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:
1. Public :
Class,Method,Field is accessible from anywhere.
2.
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.
3. Default:
Method,Field,class can be accessed only from the same package and not from
outside of it’s native package.
4. 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:
In java, 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:
In java, 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.
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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:
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. Is String a data
type in java?
Ans:
String is not a primitive data type in java. When a string is created in java,
it’s actually an object of Java.Lang.String class that gets created. After
creation of this string object, all built-in methods of String class can be
used on the string object.
Q25. What’s the
difference between an array and Vector?
Ans:
An array groups data of same primitive type and is static in nature while
vectors are dynamic in nature and can hold data of different data types.
