Without Subclass we don't Use Abstract Class?

It Correct or Not?
Explain correct answer?

Hello MengyaLiu, and welcome to the forum,
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While I’ve personally documented your feedback here at the corporate office to ensure something of this nature doesn’t happen in the future, I’m glad to hear our delivery team was able to make this right and successfully delivered your TV yesterday. Please feel free to let us know how it went and if you have any further questions for me. I’ll be glad to help.
Best wishes,
Alex|Social Media Specialist | Best Buy® Corporate
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Most commenly used abstract class in java.

hi all,
can anyone please tell me, what is most commonly used abstract class in java. this question was asked in interview.

I hate interviewers when they ask about specifics of some classes.The fact that you hate it doesn't automatically follow that it's a bad question from the perspective of the person conducting the interviewer. You don't know what the real question is.
Bad "real" question: Do you know the statistics of Java usage off the top of your head?
Good "real" question: Do you know that Object isn't Abstract and can you name a few Abstract classes off the cuff?
Even then Programmers are so literal minded - the real question may be "Talk about what you know a bit."

Why are we using Abstract class?

Why are we using Abstract class? What is specify use of Abstract class?

The way I understand it....and I may be wrong because
I am very new....is that by making the class abstract
you will add abstract methods to it. Those abstract
methods MUST be defined in in any subclass that
inherits from the abstract class thereby making the
abstract a template for subclasses.
If your animal class is abstract then you would need
an abstract method (Which is just siganture, no body)
of say "numberOfLegs". Then in your dog and cat
classes which extend animal you must define a
"numberOfLegs" method with appropriate code.it isn't mandatory for an abstract class to have abstract methods. if a class does have abstract methods, the class must be abstract. but the following is perfectly legal
public abstract class NoAbstractMethods {
public void doStuff() {
// do stuff
}a subclass of an abstract class can also be abstract, and as such need not implement any additional methods

Why use abstract classes?

Why should I use abstract classes instead of a regular class with empty method bodies? Just better design? Is there some logical or performance based reason?

Why should I use abstract classes instead of aregular
class with empty method bodies? Just better design?Is
there some logical or performance based reason?Because it describes what you're doing.
If you define a "regular" class with empty message
bodies, everyone who looks at it will say "WTF is he
trying to do" or, if they're charitable, "Look,
Harvey, someone who's trying to make Java look just
like C++!".
sigh Maybe answers like THIS are why people keep asking the same question.
Here's a couple things an abstract class does that a "regular" class with empty methods bodies doesn't:
1) An abstract class cannot be instantiated.
2) An abstract class forces it's abstract methods to be implemented.
If you were to extend a non-abstract class with empty method bodies, you wouldn't have to override the methods... you could just leave them empty. An abstract class forces it.
There's a lot more reasons... those are a couple obvious ones.

When to use abstract classes instead of interfaces with extension methods in C#?

"Abstract class" and "interface" are similar concepts, with interface being the more abstract of the two. One differentiating factor is that abstract classes provide method implementations for derived classes when needed. In C#, however,
this differentiating factor has been reduced by the recent introduction of extension methods, which enable implementations to be provided for interface methods. Another differentiating factor is that a class can inherit only one abstract class (i.e., there
is no multiple inheritance), but it can implement multiple interfaces. This makes interfaces less restrictive and more flexible. So, in C#, when should we use abstract classes
instead of interfaces with extension methods?
A notable example of the interface + extension method model is LINQ, where query functionality is provided for any type that implements IEnumerable via
a multitude of extension methods.

Hi
Well I believe Interfaces have more uses in software design. You could decouple your component implementing against interfaces so that
you have more flexibility on changing your code with less risk. Like Inversion of Control patterns where you can use interfaces and then when you decide you can change the concrete implementation that you want to use. Or other uses for interfaces is you could
use Interceptors using interfaces (Unity
Interceptor) to do different things where not all of these is feasible or at least as straightforward using abstract classes.
Regards
Aram

When to use abstract class compared to interface

hi
can some one plase advise me when to use abstract class compared to interface?
Example will be appreciated...

So an abstract class can carry implementation. This can be used to formulate a rule of thumb as to when to use it over an interface.
If you have a so called type specialization relationship between the subtypes and the supertype then they're likely to benefit from shared implementation provided by the supertype, so use class (abstract or concrete) extension in this case. Type specialization is when the supertype represents a general concept like Fruit and the subtypes are specialized forms of that like Apple and Banana.
Another common kind of relationship is called type expansion. In this case the subtypes are unlikely to have any use of implementation provided by the supertype, so use interface implementation. Type expansion is when the supertype represents a specific character the subtypes take on. For example Apple and Bicycle ure unrelated in the type specialization sense but still can share a common character like Comparable. The subtypes have been expanded to include the supertype character, namely the ability to be compared.

OC4J 9.0.4 migrate to 10.1.3.1 JSP fails using abstract class for ResultSet

Consider the following JSP code snipet:
<%
com.class.SQLDataSource detail = ((com.class.SQLDataSource)request.getAttribute("AcctList"));
%>
<%while detail.next()) {detail.getRow();%>
<tr ...>
<td ...><%= detail.getString("ACCT") %></td>
</tr>
<%}%>
blah, blah, blah
class snipet looks like this:
public abstarct class SQLDataSource extends serializable {
private ResultSet resultSet = null;
private PreparesStatement stmt = null;
public boolean next() throws SQLException {
boolean result = getResultSet().next();
return (result);
public String getString(String columnName) throws SQLExcpetion {
return getResultSet().getString(columnName);
public void execute() throws SQLException {
-- checks stmt
-- if null generates resultSet from "AcctList.sql" query
-- if not null, re-executes as-is stmt for a query only
protected ResultSet getResultSet() throws SQLException {
execute(); // see above
return resultSet;
The problem:
<%= detail.getString("ACCT") %> generates a NullPointerException
Yet, if I add debug statements in the SQLDataSouce class, they conclusively show that resultSet has 1902 rows and I can diplay the contents on these rows (on the console, of course, the JSP still generates a NullPointerException).
If I "hardcode" the creation of a ResultSet in the JSP, the pages displays properly (not the desired method).
PLEASE TAKE NOTE: This JSP/Class combination works PERFECTLY in OC4J 9.0.4 using JDeveloper 9.0.5.2. When I deploy the **EXACT SAME** JSP/Class combination in JDeveloper/OC4J 10.1.3.1, I receive the NullPointerException.
So what this feels like to me (and my VERY limited J2EE experinece) is that the abstract class SQLDataSource has somehow lost/closed/dropped/corrupted resultSet when "returning" to the JSP for display of the contents of resultSet. This is, of course, a laymans explanation of the experienced effect of running this code.
It makes me wonder if under 10.1.3.1 "something more" must be done so that the (abstract) SQLDataSource class can operate in the same way it did under 9.0.4.
Last note: this NullPointerException happens whether I deploy to standalone OC4J 10.1.3.1 or if I execute my application withing JDevloper's Embedded OC4J instance.
Any Assistance on this MOST aggrivating problem would be greatly appreciated.
Others have yet to solve it in similar posts of mine. Hopefully this more definitive description will help YOU be my personal HERO.
Ed.

repost to pique interest

When should I use abstract classes and when should I use interfaces?

Can any body tell me in which scenario we use /we go for Interface and which scenario we go for abstract class, because as per my knowledge what ever thing we can do by using Interface that thing can also done through abstract class i mean to say that the
behavior of the two class.
And other thing i also want to know that which concept comes first into the programming abstract class or Interface.
S.K Nayak

The main differences between an abstract class and an interface:
Abstract
An abstract class can contain actual working code (default functionality), and can have either virtual or abstract method.
An abstract class must be sub-classed and only the sub-classes can be instantiated. Abstract methods must be implemented in the sub-class. Virtual methods may be overridden in the sub-class (although virtual methods typically contain code, you still may
need/want to override them). A good use for an abstract class is if you want to implement the majority of the functionality that a class will need, but individual sub-classes may need slightly different additional functioality.
Interface
An interface only contains the method signatures (method name and parameters), there is no code and it is not a class.
An interface must be implemented by a class. An interface is not a class and so it cannot be sub-classed. It can only be implemented by a class. When a class implements an interface, it must have code in it for each method in the interface's definition.
I have a blog post about interfaces:
http://geek-goddess-bonnie.blogspot.com/2010/06/program-to-interface.html
(sorry, I have no blog posts specific to abstract classes)
~~Bonnie DeWitt [C# MVP]
http://geek-goddess-bonnie.blogspot.com

Using Abstract Class Methods

I just want to know that in how many ways can we use the non abstract methods of the class without extending the class(dont want to implement all abstract methods of the parent class).One way is that if the methods are static then we can use CLASSNAME.METHODNAME().Is their any other way of doing it

none at all. not a single one. by definition, a non-static method has to be invoked on a particular instance of a class, and an abstract class cannot be instantiated. somebody is probably going to say you can do this:
abstract class AbstractClass {
abstract void minceAbout();
void talkRubbish() {
// do stuff
new AbstractClass() {
void minceAbout() {}
}.talkRubbish();but that is extending the AbstractClass, no matter what their argument. why d'you ask, anyway? why not implement the methods? sounds like a design smell

EJB question: How to use abstract class in writing a session bean?

I had written an abstract class which implements the session bean as follow:
public abstract class LoggingSessionBean implements SessionBean {
protected SessionContext ctx;
protected abstract Object editRecord(Object obj) throws Exception;
public LoggingSessionBean()
super();
private final String getBeforeUpdateImage(Object obj) throws Exception {
// implement the details of extracting the backup image ...
public void setSessionContext(SessionContext ctx)
this.ctx = ctx;
private final void writeThisImageToDatabase(String aStr) {
// connect to database to write the record ...
public final Object update(final Object obj) {
try {
final String aStr = getBeforeUpdateImage(obj);
writeThisImageToDatabase(aStr);
editRecord(obj);
} catch (Exception e) {
ctx.setRollbackOnly();
This abstract class is to write the backup image to the database so that other session beans extending it only need to implement the details in editRecord(Object Obj) and they do not need to take care of the operation of making the backup image.
However, some several questions for me are:
1. If I write a class ScheduleSessionBean extending the above abstract class and the according 2 interfaces ScheduleSession and ScheduleSessionHome for this session bean (void update(Object obj); defined in ScheduleSession), do I still need to write the interfaces for LoggingSession and LoggingSessionHome?
2. If I wrote the interface LoggingSession extending EJBObject where it defined the abstract methods "void update(Object obj);" and "void setSessionContext(SessionContext ctx);", that this meant I needed to write the ScheduleSession to implement the Logging Session?
3. I used OC4J 9.0.4. How can I define the ejb-jar.xml in this case?

Hi Maggie,
1. do I still need to write
the interfaces for LoggingSession and
LoggingSessionHome?"LoggingSessionBean" can't be a session bean, because it's an abstract class. Therefore there's no point in thinking about the 'home' and 'remote' interfaces.
2. this
meant I needed to write the ScheduleSession to
implement the Logging Session?Again, not really a question worth considering, since "LoggingSessionBean" can't be an EJB.
3. I used OC4J 9.0.4. How can I define the
ejb-jar.xml in this case?Same as you define it for any version of OC4J and for any EJB container, for that matter, since the "ejb-jar.xml" file is defined by the EJB specification.
Let me suggest that you create a "Logging" class as a regular java class, and give your "ScheduleSessionBean" a member that is an instance of the "Logging" class.
Alternatively, the "ScheduleSessionBean" can extend the "Logging" class and implement the "SessionBean" interface.
Good Luck,
Avi.

Can It Be Done? - Using Java Class Files from a DB

Hello,
I am interested in storing a binary java class file in my oracle DB. Then I want to get it out at runtime and execute methods on it. I am planning on casting the object to an interface and executing the methods within. Please let me know if this is possible and where to look for more info, and a code example would be awesome.
Joe
[email protected]

I think you are going to need to serialize the objects and the classes. (Don't get stuck on semantics here, I don't mean use Java Serialization to write the Class instance, I am referring to writing the .class file to a field in the database.) Obviously the instances are what you are interested in (since you mentioned casting them to interfaces and invoking methods on them), but the class definitions will need to either be available already, or stored elsewhere in the database.

Non-abstract methods in a Abstract class

Abstract Class can contain Non-abstract methods.
and Abstract Classes are not instantiable as well
So,
What is the purpose of Non-abstract methods in a Abstract class.
since we can't create objects and use it
so these non-abstract methods are only available to subclasses.
(if the subclass is not marked as abstract)
is that the advantage that has.(availability in subclass)
??

For example, the AbstractCollection class (in
java.util) provides an implementation for many of the
methods defined in the Collection interface.
Subclasses only have to implement a few more methods
to fulfill the Collection contract. Subclasses may
also choose to override the AbstractCollection
functionality if - for example - they know how to
provide an optimized implementation based on
characteristics of the actual subclass.Another example is the abstract class MouseAdapter that implements MouseListener, MouseWheelListener, MouseMotionListener, and that you can use instead of these interfaces when you want to react to one or two types of events only.
Quoting the javadocs: "If you implement the MouseListener, MouseMotionListener interface, you have to define all of the methods in it. This abstract class defines null methods for them all, so you can only have to define methods for events you care about."

Abstract class implementation Dos Prompt

Hello everyone,
Good day! Anyone knows how to implement or use abstract class to another class. Pls.... help me. I'm still a novice programmer. Program like Bank Account with a abstract class named 'Account' and an another class 'Savings' extends the abstract class and also the third class named 'TimeDeposit' something like it.

Hello everyone,
Good day! Anyone knows how to implement or
use abstract class to another class. Pls.... help
me. I'm still a novice programmer. Program like Bank
Account with a abstract class named 'Account' and an
another class 'Savings' extends the abstract class
and also the third class named 'TimeDeposit'
something like it.One thing to remember is that your class has to include code for all methods that are marked abstract in the abstract class you are extending; and if you don't want anyone else extending your class, you should make it final. You should also check the abstract class's constructors to see if you need to call a particular one when you are constructing your class. If you do, you'll need to call super(...) in your class's constructor, and it should be the first statement.

Interfaces, Abstract Classes & Polymorphism

I have a friend taking a Java course as part of a larger degree program, and she's asked for some help with an assignment. The assignment is as follows:
This assignment is to write a simple encryption/decryption application. There will be a single application
that the user can use to encrypt or decrypt a phrase passed in on the command line, with the user deciding
which encryption/decryption scheme to use.
The first scheme is to add 1 to each character that is in the phrase, the other is to subtract 1 from each
character that is in the phrase.
To encrypt, you must be able to use the follwing syntax:
java Crypto encrypt Additive "hello"
Output:
"hello" encrypts to: "ifmmmp"
To decrypt:
java ca.bcit.cst.comp2256.a###### decrypt Additive "ifmmp"
Output:
"ifmmp" decrypts to: "hello"
Use Additive or Subtractive as the arguments to choose which encryption/decryption to
use. The problem is, I'm not entirely sure how to use abstract classes and interfaces to do what is being asked. I'm pretty sure I could do the whole program in a single for-loop, but apparently her teacher doesn't want people coming up with their own solutions for the problem. I don't need any code for how to do it, per se, I'm just wondering how one would structure a program like that to include interfaces, polymorphism and abstract classes.
Anyone have any ideas?

with the user deciding which encryption/decryption scheme to use.This is the key sentence. encryption/decryption can be done using multiple schemes. The contract for any given scheme can be defined using
public String encrypt(String input);
public String decrypt(String input);There can be multiple implementations for these methods, one set for each scheme.
The contract therefore becomes the interface and each implementation is a concrete implementation of this interface.
The problem doesn't delve deep into the kind of schemes available. If it does and there is a significant overlap between 2 schemes, an abstract class that takes care of the shared logic among 2 schemes comes into picture.
Hope this helps!

Problem heriting fro abstract class

Hi, i try to use abstract classes for the first time, i write this code:
public abstract class ClassA{
abstract void getValue();
public lass ClassB extends ClassA{
void getValue(){
}But when i compile i get this message: ClassB must be abstract, it does not implements ClassA. What is wrong please?

Should not a method called getValue() return a value?
The following compiles
// In a file called ClassA.java
public abstract class ClassA {
    public abstract int getValue();
// In a file called ClassB.java
public class ClassB extends ClassA {
    private int value;
    public void getValue() {
        return value;
}If your abstract class has only abstract methods and you don't need the method to be package-private scope. You can just use an interface.
// In a file called InterfaceA.java
public interface InterfaceA {
    public int getValue();
// In a file called ClassB.java
public class ClassB implements InterfaceA {
    private int value;
    public void getValue() {
        return value;

Without Subclass we don't Use Abstract Class?

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