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Q: What is the difference
between an Interface and an Abstract class?
A: An abstract class can have instance methods
that implement a default behavior. An Interface
can only declare constants and instance methods,
but cannot implement default behavior and all
methods are implicitly abstract. An interface
has all public members and no implementation.
An abstract class is a class which may have the
usual flavors of class members (private, protected,
etc.), but has some abstract methods.
Q: What is the purpose of garbage
collection in Java, and when is it used?
A: The purpose of garbage collection is to identify
and discard objects that are no longer needed
by a program so that their resources can be reclaimed
and reused. A Java object is subject to garbage
collection when it becomes unreachable to the
program in which it is used.
Q: Describe synchronization in respect
to multithreading.
A: With respect to multithreading, synchronization
is the capability to control the access of multiple
threads to shared resources. Without synchonization,
it is possible for one thread to modify a shared
variable while another thread is in the process
of using or updating same shared variable. This
usually leads to significant errors.
Q: Explain different way of using
thread?
A: The thread could be implemented by using runnable
interface or by inheriting from the Thread class.
The former is more advantageous, 'cause when you
are going for multiple inheritance..the only interface
can help.
Q: What are pass by reference and
passby value?
A: Pass By Reference means the passing the address
itself rather than passing the value. Passby Value
means passing a copy of the value to be passed.
Q: What is HashMap and Map?
A: Map is Interface and Hashmap is class that
implements that.
Q: Difference between HashMap and
HashTable?
A: The HashMap class is roughly equivalent to
Hashtable, except that it is unsynchronized and
permits nulls. (HashMap allows null values as
key and value whereas Hashtable doesnt allow).
HashMap does not guarantee that the order of the
map will remain constant over time. HashMap is
unsynchronized and Hashtable is synchronized.
Q: Difference between Vector and ArrayList?
A: Vector is synchronized whereas arraylist is
not.
Q: Difference between Swing and Awt?
A: AWT are heavy-weight componenets. Swings are
light-weight components. Hence swing works faster
than AWT.
Q: What is the difference between
a constructor and a method?
A: A constructor is a member function of a class
that is used to create objects of that class.
It has the same name as the class itself, has
no return type, and is invoked using the new operator.
A method is an ordinary member function of a class.
It has its own name, a return type (which may
be void), and is invoked using the dot operator.
Q: What is an Iterator?
A: Some of the collection classes provide traversal
of their contents via a java.util.Iterator interface.
This interface allows you to walk through a collection
of objects, operating on each object in turn.
Remember when using Iterators that they contain
a snapshot of the collection at the time the Iterator
was obtained; generally it is not advisable to
modify the collection itself while traversing
an Iterator.
Q: State the significance of public,
private, protected, default modifiers both singly
and in combination and state the effect of package
relationships on declared items qualified by these
modifiers.
A: public : Public class is visible in other packages,
field is visible everywhere (class must be public
too)
private : Private variables or methods may be
used only by an instance of the same class that
declares the variable or method, A private feature
may only be accessed by the class that owns the
feature.
protected : Is available to all classes in the
same package and also available to all subclasses
of the class that owns the protected feature.This
access is provided even to subclasses that reside
in a different package from the class that owns
the protected feature.
default :What you get by default ie, without any
access modifier (ie, public private or protected).It
means that it is visible to all within a particular
package.
Q: What is an abstract class?
A: Abstract class must be extended/subclassed
(to be useful). It serves as a template. A class
that is abstract may not be instantiated (ie,
you may not call its constructor), abstract class
may contain static data. Any class with an abstract
method is automatically abstract itself, and must
be declared as such.
A class may be declared abstract even if it has
no abstract methods. This prevents it from being
instantiated.
Q: What is static in java?
A: Static means one per class, not one for each
object no matter how many instance of a class
might exist. This means that you can use them
without creating an instance of a class.Static
methods are implicitly final, because overriding
is done based on the type of the object, and static
methods are attached to a class, not an object.
A static method in a superclass can be shadowed
by another static method in a subclass, as long
as the original method was not declared final.
However, you can't override a static method with
a nonstatic method. In other words, you can't
change a static method into an instance method
in a subclass.
Q: What is final?
A: A final class can't be extended ie., final
class may not be subclassed. A final method can't
be overridden when its class is inherited. You
can't change value of a final variable (is a constant).
Q: What if the main method is declared
as private?
A: The program compiles properly but at runtime
it will give "Main method not public."
message.
[ Received from Sandesh Sadhale]
Q: What if the static modifier is
removed from the signature of the main method?
A: Program compiles. But at runtime throws an
error "NoSuchMethodError".
[ Received from Sandesh Sadhale]
Q: What if I write static public void
instead of public static void?
A: Program compiles and runs properly.
[ Received from Sandesh Sadhale]
Q: What if I do not provide the String
array as the argument to the method?
A: Program compiles but throws a runtime error
"NoSuchMethodError".
[ Received from Sandesh Sadhale]
Q: What is the first argument of the
String array in main method?
A: The String array is empty. It does not have
any element. This is unlike C/C++ where the first
element by default is the program name.
[ Received from Sandesh Sadhale]
Q: If I do not provide any arguments
on the command line, then the String array of
Main method will be empty or null?
A: It is empty. But not null.
[ Received from Sandesh Sadhale]
Q: How can one prove that the array
is not null but empty using one line of code?
A: Print args.length. It will print 0. That means
it is empty. But if it would have been null then
it would have thrown a NullPointerException on
attempting to print args.length.
[ Received from Sandesh Sadhale]
Q: What environment variables do I
need to set on my machine in order to be able
to run Java programs?
A: CLASSPATH and PATH are the two variables.
[ Received from Sandesh Sadhale]
Q: Can an application have multiple
classes having main method?
A: Yes it is possible. While starting the application
we mention the class name to be run. The JVM will
look for the Main method only in the class whose
name you have mentioned. Hence there is not conflict
amongst the multiple classes having main method.
[ Received from Sandesh Sadhale]
Q: Can I have multiple main methods
in the same class?
A: No the program fails to compile. The compiler
says that the main method is already defined in
the class.
[ Received from Sandesh Sadhale]
Q: Do I need to import java.lang package
any time? Why ?
A: No. It is by default loaded internally by the
JVM.
[ Received from Sandesh Sadhale]
Q: Can I import same package/class
twice? Will the JVM load the package twice at
runtime?
A: One can import the same package or same class
multiple times. Neither compiler nor JVM complains
abt it. And the JVM will internally load the class
only once no matter how many times you import
the same class.
[ Received from Sandesh Sadhale]
Q: What are Checked and UnChecked
Exception?
A: A checked exception is some subclass of Exception
(or Exception itself), excluding class RuntimeException
and its subclasses.
Making an exception checked forces client programmers
to deal with the possibility that the exception
will be thrown. eg, IOException thrown by java.io.FileInputStream's
read() method·
Unchecked exceptions are RuntimeException and
any of its subclasses. Class Error and its subclasses
also are unchecked. With an unchecked exception,
however, the compiler doesn't force client programmers
either to catch the
exception or declare it in a throws clause. In
fact, client programmers may not even know that
the exception could be thrown. eg, StringIndexOutOfBoundsException
thrown by String's charAt() method· Checked
exceptions must be caught at compile time. Runtime
exceptions do not need to be. Errors often cannot
be.
Q: What is Overriding?
A: When a class defines a method using the same
name, return type, and arguments as a method in
its superclass, the method in the class overrides
the method in the superclass.
When the method is invoked for an object of the
class, it is the new definition of the method
that is called, and not the method definition
from superclass. Methods may be overridden to
be more public, not more private.
Q: What are different types of inner
classes?
A: Nested top-level classes, Member classes, Local
classes, Anonymous classes
Nested top-level classes- If you declare
a class within a class and specify the static
modifier, the compiler treats the class just like
any other top-level class.
Any class outside the declaring class accesses
the nested class with the declaring class name
acting similarly to a package. eg, outer.inner.
Top-level inner classes implicitly have access
only to static variables.There can also be inner
interfaces. All of these are of the nested top-level
variety.
Member classes - Member inner classes
are just like other member methods and member
variables and access to the member class is restricted,
just like methods and variables. This means a
public member class acts similarly to a nested
top-level class. The primary difference between
member classes and nested top-level classes is
that member classes have access to the specific
instance of the enclosing class.
Local classes - Local classes are
like local variables, specific to a block of code.
Their visibility is only within the block of their
declaration. In order for the class to be useful
beyond the declaration block, it would need to
implement a more publicly available interface.Because
local classes are not members, the modifiers public,
protected, private, and static are not usable.
Anonymous classes - Anonymous inner
classes extend local inner classes one level further.
As anonymous classes have no name, you cannot
provide a constructor.
Q: Are the imports checked for validity
at compile time? e.g. will the code containing
an import such as java.lang.ABCD compile?
A: Yes the imports are checked for the semantic
validity at compile time. The code containing
above line of import will not compile. It will
throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
[ Received from Sandesh Sadhale]
Q: Does importing a package imports
the subpackages as well? e.g. Does importing com.MyTest.*
also import com.MyTest.UnitTests.*?
A: No you will have to import the subpackages
explicitly. Importing com.MyTest.* will import
classes in the package MyTest only. It will not
import any class in any of it's subpackage.
[ Received from Sandesh Sadhale]
Q: What is the difference between
declaring a variable and defining a variable?
A: In declaration we just mention the type of
the variable and it's name. We do not initialize
it. But defining means declaration + initialization.
e.g String s; is just a declaration while String
s = new String ("abcd"); Or String s
= "abcd"; are both definitions.
[ Received from Sandesh Sadhale]
Q: What is the default value of an
object reference declared as an instance variable?
A: null unless we define it explicitly.
[ Received from Sandesh Sadhale]
Q: Can a top level class be private
or protected?
A: No. A top level class can not be private or
protected. It can have either "public"
or no modifier. If it does not have a modifier
it is supposed to have a default access.If a top
level class is declared as private the compiler
will complain that the "modifier private
is not allowed here". This means that a top
level class can not be private. Same is the case
with protected.
[ Received from Sandesh Sadhale]
Q: What type of parameter passing
does Java support?
A: In Java the arguments are always passed by
value .
[ Update from Eki and Jyothish Venu]
Q: Primitive data types are passed
by reference or pass by value?
A: Primitive data types are passed by value.
[ Received from Sandesh Sadhale]
Q: Objects are passed by value or
by reference?
A: Java only supports pass by value. With objects,
the object reference itself is passed by value
and so both the original reference and parameter
copy both refer to the same object .
[ Update from Eki and Jyothish Venu]
Q: What is serialization?
A: Serialization is a mechanism by which you can
save the state of an object by converting it to
a byte stream.
[ Received from Sandesh Sadhale]
Q: How do I serialize an object to
a file?
A: The class whose instances are to be serialized
should implement an interface Serializable. Then
you pass the instance to the ObjectOutputStream
which is connected to a fileoutputstream. This
will save the object to a file.
[ Received from Sandesh Sadhale]
Q: Which methods of Serializable interface
should I implement?
A: The serializable interface is an empty interface,
it does not contain any methods. So we do not
implement any methods.
[ Received from Sandesh Sadhale]
Q: How can I customize the seralization
process? i.e. how can one have a control over
the serialization process?
A: Yes it is possible to have control over serialization
process. The class should implement Externalizable
interface. This interface contains two methods
namely readExternal and writeExternal. You should
implement these methods and write the logic for
customizing the serialization process.
[ Received from Sandesh Sadhale]
Q: What is the common usage of serialization?
A: Whenever an object is to be sent over the network,
objects need to be serialized. Moreover if the
state of an object is to be saved, objects need
to be serilazed.
[ Received from Sandesh Sadhale]
Q: What is Externalizable interface?
A: Externalizable is an interface which contains
two methods readExternal and writeExternal. These
methods give you a control over the serialization
mechanism. Thus if your class implements this
interface, you can customize the serialization
process by implementing these methods.
[ Received from Sandesh Sadhale]
Q: When you serialize an object, what
happens to the object references included in the
object?
A: The serialization mechanism generates an object
graph for serialization. Thus it determines whether
the included object references are serializable
or not. This is a recursive process. Thus when
an object is serialized, all the included objects
are also serialized alongwith the original obect.
[ Received from Sandesh Sadhale]
Q: What one should take care of while
serializing the object?
A: One should make sure that all the included
objects are also serializable. If any of the objects
is not serializable then it throws a NotSerializableException.
[ Received from Sandesh Sadhale]
Q: What happens to the static fields
of a class during serialization?
A: There are three exceptions in which serialization
doesnot necessarily read and write to the stream.
These are
1. Serialization ignores static fields, because
they are not part of ay particular state state.
2. Base class fields are only hendled if the base
class itself is serializable.
3. Transient fields.
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