block?
Yes we can have multiple catch blocks under single
try block but the approach should be from specific to general. Let’s understand
this with a programmatic example.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 |
public class Example { public static void main(String args[]) { try { int a[]= new int[10]; a[10]= 10/0; } catch(ArithmeticException e) { System.out.println("Arithmetic exception in first catch
block"); } catch(ArrayIndexOutOfBoundsException e) { System.out.println("Array index out of bounds in second catch
block"); } catch(Exception e) { System.out.println("Any exception in third catch block"); } } |
Q13. What are the important methods of Java
Exception Class?
Methods are defined in the base class Throwable.
Some of the important methods of Java exception class are stated below.
1.
String getMessage() – This method
returns the message String about the exception. The message can be provided
through its constructor.
2.
public StackTraceElement[] getStackTrace() – This method returns
an array containing each element on the stack trace. The element at index 0
represents the top of the call stack whereas the last element in the array
represents the method at the bottom of the call stack.
3.
Synchronized Throwable getCause() – This method
returns the cause of the exception or null id as represented by a Throwable
object.
4.
String toString() – This method returns the information
in String format. The returned String contains the name of Throwable class and
localized message.
5.
void printStackTrace() – This method
prints the stack trace information to the standard error stream.
Q14.
What is OutOfMemoryError in Java?
OutOfMemoryError is the subclass of java.lang.Error which generally
occurs when our JVM runs out of memory.
Q15.
What is a Thread?
A thread is the smallest piece of programmed
instructions which can be executed independently by a scheduler. In Java, all
the programs will have at least one thread which is known as
the main thread. This main thread is created by the JVM when the
program starts its execution. The main thread is used to invoke the main() of
the program.
Q16.
What are the two ways to create a thread?
In Java, threads can be created in the following two ways:-
·
By implementing the Runnable interface.
·
By extending the Thread
Q17. What are the different types of garbage
collectors in Java?
Garbage collection in Java a program which helps in
implicit memory management. Since in Java, using the new keyword you can create
objects dynamically, which once created will consume some memory. Once the job
is done and there are no more references left to the object, Java using garbage
collection destroys the object and relieves the memory occupied by it. Java
provides four types of garbage collectors:
·
Serial Garbage Collector
·
Parallel Garbage Collector
·
CMS Garbage Collector
·
G1 Garbage Collector