Using abstraction

I'm a novice, discovering how to use and develop abstract classes.
First, a question. Because the methods declared in my abstract classes are general, they employ Object as the parameter type. That seems to mean that I need a prologue such as the following in all of my concrete implementations of these methods:
public String CompGetT( int iParN, Object iCell) {
BZCellE lCell = null;
if( ! iCell instanceof BZCellE) return null;
lCell = ( BZCellE) iCell;
Is the use of such a prologue a standard technique, when implementing abstract classes? If not, what is the alternative?
I think I've learned something about the design process in recent months. Most texts on on the art of programming suggest that programming is a linear process, proceding from the design to the implementation, from the abstract to the specific. The Java class hierarchy reflects this same sequence -- abstract classes are "above" or "super" to the concrete classes.
In the real world, however (or in my world, at least), programs evolve. I write a concrete class, then I decide that I want to have an enhanced version of that class, so I extend it. Then I decide I want to make the second class a variant. SO I GO BACK and create an abstract class that can serve as the common denominator, and I make both concrete classes extensions of the third class. If I decide I want a fourth class that has some but not all of the methods common to the first two, then I create a second abstract class, even further up the hierarchy.
The programming texts suggest that we start with a "Divine Plan for Creation", and then proceed methodically downwards. Practice suggests that we start with something that works, then let it evolve, then retrofit the plan and derive the abstractions from the concrete. I'd like to see more texts that address programming PRACTICE in this less-than-perfect world.

Thanks for the information. Refactoring? -- nice to know there's a term for it. I guess I'm not crazy after all!
Still wondering whether there's an alternative to using Object (or some general class) as the paremter in the concrete version of the polymorphic method. Intuitively, it seems like the concrete class ought to be able to make the class of the parameter very specific, but when I try to do that, the concrete method no longer matches the signature of the method in the abstract class, and, as I recall, I get a compilation error.

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How to Add and delete a row while using Abstract Table Model

Hi,
I need to do the following functionalities in JTable.I've done tht but a small problem.
1. Adding a row (Using addRow() method from DefaultTableModel).
2. Deleting a row(Using setRowCount() method from Default Table Model).
3. Sorting the table based on the selection of column(Using TableSorter which is using AbstracTableModel).
As the sorting is mandatory i've to change my model to Abtract Table Model
The problem is this Abstract Table Model doesn't have any methods to Add a row or deleting a row (setRowCount()).If anybody has written any utility method for this using Abstract Table Model help me.

Using TableSorter which is using AbstracTableModel).If your talking about the TableSorter class from the Swing tutorial, then you create the TableSorter class by passing it a TableModel. There is no reason you can't use the DefaltTableModel.
I changed the code in TableSorterDemo as follows:
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                                        "Last Name",
                                        "Sport",
                                        "# of Years",
                                        "Vegetarian"};
        Object[][] data = {
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             "Snowboarding", new BigDecimal(1), new Boolean(false)},
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               public Class getColumnClass(int c)
                    return getValueAt(0, c).getClass();
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//        TableSorter sorter = new TableSorter(new MyTableModel()); //ADDED THIS

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.

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 AWT (Abstract Window Toolkit) is use Abstract word? What AWT hiding?

Hi i just want to know why sun microsystem used Abstract keyword while defining AWT and why not other classes like String, Exception, etc are not Abstract????
Kindly Explain me

Encephalopathic wrote:
masijade. wrote:
AWT is not a Class, it is an entire API, so the question makes no sense.I'm not so sure about that. It's got me to thinking, just what were they hiding?The native code of the underlying windowing platform

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.

Why am i able to use abstract methods id they aren't yet implemented?

For example, why i can use this [http://docs.oracle.com/javase/7/docs/api/java/nio/CharBuffer.html#slice] if this is abstract?
Edited by: 947971 on 13-set-2012 10.38

947971 wrote:
Yes, but if CharBuffer has a method declared:
public abstract CharBuffer slice()So i can't use the method slice() because it's not implemented.
And why , indeed, i'm able to use it?Because it is implemented by the actual class being used. You can't instantiate abstract classes, so there is a subclass of CharBuffer which does implement the method.
If you take the interface Comparable, you see that it has an (implicitly)abstract method compareTo. A class that implements Comparable is Integer, so you can easily write:
Comparable<Integer> i = new Integer(100);
i.compareTo(new Integer(55));

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

ORA-00904: invalid column name in select query by using abstract datatype

Hi,
I had created abstract datatype as PERSON_TY and ADDRESS_TY ,
after inserting the record . i'm tryng to select the column but i got the error , even i refferd all those thing. they are given that same please look this finde me a result.
SQL> DESC PERSON_TY
Name Null? Type
NAME VARCHAR2(25)
ADDRESS ADDRESS_TY
SQL> DESC ADDRESS_TY
Name Null? Type
STREET VARCHAR2(30)
CITY VARCHAR2(25)
STATE CHAR(2)
COUNTRY VARCHAR2(15)
SQL> SELECT * FROM EMPLOYE
2 ;
EMP_CODE
PERSON(NAME, ADDRESS(STREET, CITY, STATE, COUNTRY))
10
PERSON_TY('VENKAT', ADDRESS_TY('112: BLUE MOUNT', 'CHENNAI', 'TN', 'INDIA'))
20
PERSON_TY('SRINI', ADDRESS_TY('144: GREEN GARDEN', 'THAMBARAM', 'TN', 'INDIA'))
SQL> SELECT PERSON.NAME FROM EMPLOYE
2 ;
SELECT PERSON.NAME FROM EMPLOYE
ERROR at line 1:
ORA-00904: invalid column name
regards
venki

SELECT PERSON.NAME FROM EMPLOYEIf you look in the documentation, you will see that we need to alias the table in order to make this work:
select e.person.name from employees e
/Cheers, APC
Blog : http://radiofreetooting.blogspot.com

NotSerializable Exception using abstract processor

Hi all,
I'm new to Coherence and I'm using it on a new project.
I'm using a Coherence client in an OSGI enviroment and I've to satisfy some specific interfaces that are not serializable.
I'm agnostic on what are the maps that will be used, so I can't use a POF file.
So, I'm using a specific library based on jboss serialization for serialization/deserialization stuff.
I've found a serialization problem using an agent, in particular an AbstractProcessor.
This is the piece of code:
               cache.invoke(key.serialize(),
                    new AbstractProcessor() {
                                        @Override
                                        public Object process(
                                                  InvocableMap.Entry entry) {
                                             ModifiableKeyValueImpl modifiable =
                                                       new ModifiableKeyValueImpl(
                                                                 key, toValue(entry.getValue()));
                              op.perform(modifiable);
                              if (modifiable.isModified() && modifiable.getValue() != null) {
                                   entry.setValue(modifiable.getValue().serialize());
                                             return entry.getValue();
2 words on the code:
I want to make an update using an agent, so I'm using the AbstractProcessor.
I get the Entry for the specific key and wrap key and value in a specific class in order to permit to the caller of the method to complete a specific task (the update of the value).
When the value is modified I take it and I change the entry value.
And this is the stacktrace:
2012-03-10 19:02:25.480/3.483 Oracle Coherence GE 3.7.1.0 <Error> (thread=main, member=3): Processor is not serializable: package.CoherenceMyMap$1@4f536eec
(Wrapped) java.io.NotSerializableException: package.CoherenceMyMap
     at com.tangosol.util.Base.ensureRuntimeException(Base.java:288)
     at com.tangosol.util.Base.ensureRuntimeException(Base.java:269)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.packetProcessor.PacketPublisher.packetizeMessage(PacketPublisher.CDB:23)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.packetProcessor.PacketPublisher$InQueue.add(PacketPublisher.CDB:11)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.Grid.dispatchMessage(Grid.CDB:62)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.Grid.post(Grid.CDB:31)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.Grid.send(Grid.CDB:1)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.Grid.poll(Grid.CDB:13)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.Grid.poll(Grid.CDB:11)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.grid.partitionedService.PartitionedCache$BinaryMap.invoke(PartitionedCache.CDB:30)
     at com.tangosol.util.ConverterCollections$ConverterInvocableMap.invoke(ConverterCollections.java:2282)
     at com.tangosol.util.ConverterCollections$ConverterNamedCache.invoke(ConverterCollections.java:2748)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.grid.partitionedService.PartitionedCache$ViewMap.invoke(PartitionedCache.CDB:11)
     at com.tangosol.coherence.component.util.SafeNamedCache.invoke(SafeNamedCache.CDB:1)
     at com.tangosol.net.cache.NearCache.invoke(NearCache.java:432)
at package.CoherenceMyMap.update(CoherenceMyMap.java:257)
     at package.MyMapAdapter$2.update(MyMapAdapter.java:230)
     at package.adapter.MyMapAdapterTest.testUpdateOnMyMap(MyMapAdapterTest.java:245)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at junit.framework.TestCase.runTest(TestCase.java:168)
     at junit.framework.TestCase.runBare(TestCase.java:134)
     at junit.framework.TestResult$1.protect(TestResult.java:110)
     at junit.framework.TestResult.runProtected(TestResult.java:128)
     at junit.framework.TestResult.run(TestResult.java:113)
     at junit.framework.TestCase.run(TestCase.java:124)
     at junit.framework.TestSuite.runTest(TestSuite.java:243)
     at junit.framework.TestSuite.run(TestSuite.java:238)
     at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:83)
     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:49)
     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: java.io.NotSerializableException: package.COherenceMyMap
     at java.io.ObjectOutputStream.writeObject0(Unknown Source)
     at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
     at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
     at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
     at java.io.ObjectOutputStream.writeObject0(Unknown Source)
     at java.io.ObjectOutputStream.writeObject(Unknown Source)
     at com.tangosol.util.ExternalizableHelper.writeSerializable(ExternalizableHelper.java:2253)
     at com.tangosol.util.ExternalizableHelper.writeObject(ExternalizableHelper.java:2440)
     at com.tangosol.io.DefaultSerializer.serialize(DefaultSerializer.java:53)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.Service.writeObject(Service.CDB:1)
     at com.tangosol.coherence.component.net.Message.writeObject(Message.CDB:1)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.grid.partitionedService.PartitionedCache$InvokeRequest.write(PartitionedCache.CDB:7)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.service.Grid.serializeMessage(Grid.CDB:14)
     at com.tangosol.coherence.component.util.daemon.queueProcessor.packetProcessor.PacketPublisher.packetizeMessage(PacketPublisher.CDB:17)
     ... 34 more
I know that I'm asking a lot :)
Is there someone how can give me some indications?
Edited by: 920029 on Mar 10, 2012 10:27 AM

Hi, thanks for the reply.
I changed my code building an external class that extends AbstractProcessor.
The issue remains the same.
I'm thinking that the problem is that some elements used in the process method of the processor aren't serializable (I can't change it, they are specific api).
Can be this the problem? Is there a way to by-pass this?
Thanks in advance!

Using abstraction

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