Abstract Class Misunderstanding

I have a GUI Main class that has action listeners that need to call sub classes passing in data entered by the interacting user. The subclass being called from the GUI is an Abstract class. Since abstract classes can't be instantiated, how do I pass down the data entered by the user? If this does not make sense, please reply so and I'll post code as an example.
Thanks in advance.

You pass an instance of a subclass of the abstract class, one that is not itself abstract.

Similar Messages

Abstract class confusion

I am writing a J2ME application, and I want to write a Record management system (RMS) that I can use in other J2ME programs. I have a Record class that RMS classes need to know about. The idea is that the Record class is abstract, and the implementation is defined by a subclass - depending on what the application needs to store. Any Record must provide the following:
1) A constructor which takes a single byte[] parameter, which is the data read in from the RecordStore by the RMS. The implementation depends on what information is stored in this type of Record - it might be a String, series of ints, etc...
2) A method getStorageFormat() which returns a btye[] representing the data to be stored in the RecordStore. Again, this is dependent on the application.
I think that I have to use an abstract class here, so I have written an abstract class called Record - here's what it looks like:
public abstract class Record {
     public Record(byte[] bytes) { }; //Should this be an empty constructor?
     public abstract byte[] getStorageFormat();
}My problem is that I can't define a subclass of Record that works! Here's an example:
public class MyRecord extends Record {
       String data1, data2, data3; //Some data that is stored in this record
       public MyRecord(byte[] bytes) {
              String data = new String(bytes);
              // do some processing to extract the data...
       public byte[] getStorageFormat() {
              String sep = ","; //A seperator
              String tmp = data1 + sep + data2 + sep + data3; //put all the data together with
              return tmp.getBytes();
}}I get an error in Eclipse with the MyRecord constructor that says:" The implicit super constructor Record() is undefined. Must explicity invoke another constructor". I tried super() but I get the same error. Can anyone please tell me what I am doing wrong? It seems to me that I am misunderstanding something about how abstract classes work.

Because you defined a ctor that takes args, the implicit one that takes no args--super()--is no longer implicitly supplied. You have to either put a no-arg ctor into the parent class, or call the one that you already have.
public Record(byte[] bytes) { }; //Should this be an empty constructor?
public abstract byte[] getStorageFormat();
}No. You don't want a ctor that takes args and does nothing. If you're taking that arg, then you should use it to initialize the state of the object.
Here are the rules for constructors--"ctors" because I'm lazy. Also, because I'm lazy, "super(...)" and "this(...)" mean any super or this call, regardless of how many args it takes, including those that take no args.
1) Every class has at least one ctor.
1.1) If you do not define an explicit constructor for your class, the compiler provides a implicit constructor that takes no args and simply calls super().
1.2) If you do define one or more explicit constructors, regardless of whether they take args, then the compiler no longer provides the implicit no-arg ctor. In this case, you must explicitly define a
public MyClass() {...}
if you want one.
1.3) Constructors are not inherited.
2) The first statement in the body of any ctor is either a call to a superclass ctor
super(...)
or a call to another ctor of this class
this(...)
2.1) If you do not explicitly put a call to super(...) or this(...) as the first statement in a ctor that you define, then the compiler implicitly inserts a call to super's no-arg ctor
super()
as the first call. The implicitly called ctor is always super's no-arg ctor, regardless of whether the currently running ctor takes args.
2.2) There is always exactly one call to either super(...) or this(...) in each constructor, and it is always the first call. You can't put in more than one, and if you put one in, the compiler's implicitly provided one is removed.

Instanciate generic subclass of an abstract class

Hello,
I'm writing a litle file manager API for my application, organised like a Database: a table filled by records.
I want my table to be able to be able to manage different types of records (but one record type by table!), so I have this hierarchy:
the abstract record class:
public abstract class Record {
     public Record() {
     public abstract void readFrom(DataInput in, int length) throws IOException;
     public abstract void writeTo(DataOutput out) throws IOException;
     public abstract int prepareToWrite();
}the abstract atomic record class, extending the abstract record class:
public abstract class AtomicRecord<T> extends Record {
     public AtomicRecord() {
     public AtomicRecord(T value) {
          super();
     public abstract T getValue();
     public abstract void setValue(T value);
}and, for example, the StringRecord class:
public class StringRecord extends AtomicRecord<String> {
     code doesn't matter.
}now, I want to make a table of generic records:
public class RandomRecordAccessAppendableFileTable<RecordType extends Record>{
     public final RecordType getRecord(long id) throws IOException {
          RecordType record = new RecordType();
}problem: the RecordType is obviously not instanciable, as it extends Record, and not StringRecord. But I still want to instanciate a new StringRecord from a RandomRecordAccessAppendableFileTable<StringRecord> and a FooRecord from a RandomRecordAccessAppendableFileTable<FooRecord>.
Is there a way I can do this?
Thank you in advance.

This is just my opinion, so you might ignore it: The design you are targeting is a result of misunderstanding what Generics are for and what Generics may provide in their current state. Their only purpose is to provide compile-time type safety. Creating instances the way you want to do seems more runtime focused, i.e., when generic information is gone.
Maybe, you should consider passing a factory at construction time that enables to create a record of a specific type. Using the class object requires reflection, which will introduce implicit contracts to record classes and potential runtime failures.

Abstract class and RMI

Hi!
I have a desgin where I have an abstract class that in its constructor registers itself in a registry and define three abstract methods. Then I have a class that extends the abstract class and implements the three methods.
One of the methods inserts som text in a textArea defined in the child of the abstract class but when I call this method remotly I get a nullpointer exception saying that the textArea has not been initialized. The child ofcause calls super in its constructor.
So my question is can you not extend an abstract class that register it self in the constructor and then call methods remotly that uses variables that have been initialized in a childs constructor?
Have your any idea to why I get the nullpointer exception?
Peter

>
This sounds like there might be a misunderstanding
about the nature of methods and variables. If, for
instance, your client looks something like this:
RObject roo = ...// get via RMI
TextArea text = roo.getTextArea();
text.setText("...");
My design is more like this:
public interface Subscriber extends Remote {
public void notifySubscriber();
public abstract class A implements Subscriber {
a() {
    server.addSubscriber(this);
public abstract method1();
// some more abstract methods
public void notifySubscriber() {
    // do some stuff
    method1();
public class B extends A {
JTextArea text;
A() {
    super();
    text = new JTextArea();
public void method1() {
    // do some stuff
    // then text is null when called remotly
    text.setText("Hello");
}Hope this makes my design more clear and you can see the problem.
Thanks in advance

What is the diff b/w Abstract class and an interface ?

Hey
I am always confused as with this issue : diff b/w Abstract class and an interface ?
Which is more powerful in what situation.
Regards
Vinay

Hi, Don't worry I am teach you
Abstract class and Interface
An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
Edited by SASIKUMARA
SIT INNOVATIONS- Chennai
Message was edited by:
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Abstract class with set and get methods

hi
how to write set and get methods(plain methods) in an abstartc class
ex: setUsername(String)
String getUsername()
and one class is extending this abstract class and same methods existing in that sub class also..... how to write......plz provide some ideas
am new to programming....
asap
thnx in advance

yes... as i told u.... i am new to coding......
and my problem is ..... i have 2 classes one is abstract class without abstract methods.. and another class is extending abstract class.....
in abstract class i have 2 methods...one is setusername(string) and getusername() ..... how to write these two methods.... in abstract class i have private variables username...... when user logins ..... i need to catch the user name and i need to validate with my oracle database and i need to identify the role of that user and based on role of that user i need to direct him to appropriate jsp page.......
for that now i am writing business process classes..... the above mentioned two classes are from business process.....
could u help me now
thnx in advance

Calling a super.ssuper.method but your super is a abstract class.

Dear guys,
Is that possible to invoke your super's super's method but your super is a abstract class?
like:
class GO { public void draw() { } }
abstract class GORunner extends GO {}
class GOCounter extends GORunner {
public void draw() {
super.super.draw();
I want to do this because I would like to take advantages of the abstract as an layer to achieve some polymorphism programming Therefore, in the later stage of the programming some code may only refer to GORunner but actually it is holding a GOCounter object.
Thank!!

BTW you don't need to write this
public void draw() {
super.draw();
}It works but its basically the same as not having it at all.

Calling a method from an abstract class in a seperate class

I am trying to call the getID() method from the Chat class in the getIDs() method in the Outputter class. I would usually instantiate with a normal class but I know you cant instantiate the method when using abstract classes. I've been going over and over my theory and have just become more confused??
Package Chatroom
public abstract class Chat
   private String id;
   public String getID()
      return id;
   protected void setId(String s)
      id = s;
   public abstract void sendMessageToUser(String msg);
Package Chatroom
public class Outputter
public String[] getIDs()
     // This is where I get confused. I know you can't instantiate the object like:
        Chat users=new Chat();
        users.getID();
I have the two classes in the package and you need to that to be able to use a class' methods in another class.
Please help me :(

I have just looked over my program and realised my class names are not the most discriptive, so I have renamed them to give you a clearer picture.
package Chatroom
public abstract class Chatter
private String id;
public String getID()
return id;
protected void setId(String s)
id = s;
I am trying to slowly build a chatroom on my own. The Chatter class is a class that will be used to represent a single logged in user and the user is given an ID when he logs in through the setId and getID() methods.
package Chatroom;
import java.util.Vector;
public class Broadcaster
private Vector<Chatter> chatters = new Vector<Chatter>();
public String[] getIDs()
// code here
The Broadcaster class will keep a list of all logged-in users keeps a list of all the chats representing logged-in users, which it stores in a Vector.I am trying to use the getIDs() method to return an array of Strings comprising the IDs of all logged-in users, which is why im trying to use the getID() method from the Chat class.
I apologise if I come across as clueless, it's just I have been going through books for about 4 hours now and I have just totally lossed all my bearings

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.

".. is an abstract class. It can't be instantiated"

Does anyone have an idea of how I can get rid of the above error message? Here is a bit of my code:
import java.io.*;
import java.util.Vector;
class Project3 {
private BiTree company = new BiTree();
public static void main(String[] args) throws IOException {
Project3 run = new Project3();
run.Command();
public void Command() throws IOException {}
public interface comparable {
//not sure if need this in this class, or at all
public int compare(Object object1,Object object2);
public void add() {
Employee newOne = new Employee();
System.out.println("Enter name");
BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in));;
String name = stdin.readLine();
System.out.println("Enter title");
BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in));;
String title = stdin.readLine();
System.out.println("Enter payrate");
BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in));;
payrate = Double.parseDouble.stdin.readLine();
newOne.setTitle(title);
newOne.setName(name);
newOne.setPayrate(payrate);
company.insert(newOne);
}

It is not possible to create instances of an abstract class.
I hope this example helps,
abstract class Vehicle
public static void main(String arg[])
Vehicle a;
a = new Automobile(); // ok, since Automobile is a kind of Vehicle
// The class Automobile, a special case of Vehicle
class Automobile extends Vehicle

Abstract Class can't be instantiated

Hi I'm getting the following error messages while compiling. Does any one have any idea how to get RID of it?
ERROR MESSAGES:
EditableHeaderTableExample2.java:48: inner class EditableHeaderTableExample2. MyComboRenderer is an abstract class. It can't be instantiated.
MyComboRenderer renderer = new MyComboRenderer(items);
^
EditableHeaderTableExample2.java:79: Method redefined with different return type: javax.swing.table.TableCellRenderer getTableCellRendererComponent(javax.swing.JTable, java.lang.Object, boolean, boolean, int, int) was java.awt.Component getTableCellRendererComponent(javax.swing.JTable, java.lang.Object, boolean, boolean, int, int)
public TableCellRenderer getTableCellRendererComponent(
^
2 errors
Here is the code:
EditableHeaderTableExample2.java
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import javax.swing.JComponent;
import javax.swing.JComboBox;
import java.util.*;
import javax.swing.*;
import javax.swing.table.*;
import javax.swing.border.*;
import javax.swing.JTable;
import javax.swing.plaf.metal.*;
import javax.swing.JFrame;
import javax.swing.table.TableColumn;
import javax.swing.table.TableColumnModel;
import javax.swing.DefaultCellEditor;
import javax.swing.table.TableCellRenderer;
import javax.swing.table.DefaultTableCellRenderer;
public class EditableHeaderTableExample2 extends JFrame {
public static void main(String[] args) {
EditableHeaderTableExample2 frame = new EditableHeaderTableExample2();
frame.setSize(400,300);
     frame.setLocation(100,100);
     frame.setVisible(true);
public EditableHeaderTableExample2(){
JTable table = new JTable(10,10);
TableColumnModel columnModel = table.getColumnModel();
table.setTableHeader(new EditableHeader(columnModel));
String[] items = {"Dog","Cat"};
JComboBox combo = new JComboBox();
for (int i=0;i<items.length;i++) {
combo.addItem(items);
MyComboRenderer renderer = new MyComboRenderer(items);
EditableHeaderTableColumn col;
// column 1
col = (EditableHeaderTableColumn)table.getColumnModel().getColumn(1);
col.setHeaderValue(combo.getItemAt(0));
col.setHeaderRenderer(renderer);
col.setHeaderEditor(new DefaultCellEditor(combo));
// column 3
col = (EditableHeaderTableColumn)table.getColumnModel().getColumn(3);
col.setHeaderValue(combo.getItemAt(0));
//col.setHeaderRenderer(renderer);
col.setHeaderEditor(new DefaultCellEditor(combo));
JScrollPane pane = new JScrollPane(table);
getContentPane().add(pane);
class MyComboRenderer extends JComboBox implements TableCellRenderer
MyComboRenderer(String[] items) {
for (int i=0;i<items.length;i++) {
addItem(items[i]);
public TableCellRenderer getTableCellRendererComponent(
JTable table, Object value,
boolean isSelected, boolean hasFocus,
int row, int column) {
setSelectedItem(value);
return this;

This is not a suggestion, it's a requirement. When you say
class MyComboRenderer extends JComboBox implements TableCellRendererthe "implements TableCellRenderer" part is a promise that your class will include a method whose signature is
    Component getTableCellRendererComponent(JTable table, Object value,
                             boolean isSelected, boolean hasFocus,
                             int row, int column);

Difference between abstract class and the normal class

Hi...........
can anyone tell me use of abstract class instead of normal class
The main doubt for me is...
1.why we are defining the abstract method in a abstract class and then implementing that in to the normal class.instead of that we can straight way create and implement the method in normal class right...../

Class vs. interface
Some say you should define all classes in terms of interfaces, but I think recommendation seems a bit extreme. I use interfaces when I see that something in my design will change frequently.
For example, the Strategy pattern lets you swap new algorithms and processes into your program without altering the objects that use them. A media player might know how to play CDs, MP3s, and wav files. Of course, you don't want to hardcode those playback algorithms into the player; that will make it difficult to add a new format like AVI. Furthermore, your code will be littered with useless case statements. And to add insult to injury, you will need to update those case statements each time you add a new algorithm. All in all, this is not a very object-oriented way to program.
With the Strategy pattern, you can simply encapsulate the algorithm behind an object. If you do that, you can provide new media plug-ins at any time. Let's call the plug-in class MediaStrategy. That object would have one method: playStream(Stream s). So to add a new algorithm, we simply extend our algorithm class. Now, when the program encounters the new media type, it simply delegates the playing of the stream to our media strategy. Of course, you'll need some plumbing to properly instantiate the algorithm strategies you will need.
This is an excellent place to use an interface. We've used the Strategy pattern, which clearly indicates a place in the design that will change. Thus, you should define the strategy as an interface. You should generally favor interfaces over inheritance when you want an object to have a certain type; in this case, MediaStrategy. Relying on inheritance for type identity is dangerous; it locks you into a particular inheritance hierarchy. Java doesn't allow multiple inheritance, so you can't extend something that gives you a useful implementation or more type identity.
Interface vs. abstract class
Choosing interfaces and abstract classes is not an either/or proposition. If you need to change your design, make it an interface. However, you may have abstract classes that provide some default behavior. Abstract classes are excellent candidates inside of application frameworks.
Abstract classes let you define some behaviors; they force your subclasses to provide others. For example, if you have an application framework, an abstract class may provide default services such as event and message handling. Those services allow your application to plug in to your application framework. However, there is some application-specific functionality that only your application can perform. Such functionality might include startup and shutdown tasks, which are often application-dependent. So instead of trying to define that behavior itself, the abstract base class can declare abstract shutdown and startup methods. The base class knows that it needs those methods, but an abstract class lets your class admit that it doesn't know how to perform those actions; it only knows that it must initiate the actions. When it is time to start up, the abstract class can call the startup method. When the base class calls this method, Java calls the method defined by the child class.

A Singleton variable in an Abstract Class

Hello there people
I have a quick question. I have made an abstract class in which I want to put an identifier (ID) int to be incremented whenever I make any of the concrete class instances that extend this abstract class.
whould that declairation be as follows?:
public static int IDAdditionally, I have a quick question about abstract constructors. If constructors are defined in the concrete classes, are their declairations needed in the abstract class?
and finally, if I wanted to use the int ID (as mentioned above) then would I have to use constructor from the abstract class?
thanks in advance for the help

I think the OP would want a constructor in the abstract class to update that variable. Otherwise, he'd have to remember to update it in the constructor for all concrete classes. But, you don't declare constructors for the concrete class in the abstract class. You only declare constructors for the abstract class in the abstract class.

Abstract method called in an abstract class

Hello,
I am writing some code that I'd like to be as generic as possible.
I created an abstract class called Chromozome. This abstract class has a protected abstract method called initialize().
I also created an abstract class called Algorithm which contains a protected ArrayList<Chromozome>.
I would like to create a non abstract method (called initializePopulation()) which would create instances of Chromozome, call their method initialize() and full the ArrayList with them.
In a practical matter, only subclass of Algorithm will be used, using an ArrayList of a subclass of Chromozome implementing their own version of initialize.
I have been thinking of that and concluded it was impossible to do. But I'd like to ask more talented peaple before forgetting it !
Thanks,
Vincent

Ok, let's it is not impossible, juste that I had no idea of how doing it :-)
The difficulty is that Algorithm will never have to deal with Chromozome itself, but always with subclass of Chromozome. This is usually not an issue, but in that case, Algorithm is required to create instances of the desired subclass of Chromozome, but without knowing in advance wich subclass will be used (I hope what I say makes any sense).
Actually I may have found a way in the meantime, but maybe not the best one.
I created in Algorithm an abstract method :
protected abstract Chromozome createChromozome()The method initializePopulation will call createChromozome instead of calling directly the constructor and the initialize() method of Chromozome.
Then subclass of Algorithm will implement the method createChromozome using the desired subclass of Chromozome.

Casting & abstract class & final method

what is casting abstract class & final method in ABAP Objects give some scenario where actually use these.

Hi Sri,
I'm not sure we can be any more clear.
An Abstract class can not be instantiated. It can only be used as the superclass for it's subclasses. In other words it <b>can only be inherited</b>.
A Final class cannot be the superclass for a subclass. In other words <b>it cannot be inherited.</b>
I recommend the book <a href="http://www.sappress.com/product.cfm?account=&product=H1934">ABAP Objects: ABAP Programming in SAP NetWeaver</a>
Cheers
Graham

Abstract Class Misunderstanding

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