Tuesday, 18 July 2017
Wednesday, 5 July 2017
Java Interview Questions and Answers
Java Interview Questions and Answers:
Q1. What is the difference between an Inner Class and a Sub-Class?
Ans: An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class.
A sub-class is a class which inherits from another class called super class. Sub-class can access all public and protected methods and fields of its super class.
Q2. What are the various access specifiers for Java classes?
Ans: In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are:
Public : Class,Method,Field is accessible from anywhere.
Protected:Method,Field can be accessed from the same class to which they belong or from the sub-classes,and from the class of same package,but not from outside.
Default: Method,Field,class can be accessed only from the same package and not from outside of it’s native package.
Private: Method,Field can be accessed from the same class to which they belong.
Q3. What’s the purpose of Static methods and static variables?
Ans: When there is a requirement to share a method or a variable between multiple objects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects.
Q4. What is data encapsulation and what’s its significance?
Ans: Encapsulation is a concept in Object Oriented Programming for combining properties and methods in a single unit.
Encapsulation helps programmers to follow a modular approach for software development as each object has its own set of methods and variables and serves its functions independent of other objects. Encapsulation also serves data hiding purpose.
Q5. What is a singleton class? Give a practical example of its usage.
A singleton class in java can have only one instance and hence all its methods and variables belong to just one instance. Singleton class concept is useful for the situations when there is a need to limit the number of objects for a class.
The best example of singleton usage scenario is when there is a limit of having only one connection to a database due to some driver limitations or because of any licensing issues.
Q6. What are Loops in Java? What are three types of loops?
Ans: Looping is used in programming to execute a statement or a block of statement repeatedly. There are three types of loops in Java:
1) For Loops
For loops are used in java to execute statements repeatedly for a given number of times. For loops are used when number of times to execute the statements is known to programmer.
2) While Loops
While loop is used when certain statements need to be executed repeatedly until a condition is fulfilled. In while loops, condition is checked first before execution of statements.
3) Do While Loops
Do While Loop is same as While loop with only difference that condition is checked after execution of block of statements. Hence in case of do while loop, statements are executed at least once.
Q7. What is the difference between double and float variables in Java?
Ans: float takes 4 bytes in memory while Double takes 8 bytes in memory. Float is single precision floating point decimal number while Double is double precision decimal number.
Q8. What is Final Keyword in Java? Give an example.
Ans: A constant is declared using the keyword Final. Value can be assigned only once and after assignment, value of a constant can’t be changed.
In below example, a constant with the name const_val is declared and assigned avalue:
Private Final int const_val=100
When a method is declared as final,it can NOT be overridden by the subclasses.This method are faster than any other method,because they are resolved at complied time.
When a class is declares as final,it cannot be subclassed. Example String,Integer and other wrapper classes.
Q9. What’s the base class in Java from which all classes are derived?
Ans: java.lang.object
Q10. Can main() method in Java can return any data?
Ans: In java, main() method can’t return any data and hence, it’s always declared with a void return type.
Q11. What are Java Packages? What’s the significance of packages?
Ans: In Java, package is a collection of classes and interfaces which are bundled together as they are related to each other. Use of packages helps developers to modularize the code and group the code for proper re-use. Once code has been packaged in Packages, it can be imported in other classes and used.
Q12. Can we declare a class as Abstract without having any abstract method?
Ans: Yes we can create an abstract class by using abstract keyword before class name even if it doesn’t have any abstract method. However, if a class has even one abstract method, it must be declared as abstract otherwise it will give an error.
Q13. What’s the difference between an Abstract Class and Interface in Java?
Ans: The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers (public, private etc) with or without concrete implementation.
Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn’t require implementation of all the methods of its super class.
A class can implement multiple interfaces but it can extend only one abstract class.
Q14. What are the performance implications of Interfaces over abstract classes?
Ans: Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces. Another key factor for developers to take into consideration is that any class can extend only one abstract class while a class can implement many interfaces.
Use of interfaces also puts an extra burden on the developers as any time an interface is implemented in a class; developer is forced to implement each and every method of interface.
Q15. Does Importing a package imports its sub-packages as well in Java?
Ans: In java, when a package is imported, its sub-packages aren’t imported and developer needs to import them separately if required.
For example, if a developer imports a package university.*, all classes in the package named university are loaded but no classes from the sub-package are loaded. To load the classes from its sub-package ( say department), developer has to import it explicitly as follows:
Import university.department.*
Q16. Can we declare the main method of our class as private?
Ans: In java, main method must be public static in order to run any application correctly. If main method is declared as private, developer won’t get any compilation error however, it will not get executed and will give a runtime error.
Q17. How can we pass argument to a function by reference instead of pass by value?
Ans: In java, we can pass argument to a function only by value and not by reference.
Q18. How an object is serialized in java?
Ans: In java, to convert an object into byte stream by serialization, an interface with the name Serializable is implemented by the class. All objects of a class implementing serializable interface get serialized and their state is saved in byte stream.
Q19. When we should use serialization?
Ans: Serialization is used when data needs to be transmitted over the network. Using serialization, object’s state is saved and converted into byte stream .The byte stream is transferred over the network and the object is re-created at destination.
Q20. Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling?
Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.
Q21. Is there any way to skip Finally block of exception even if some exception occurs in the exception block?
Ans: If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block:
System.exit(0);
Q1. What is the difference between an Inner Class and a Sub-Class?
Ans: An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class.
A sub-class is a class which inherits from another class called super class. Sub-class can access all public and protected methods and fields of its super class.
Q2. What are the various access specifiers for Java classes?
Ans: In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are:
Public : Class,Method,Field is accessible from anywhere.
Protected:Method,Field can be accessed from the same class to which they belong or from the sub-classes,and from the class of same package,but not from outside.
Default: Method,Field,class can be accessed only from the same package and not from outside of it’s native package.
Private: Method,Field can be accessed from the same class to which they belong.
Q3. What’s the purpose of Static methods and static variables?
Ans: When there is a requirement to share a method or a variable between multiple objects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects.
Q4. What is data encapsulation and what’s its significance?
Ans: Encapsulation is a concept in Object Oriented Programming for combining properties and methods in a single unit.
Encapsulation helps programmers to follow a modular approach for software development as each object has its own set of methods and variables and serves its functions independent of other objects. Encapsulation also serves data hiding purpose.
Q5. What is a singleton class? Give a practical example of its usage.
A singleton class in java can have only one instance and hence all its methods and variables belong to just one instance. Singleton class concept is useful for the situations when there is a need to limit the number of objects for a class.
The best example of singleton usage scenario is when there is a limit of having only one connection to a database due to some driver limitations or because of any licensing issues.
Q6. What are Loops in Java? What are three types of loops?
Ans: Looping is used in programming to execute a statement or a block of statement repeatedly. There are three types of loops in Java:
1) For Loops
For loops are used in java to execute statements repeatedly for a given number of times. For loops are used when number of times to execute the statements is known to programmer.
2) While Loops
While loop is used when certain statements need to be executed repeatedly until a condition is fulfilled. In while loops, condition is checked first before execution of statements.
3) Do While Loops
Do While Loop is same as While loop with only difference that condition is checked after execution of block of statements. Hence in case of do while loop, statements are executed at least once.
Q7. What is the difference between double and float variables in Java?
Ans: float takes 4 bytes in memory while Double takes 8 bytes in memory. Float is single precision floating point decimal number while Double is double precision decimal number.
Q8. What is Final Keyword in Java? Give an example.
Ans: A constant is declared using the keyword Final. Value can be assigned only once and after assignment, value of a constant can’t be changed.
In below example, a constant with the name const_val is declared and assigned avalue:
Private Final int const_val=100
When a method is declared as final,it can NOT be overridden by the subclasses.This method are faster than any other method,because they are resolved at complied time.
When a class is declares as final,it cannot be subclassed. Example String,Integer and other wrapper classes.
Q9. What’s the base class in Java from which all classes are derived?
Ans: java.lang.object
Q10. Can main() method in Java can return any data?
Ans: In java, main() method can’t return any data and hence, it’s always declared with a void return type.
Q11. What are Java Packages? What’s the significance of packages?
Ans: In Java, package is a collection of classes and interfaces which are bundled together as they are related to each other. Use of packages helps developers to modularize the code and group the code for proper re-use. Once code has been packaged in Packages, it can be imported in other classes and used.
Q12. Can we declare a class as Abstract without having any abstract method?
Ans: Yes we can create an abstract class by using abstract keyword before class name even if it doesn’t have any abstract method. However, if a class has even one abstract method, it must be declared as abstract otherwise it will give an error.
Q13. What’s the difference between an Abstract Class and Interface in Java?
Ans: The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers (public, private etc) with or without concrete implementation.
Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn’t require implementation of all the methods of its super class.
A class can implement multiple interfaces but it can extend only one abstract class.
Q14. What are the performance implications of Interfaces over abstract classes?
Ans: Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces. Another key factor for developers to take into consideration is that any class can extend only one abstract class while a class can implement many interfaces.
Use of interfaces also puts an extra burden on the developers as any time an interface is implemented in a class; developer is forced to implement each and every method of interface.
Q15. Does Importing a package imports its sub-packages as well in Java?
Ans: In java, when a package is imported, its sub-packages aren’t imported and developer needs to import them separately if required.
For example, if a developer imports a package university.*, all classes in the package named university are loaded but no classes from the sub-package are loaded. To load the classes from its sub-package ( say department), developer has to import it explicitly as follows:
Import university.department.*
Q16. Can we declare the main method of our class as private?
Ans: In java, main method must be public static in order to run any application correctly. If main method is declared as private, developer won’t get any compilation error however, it will not get executed and will give a runtime error.
Q17. How can we pass argument to a function by reference instead of pass by value?
Ans: In java, we can pass argument to a function only by value and not by reference.
Q18. How an object is serialized in java?
Ans: In java, to convert an object into byte stream by serialization, an interface with the name Serializable is implemented by the class. All objects of a class implementing serializable interface get serialized and their state is saved in byte stream.
Q19. When we should use serialization?
Ans: Serialization is used when data needs to be transmitted over the network. Using serialization, object’s state is saved and converted into byte stream .The byte stream is transferred over the network and the object is re-created at destination.
Q20. Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling?
Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.
Q21. Is there any way to skip Finally block of exception even if some exception occurs in the exception block?
Ans: If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block:
System.exit(0);
Q22. Can a class have multiple constructors?
Ans: Yes, a class can have multiple constructors with different parameters. Which constructor gets used for object creation depends on the arguments passed while creating the objects.
Q23. Can we override static methods of a class?
Ans: We cannot override static methods. Static methods belong to a class and not to individual objects and are resolved at the time of compilation (not at runtime).Even if we try to override static method,we will not get an complitaion error,nor the impact of overriding when running the code.
Q24. How we can execute any code even before main method?
Ans: If we want to execute any statements before even creation of objects at load time of class, we can use a static block of code in the class. Any statements inside this static block of code will get executed once at the time of loading the class even before creation of objects in the main method.
Q25. Can we call a non-static method from inside a static method?
Ans: Non-Static methods are owned by objects of a class and have object level scope and in order to call the non-Static methods from a static block (like from a static main method), an object of the class needs to be created first. Then using object reference, these methods can be invoked.
Tuesday, 4 July 2017
Spring Framework
Spring Framework Introduction
There are a lot of frameworks available in the market to develop enterprise applications. Each framework is having its own specific features to develop enterprise applications. For example, developers can use struts framework to implement Model-View- Controller (MVC) Architecture instead of manually separating servlet and jsp’s.
A typical enterprise application may use the following set of frameworks and services.
1. Hibernate framework for database access mechanism.
2. Struts framework for developing front-end layer.
3. Enterprise java beans (EJB) for services like transactions, security and messaging.
If the business requirement increases, the list of frameworks and services that an enterprise application uses also will grow. If an application using different frameworks and services become difficult to maintain the code. And it becomes difficult to test the application
We need a solution where we can develop application that can use any number of frameworks and services but still code remain maintainable and the code that we write should not be tight coupled.
Spring framework provided a light-weight solution to develop maintainable and reusable enterprise applications.
Spring is a modular framework i.e. spring framework can be used for all layer (spring –JBC, Spring MVC, Spring ORM etc.) implementations for developing enterprise applications.
Spring provides a facility to integrate various frameworks and technologies, and services in the application.
The main reason for using spring framework is to keep the code as simple as possible. Spring framework uses the POJO (Plain Old Java Objects) to develop enterprise applications. For this reason, spring framework can also be called as Plain Old Java Object (POJO) framework.
Spring is a open-source framework (http://www.springsource.org) developed by Spring Source a division of VMware.
We can call spring framework in the following 2 ways.
Container: spring framework can be described as light-wight container, as it does not require installation, configuration, start, and stop activities. It is just a simple collection of few Java Archive (JAR) files that need to be added to the classpath.
Framework: Spring framework can be described as an Application Programming Interfaces(API) .It contains a large number of classes, methods, interfaces, annotations, Extensible Markup Language(XML) tags that can be used in the application.
Cursors in Collection framework
Cursors in Collection framework
To retrieve objects one by one from Collection we have to use cursors.
There are 3 types of cursors available
1. Enumeration
2. Iterator
3. ListIterator
Enumeration
This interface has introduced in 1.0 version it contains the following 2 methods.
public boolean hasMoreElements()
public Object nextElement()
Example:
Limitations of Enumeration
It is applicable only for legacy classes and it is not a universal cursor.
While iterating the objects by using enumeration we can get only read access
and we can’t perform removal or replacement operations.
To overcome these problems we should go for Iterator interface.
Iterator
It is Introduced in 1.2 version.
We can get Iterator Object for any collection implemented class hence it is universal cursor.
while iterating objects, we are allowed perform removal operation in addition to read operation.
This interface contains the following 3 methods.
public boolean hasNext()
public Object next()
public void remove()
Example:
To over come these limitations we should go for ListIterator interface.
ListIterator
It has introduced in 1.2 version and it is child interface of Iterator.
It is a bi-directional cursor,i.e we can move either to the forward or backward direction.
While Iterating we can perform read,remove,replace and addition of new objects.
This interface defines the following 9 methods.
public boolean hasNext();
public boolean hasPrevious();
public Object next();
public Object previous();
public int nextIndex();
If there is no next element it returns size of the list.
public int previousIndex();
If there is no previous element it returns -1 .
public void remove();
public void set(Object new)
public void set(Object new)
public void add(Object new)
Example:
Note:The most powerful cursor is listIterator.
But it’s main limitation is it is applicable only for list implemented classes
(ArrayList, LinkedList, Vector, Stack).
To retrieve objects one by one from Collection we have to use cursors.
There are 3 types of cursors available
1. Enumeration
2. Iterator
3. ListIterator
Enumeration
This interface has introduced in 1.0 version it contains the following 2 methods.
public boolean hasMoreElements()
public Object nextElement()
Example:
Limitations of Enumeration
It is applicable only for legacy classes and it is not a universal cursor.
While iterating the objects by using enumeration we can get only read access
and we can’t perform removal or replacement operations.
To overcome these problems we should go for Iterator interface.
Iterator
It is Introduced in 1.2 version.
We can get Iterator Object for any collection implemented class hence it is universal cursor.
while iterating objects, we are allowed perform removal operation in addition to read operation.
This interface contains the following 3 methods.
public boolean hasNext()
public Object next()
public void remove()
Example:
Enumeration and Iterator are single directional cursors. They can always move towords forward direction only.
By using Iterator we can perform read and remove operations. And we can’t perform any replace or addition of new objectsTo over come these limitations we should go for ListIterator interface.
ListIterator
It has introduced in 1.2 version and it is child interface of Iterator.
It is a bi-directional cursor,i.e we can move either to the forward or backward direction.
While Iterating we can perform read,remove,replace and addition of new objects.
This interface defines the following 9 methods.
public boolean hasNext();
public boolean hasPrevious();
public Object next();
public Object previous();
public int nextIndex();
If there is no next element it returns size of the list.
public int previousIndex();
If there is no previous element it returns -1 .
public void remove();
public void set(Object new)
public void set(Object new)
public void add(Object new)
Example:
Note:The most powerful cursor is listIterator.
But it’s main limitation is it is applicable only for list implemented classes
(ArrayList, LinkedList, Vector, Stack).
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