In abstract class concrete methods are also there. concrete 
methods are the one which has implementation. so in future 
if any programmer wants to give concrete methods can use 
abstract class not interface. 
In abstract class for methods and variables 
public,private,protected modifier can be use. but in 
interface public and abstract are permitted.

you can write as like for regular class.

First We cannot force the garbage collection to garbage the 
object. Garbage collection can never be forced.

So by calling System.gc() will not ensure you that the 
object will be garbage collected.

say me when this eill actually happen.

Maverickhari,

Garbage collector is system created thread which runs 
automatically. 

We are not sure when the garbage collection is going to 
happen. this totally depends upon the JVM. Like connection 
pool all the the objects are created in pool JVM will check 
if there is no free memory in pool then it searches for the 
objects which are no longer in use and will garbage collect 
that and allocate to some other object. 

hope this will clear

log4j can be used for writing the log messages to various 
kinds of destinations like a file, database, console etc...

It is always advisable to use some sort of logging API to 
generate the log messages.the log messages helps the end 
user as well as the developer of the application, to kow 
about what the application has done and why the application 
has failed, etc...

It is developed by Apache and this is used as part of 
various products like hibernate, struts, strings etc...

Java is apure object-oriented progrmaing language.here one 
beter feature we have i.e multiple inheritance and pointer 
concepets are not used.

In java the primitivw datatypes like(int,float,double,short)
these values can be represented in objects of classes.

there are following diff between java and c++
1) one of the most important diff is there is no pointre in 
java but pointer is hidden from programmer.
2) java is pure object oriented programming language where 
as c++ is not pure object oriented language is also support 
c feature.
3) in java everything should be within class includin  main 
function where as it can be in calss or not.
4) java doesn't support operator overloading where as c++ 
souuprt
5)there is no global variable in java where as it is in c++
6) there is no distructor in java but threr is finally key 
work 
7)there is no multipal inharitance but is can be achived by 
interface

there are following diff between java and c++
1) one of the most important diff is there is no pointre in 
java but pointer is hidden from programmer.
2) java is pure object oriented programming language where 
as c++ is not pure object oriented language is also support 
c feature.
3) in java everything should be within class includin  main 
function where as it can be in calss or not.
4) java doesn't support operator overloading where as c++ 
souuprt
5)there is no global variable in java where as it is in c++
6) there is no distructor in java but threr is finally key 
work 
7)there is no multipal inharitance but is can be achived by 
interface

yes it is very much compulsory to have a one abstract mthod 
in abstract class
otherwise will be no longer abstract

No.  It is not compulsory that an abstract class
should have atleast one abstract method.

One can declare a class as abstract still with full
implementation of methods.  This avoids the instantiation
the class

No,It is not mandatory, but sub class of abstract class 
should be override by any one of the method from abstract 
class .

Ex:i)  javax.servlet.http.HttpServlet
   ii) org.apache.struts.actions.DispatchAction, etc.,

an abstract class contains the concrete methods as well 
abstract methods. It may instance variables and static 
final variables
an interface contains only abstract methods. all the 
variables are by default static and final.
an abstract class contains constructor where as an 
interface does not have constructors...

abstract class A
{
protected abstract void show();
}

public class xx extends A
{
protected void show()
{
System.out.println("karthik ok");

}

static void main(String aa[])
{
xx ob=new xx();
ob.show();
}
}


But in Interface


interface A
{
protected abstract void show();
}



public class xx implements A
{
protected void show()
{
System.out.println("karthik ok");

}

static void main(String aa[])
{
xx ob=new xx();
ob.show();
}
}

modifier protected not allowed here
protected abstract void show(); in Interface
Because Default public method


But in Abstract class we can have protected


interface A
{
int x;//Error we should need initial value
}