1)How many ways are there to create an object?
i) Using ‘new’ operator:
Test s=new Test();
ii) Factory method:
Thread
t=Thread.currentThread();
iii) newInstance():
Class c=Class.forName(“Test”);
Object
obj=c.newInstance(); àcreates
Test class Object.
Test t=(Test)obj;
iv) clone():
Test t1=new
Test(10,20);
Test
t2=t1.clone();
v) Deserialization:
FileInputStream fis=new FileInputStream(“Test.txt”);
ObjectInputStream ois=new
ObjectInputStream(fis);
UserDefSerCls uds=new UserDefSerCls(,” ”,);
Ois.readObject(uds);
Ois.close();
2) What are the
differences b/w HashMap and HashSet?
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HashMap
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HashSet
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It stores the data in key,value format.
It allows duplicate elements as values.
It implements Map
It allows only
one NULL key.
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It stores
grouped data .
It does not
allow any duplicates.
It implements Set.
It does not allow
NULL .
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3) What is the
difference b/w wait(-), sleep(-)& wait(-),yield()?
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Wait(-)
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Sleep(-)
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Running
to Waiting/
Sleeping/Block.
It makes the current thread to sleep up to the given seconds and it could sleep less than the
given seconds if it receives notify()/notifyAll() call.
In this case, the locks
will be released before going into
waiting state so that the other threads that are waiting on that object will use it.
Causes current thread to wait until either another
thread invokes the
notify() method or the notifyAll() method for this
object, or a specified amount of time has elapsed. |
Do
It makes the current thread to sleep for exactly the given seconds.
In case of
sleep(), once the thread is entered
into
synchronized block/method, no other
thread will be able to enter into that
method/block.
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Yield()à when a task invokes
yield(), it changes from Running state
to Runnable state.
*4) Can we create a userdefined immutable class?
Yes.
i)
Make the
class as final and
ii)
make the data
members as private and final.
*5) What are the differences b/w String and
StringBuffer?
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String
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StringBuffer
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It is immutable.
It won’t
give any additional space.
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It is mutable.
gives 16
additional characters memory space.
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6) Which “Collections F/W” classes did you use in your project?
List, ArrayList, LinkedList—add()
Set, TreeSet,
HashSet--
HashMap—put(),
get(), entrySet(), keyset()
Map.Entry
Iterator----hasMoreElements(),
next()
ListIterator.
7) Can you write the simple code for HashMap?
HashMap<String,String> hm=new
HashMap<String,String>();
hm.put(“key1”,”value1”);
hm.put(“key2”,”value2”);
hm.put(“key3”,”value3”);
Set
set=hm.keySet(); // gives
keys Set i.e., {key1,key2,key3}
Iterator<string>
itr=set.iterator();
while(itr.hasNext()){
//true….false
String empkeys=itr.next();
//for keys
String empvalnames=hm.get(empkeys); //gives values
by taking keys.
System.out.println(“empname”+empvalnames+”empid”+empkeys);
}
8) What are thread
states?
i) Start: Thread thread=new Thread();
ii) Runnable:
looking for its turn to be picked for
execution by the Thread Schedular based
on thead priorities.
(setPriority())
iii) Running: The Processor
is actively executing the thread code. It runs until it becomes
blocked, or voluntarily gives up its turn with Thread.yield().
Because
of Context Switching overhead, yield()
should not be used very
frequently.
iv) Waiting: A
thread is in a blocked state while it
waits for some external processing such
as file I/O to finish.
Sleepingàsleep(): Java threads are forcibly put to sleep (suspended) with this overloaded method.
Thread.sleep(milliseconds);
Thread.sleep(milliseconds,nanoseconds);
Blocking
on I/O: Will move to runnable after I/O condition like reading bytes of
data etc changes.
Blocked on Synchronization: will move to
Runnable when a Lock is acquired.
v) Dead: The thread
is finished working.
How to avoid Deadlock:
1) Avoid
a thread holding multiple locks----
If no thread attempts to hold more than one
lock ,then no deadlock occurs.
2) Reordering lock acquisition:--
If we require threads to alway acquire locks in a particular order, then no
deadlock occurs.
9) What are differences b/w concrete,abstract
classes and interface & they are given?
Concrete class: A class of
only Concrete methods is
called Concrete Class.
For this, object instantiation is possible directly.
A class can extends one class and implements many interfaces.
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Abstract class:
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Interface:
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*A class of only
Concrete or only Abstract or both.
*Any java class can extend only one abstract class.
*It won’t force
the programmer to implement/override
all its methods.
*It takes less
execution time than interface.
* It allows constructor.
This class can’t be
instantiated directly.
A class must be abstract when it
consist at least one abstract method.
It gives less scope
than an Interface.
It allows both
variable & constants
declaration.
It allows methods
definitions or declarations
whenever we want.
It gives reusability
hence it can’t be declared as
“final”.
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only abstract methods.
A class can implements any no. of interfaces
(this gives multiple
interface inheritance )
It forces the
programmer to implement all its
methods
Interface takes more
execution time due to its complex
hierarchy.
* It
won’t allow any constructor.
It can’t be instantiated but it can refer to its subclass objects.
It gives more scope
than an abstract class.
By default, methodsàpublic
abstract
variablesàpublic
static final.
It allows methods
declarations whenever we want . But it
involves complexity.
Since they give reusability hence they must not be declared as “final”.
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10) Can
you create a userdefined immutable class like String?
Yes, by making the class as final and its data
members as private, final.
11) Name some struts supplied tags?
a) struts_html.tld
b) struts_bean.tld
c) struts_logic.tld d) struts_nested.tld
d) struts_template.tld e) struts_tiles.tld.
12) How to retieve the objects from an
ArrayList?
List list=new
ArrayList(); // List<String> list=new
ArrayList<String>();
list.add(“element1”); list.add(“element1”);
list.add(“element2”);
list.add(“element2”);
list.add(“element3”);
list.add(“element3”);
// Iterator<String>
iterator=list.iterator();
for(String str:list)
s.o.p(str); }
Iterator iterator=list.iterator();
while(iterator.hasNext()){
String str=(String)itr.next();
S.o.p(string);
}
}
13) Can I take a class as “private”?
Yes, but it is declare to inner class, not
to outer class. If we are declaring "private"
to outer class nothing can be accessed from that outer class to inner
class.
16) How can you clone an object?
A a1=new ();
B a11=a1.clone();
17) What methods are there in Cloneable
interface?
Since
Cloneable is a Marker/Tag interface, no methods present.When a class
implements this interface that class obtains some special behavior and that will be
realized by the JVM.
20) Which JSP methods can be overridden?
jspInit() & jspDestroy().
21) Explain the JSP life-cycle methods?
1. Page translation: The page is parsed & a Java file containing the
corresponding servlet is
created.
2. Page compilation: The Java file is compiled.
3. Load class: The compiled class is loaded.
4 .Create instance: An instance of the servlet is created.
5. Call jspInit(): This method is called before any other
method to allow initialization.
6. Call _jspService(): This method is called for each request. (can’t be overridden)
7. Call jspDestroy(): The container calls this when it decides take the instance out of service.
2. Page compilation: The Java file is compiled.
3. Load class: The compiled class is loaded.
4 .Create instance: An instance of the servlet is created.
5. Call jspInit(): This method is called before any other
method to allow initialization.
6. Call _jspService(): This method is called for each request. (can’t be overridden)
7. Call jspDestroy(): The container calls this when it decides take the instance out of service.
It is the last method
called n the servlet instance.
22) What
Design Patterns are you using in your project?
i) MVCàseparates
roles using different technologies,
ii) Singleton Java classàTo
satisfy the Servlet Specification, a servlet must be a Single
Instance multiple thread
component.
iii)
Front Controllerà A Servlet developed as FrontController can traps only the requests.
iv) D.T.O/ V.OàData Transfer
Object/Value object is there to send
huge amount of data
from one lalyer to another layer.
It avoids
Network Round trips.
v) IOC/Dependency Injectionà
F/w s/w or container can automatically instantiates the
objects implicitly and
injects the dependent data to
that object.
vi) Factory methodà
It won’t allows object instantiation from out-side of the calss.
vii) View Helperà
It is there to develop a JSP without Java code so that readability,
re-usability will become easy.
23) What is Singleton Java class & its
importance?
A Java class that allows to create only one object per JVM is called
Singleton Java class.
Ex: In Struts f/w, the ActionServlet is a Singleton Java class.
Use: Instead of creating multiple objects
for a Java class having same data, it is recommended to create only one object &
use it for multiple no. of times.
24) What are the differences b/w perform() &
execute()?
perform() is an deprecated method.
25) What are the drawbacks of Struts? Is it
MVC-I or II? Struts is MVC-II
1)
Struts 1.x components are API
dependent. Because Action &
FormBean classes must
extend from the Struts 1.x APIs pre-defined classes.
2) Since applications are API dependent hence their “Unit testing” becomes complex.
3) Struts allows only JSP technology
in developing View-layer components.
4) Struts application Debugging
is quite complex.
5) Struts gives no built-in AJAX
support. (for asynchronous communication)
Note: In
Struts 1.x, the form that comes on the Browser-Window can be displayed
under no control of F/W s/w & Action class.
26) What are the differences b/w struts, spring
and hibernate?
Struts: allows to develop only webapplications and it can’t support POJO/POJI model programming.
Spring: is useful in developing all types of Java applications and support POJO/POJI model programming.
Hibernate: is used to develop DB
independent persistence logic It also supports POJO/POJI model programming.
27) What are differences b/w Servlets & Jsp?
Servlets:
i) It
requires Recompilation and
Reloading when we do modifications in a Servlet.(some servers)
ii) Every Servlet must be configured in
“web.xml”.
iii) It is providing
less amount of implicit objects support.
iv) Since it
consists both HTML & B.logic hence modification in one logic may disturbs
the other
logic.
v) No implicit
Exception Handling support.
vi) Since it
requires strong Java knowledge hence non-java programmers shows no interest.
vii) Keeping HTML
code in Servlets is quite complex.
JSP:
i) No
need of Recompilation & Reloading when we do modifications in a
JSP page.
ii) We need
not to Configure a JSP in a “web.xml”.
iii) It is providing huge amount of Implicit Objects support.
iv) Since Html & Java code are separated
hence no disturbance while changing
logic.
v) It is providing implicit XML Exception
Handling support.
vi) It is easy to learn & implement since it
is tags based.
vii) It allows to develop custom tags.
viii) It gives JSTL support.JSP Standard Tag LibraryàIt provides more tags
that will help to develop a JSP without
using Java code .
ix) It gives all
benefits of Servlets.
28) What is the difference b/w ActionServlet
& Servlet?
ActionServlet: It is a predefined Singleton Java class and it
traps all the requests
coming to
Server. It acts as a Front-Controller in Struts 1.x.
Servlet: a Java class that extends HttpServlet/GenericServlet or implements Servlet
interface and is a Server side
technology to develop dynamic web pages.
It is a Single instance multiple thread component.
It need not be a Singleton Java class.
29) What are Access Specifiers & modifiers?
i) public- à Universal
access specifier.
--can be used along with: class, innerclass, Interface, variable, constructor, method.
ii) protectd à Inherited
access specifier.
--can be used along
with: innerclass, variable, method, constructor.
iii)
default à Package
access specifier(with in the package).
--can be used along with: class, innerclass, interface, variable, constructor, method.
iv)
private à Native access specifier.
-- can be used
along with: innerclass, variable,
constructor, method.
à Some other access modifiers:
i)
static------------innerclass,variable,block,method.
ii) abstract—----- class,method.
iii)
final--------------class,variable,method.
iv) synchronized---block,method.
v) transient---------variable.
vi) native-------------method.
vii) volatile-----------variable
viii) strict
fp-----------variable
30) Which JSP tag
is used to display error validation?
In Source page:
<%@page
errorPage=”error-page-name.jsp”>
In Error page:
<%@page
isErrorPage=”true”>