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Written By : Amit
Title :
Java Interview Questions Answer

Description :
Discuss these
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Written By : Girish
Title :
ThreadPoolExecutor Example code

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Explained with question answer
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Written By : Sameer1
Title :
Some Questions on

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Tags/Keywords : interview questions Java, Java Questions
Author : Amit
Date (Year/Month/Date): 2009-01-25
Java Interview Questions and answer

 



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Java Interview Question 1:
How can you invert a String
"Questions and Answers of
technology" to another String
"technology of Answers and
Questions"?

Java Interview answer
Using java.util.StringTokenizer, the default
constructor uses following default delimitors
1. space (" ")
2. tab ("\t")
3. new line ("\n")
4. carriage return ("\r")
5. form feed ("\f")

Now StringTokenizer's nextToken() method
returns following words:
1. Questions
2. and
3. Answers
4. of
5. technology

Now placing these words in any order
is quite simple.

Another suggestion is to use String.split
as this method can split a string in array
and thus returning all the words.




Java Interview Question  2:
What are the basic differences
between ArrayList and LinkedList?

As the name suggest ArrayList is a
List implementation arround an Array
(internally handled) and LinkedList
is a List which has all the items
linked together. So when an item is
added/removed from an ArrayList,
it internally creates another Array
and transfer all the items to the
new Array in case the capacity of the
ArrayList is altered. Whereas in case
of LinkedList, all items has internal
links from one item to another. If an
item is added or removed from LinkedList,
then only the link is added or changed
from one item to another.

So if size of ArrayList or LinkedList
is large, then addition or deletion
or modification of items arround
start or towards end of the ArrayList,
results in lower performance.
But in case of LinkedList, as link of
the next or previous item only changes,
performance is good.

In case of retrieval/get operation,
if any item is retrieved from the ArrayList,
then it is just getting the item value
from the Array with an index number,
so performace is good.

But in case of LinkedList, item is
queried from the end which is closer
to the index of the item queried.
Suppose LinkedList is very large,
and addition or removal from the middle,
so relatively poor performance.


Java Interview Question 3:
Is it mandatory to have all the objects
in an object hierarchy to implement
java.io.Serializable to make the entire
object graph persistable into a Hard disk?
Or can this be possible, like

class A
......
class B extends A implements
java.io.Serializable.

In this example, if B is persisted,
will A also persist with values preserved?

Java Interview answer
No, in an object hierarchy if any of the
class implements java.io.Serializable,
then all subclass to this class in object
graph persist. It is not required that all
the subclass to implement java.io.Serializable,
but mandatory for the supe class (like A)
to implement Serializable

Java Interview Question 4:
In case of the following code, what is
your observation about serialization and
de-serialization?
class A
{
 public String name="";
 public A(String name){
 this.name = name;
 }
}
class B extends A implements java.io.Serializable
{
 public B(String name)
 {
 super(name);
 }
}


Java Interview answer
This will throw java.io.InvalidClassException
exception while deserializing, as A is not
Serializable, and doesn't have a default
blank constructor, so the deserialization
process cann't happen. Correction is
either make A implements Serializable or
add a default A() blank constructor in class A.

Java Interview Question 5:
Where to use abstract and interface in a
design context/scenario?

Java Interview Answer
If the requirement is to provide a specification
of contract between different modules/functionally,
whereby complete implementation is provided at runtime,
then interface is of choice. For example, JDBC API,
it has most of the interfaces defined, and the
implementation is left to the JDBC driver provider.
Each vendor provided its own implementation of
Connection, Statement and ResultSet, and these
implementations are independent of other vendor's
implementation.

In case there is a functionality (default) is provided,
but there is an option for overriding the default
implementation, and controlling lifecycle in a separate
module, then abstract class is to be considered.
For example, HttpServlet, which is an abstract class,
but it provides default implementation of service method,
doGet, doPost and many more. But developer can provide
his/her own functionality by overriding service,
doGet and doPost etc methods or not. And HttpServlet
lifecycle is handled by web container, not the
developer (As developer cannot create
instance of HttpServlet class using "new" operator).



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