1. What is the default value
of the local variables?
·
The local variables are not
initialized to any default value, neither primitives nor object references.
2. What is constructor?
·
Constructor is just like a
method that is used to initialize the state of an object. It is invoked at the
time of object creation.
3. What is the purpose of
default constructor?
·
Default constructor provides
the default values to the objects. The java compiler creates a default
constructor only if there is no constructor in the class
4. Does constructor return
any value?
·
Yes, that is current instance
(You cannot use return type yet it returns a value).
5. What is static variable?
·
Static variable is used to
refer the common property of all objects (that is not unique for each object)
e.g. company name of employees, college name of students etc.
·
Static variable gets memory
only once in class area at the time of class loading.
6. What is static method?
·
A static method belongs to the
class rather than object of a class.
·
A static method can be invoked
without the need for creating an instance of a class.
·
Static method can access static
data member and can change the value of it.
7. What is this in java?
·
It is a keyword that that
refers to the current object
8. Which class is the superclass
for every class?
·
Object class
9. What is super in java?
·
It is a keyword that refers to
the immediate parent class object
10. What is method
overloading?
·
If a class has multiple methods
by same name but different parameters, it is known as Method Overloading. It
increases the readability of the program
11. What is method overriding:
·
If a subclass provides a
specific implementation of a method that is already provided by its parent
class, it is known as Method Overriding. It is used for runtime polymorphism
and to provide the specific implementation of the method
12. What is final variable?
·
If you make any variable as
final, you cannot change the value of final variable(It will be constant).
13. What is final method?
·
Final methods can't be
overridden.
14. What is final class?
·
Final class can't be inherited
15. What is abstraction?
·
Abstraction is a process of
hiding the implementation details and showing only functionality to the user
16. What is the difference
between abstraction and encapsulation?
·
Abstraction hides the implementation
details whereas encapsulation wraps code and data into a single unit
17. Is it possible to
instantiate the abstract class?
·
No, abstract class can never be
instantiated.
18. What is interface?
·
Interface is a blueprint of a
class that has static constants and abstract methods. It can be used to achieve
fully abstraction and multiple inheritance.
19. What is difference between
abstract class and interface?
Abstract class
|
Interface
|
An abstract class can have method body (non-abstract methods).
|
Interface have only abstract methods.
|
An abstract class can have instance variables.
|
An interface cannot have instance variables.
|
An abstract class can have constructor.
|
Interface cannot have constructor.
|
An abstract class can have static methods.
|
Interface cannot have static methods.
|
You can extend one abstract class.
|
You can implement multiple interfaces.
|
20.
Do I need to import java.lang package any time? Why?
No. It is by default loaded internally by the JVM.
21. What is Exception
Handling?
·
Exception Handling is a
mechanism to handle runtime errors.It is mainly used to handle checked
exceptions.
22. What is difference between
Checked Exception and Unchecked Exception?
·
Checked Exception: The classes that
extend Throwable class except RuntimeException and Error are known as checked
exceptions e.g.IOException, SQLException etc. Checked exceptions are checked at
compile-time.
·
Unchecked Exception: The classes that
extend RuntimeException are known as unchecked exceptions e.g.
ArithmeticException, NullPointerException etc. Unchecked exceptions are not
checked at compile-time.
23. What is the base class for
Error and Exception?
·
throwable
24. What is finally block?
·
finally block is a block that
is always executed
25.
What is difference 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.
|
26. What is difference between
final, finally and finalize?
·
final: final is a keyword, final can be
variable, method or class.You, can't change the value of final variable, can't
override final method, can't inherit final class.
·
finally: finally block is used in
exception handling. finally block is always executed.
·
finalize():finalize() method is used in
garbage collection.finalize() method is invoked just before the object is
garbage collected.The finalize() method can be used to perform any cleanup
processing.
27. What is the purpose of the
Runtime class?
·
The purpose of the Runtime
class is to provide access to the Java runtime system.
28. How will you invoke any
external process in Java?
·
By Runtime.getRuntime().exec(?)
method.
29.
What is the difference between the Reader/Writer class hierarchy and
theInputStream/OutputStream class hierarchy?
·
The Reader/Writer class
hierarchy is character-oriented, and the InputStream/OutputStream class
hierarchy is byte-oriented.
30. What is thread?
·
A thread is a lightweight
subprocess.It is a separate path of execution.It is called separate path of
execution because each thread runs in a separate stack frame.
31. What is the difference
between List and Set?
·
List can contain duplicate
elements whereas Set contains only unique elements.
32. What is the difference
between Set and Map?
·
Set contains values only
whereas Map contains key and values both.
33. What is the difference
between HashSet and HashMap?
·
HashSet contains only values
whereas HashMap contains entry (key,value). HashSet can be iterated but HashMap
need to convert into Set to be iterated.
34.
What is the difference between HashMap and Hashtable?
HashMap
|
Hashtable
|
HashMap is not synchronized.
|
Hashtable is synchronized.
|
HashMap can contain one null key and multiple
null values.
|
Hashtable cannot contain any null key or null
value.
|
35. What is the advantage of
Properties file?
·
If you change the value in
properties file, you don't need to recompile the java class. So, it makes the application
easy to manage.
36. How you will define the
constant in java
·
Final Keyword
37. Main method can be
override or overload?
·
Cannot override main method but it can be overloaded.
38.
From main method if we remove (String [] args) it will work or not?
Public static void main ( )
{
}
·
Yes, Overloading
39.
If I rearrange the order of main method it will work or not?
Static public Void main (String[] args)
{
}
·
Yes, this will work
40. Why it is called main
method not any other method is not main?
·
JVM looks for a method called
main to start execution
41. How to come out of the for
loop?
·
break;
42.
Hierarchy of the exception handling?
Throwable
Error Exception
IOException RuntimeException
43. What is the difference
between String and StringBuffer?
·
String is immutable, meaning that
when you perform an operation on a String you are really creating a whole new
String.
·
StringBuffer is mutable, and
you can append to it as well as reset its length to 0.
·
StringBuffer is faster than
String when performing simple concatenations.
44. What is default Boolean
value in java?
·
False
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