Abstract class - subclass

a quik question: if I have a abstract class define a method, and a subclass implementing it, and a third class with an instance casted as the abstract class, would the following method in the subclass then return the instance casted as subclass? :
public Subclass getInstance() {
return this;
Stig.

oops.... cant return SubClass....

Similar Messages

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.

Invoking method of final subclass of abstract class is slow

hi,
I've diagnosed a performance regression in my code which boils down to the example at the end of the post.
Would it be possible to have the vm dynamically figure out that within the hotspot in the test method, its
safe to ditch the dynamic dispatch and skip that overhead?
I thought marking the method as final would allow the vm to peform some run-time optimisations - I'm pretty sure I've been able to measure that in different situations; marking the test method's parameter (a) as final, also doesn't help.
Rather than have an abstract class, its looking like the performance requirements will dictate some copy and paste to
help the vm along - does anyone know if I'm missing something here?
thanks in advance,
asjf
The output I see (using 6.0 update 10) is:
B foo 28
B bar 95
C foo 29
C bar 86
D foo 26
D bar 92
E foo 29
E bar 82
public class MethodCallPerfTest {
     public static void main(String[] args) {
          test(new B());
          test(new C());
          test(new D());
          test(new E());
     public static void test(A a) {
               long start= System.currentTimeMillis();
               for(int i=0; i<8000000; i++)
                    a.foo();
               System.out.println(a.getClass().getSimpleName()+" foo "+(System.currentTimeMillis() - start));
               long start= System.currentTimeMillis();
               for(int i=0; i<8000000; i++)
                    a.bar();
               System.out.println(a.getClass().getSimpleName()+" bar "+(System.currentTimeMillis() - start));
abstract class A {
     int j;
     final void foo() {j++;}
     abstract void bar();
final class B extends A {
     int k;
     void bar() {k++;}
final class C extends A {
     int l;
     final void bar() {l++;}
class D extends A {
     int m;
     final void bar() {m++;}
class E extends A {
     int n;
     void bar() {n++;}
}

is it because references to the final fields are resolved at compile-time ?
such that the compiler must know the value of USE in:if(USE){
// a
// b at compile time so it can remove either "b" or "a". I think so. hmm.

Geting subclass info from abstract class

Does ne one know how i can retreive informtion from the subclasses of an abstract class by creatinig an instance of the abstract class.
are Methods such as getClass used?

Does ne one know how i can retreive informtion from
the subclasses of an abstract class by creatinig an
instance of the abstract class.You can never create an instance of the abstract class. That is why they are called abstract.
>
are Methods such as getClass used?
Coming to your actual question, a superclass will not know anything about its children. So you will have to do something else get that information.
Maybe inspecting all the classes in a classloader and checking its parent will be an option. I do not think this is possible, because ClassLoader does not provide a mechanism to get all the classes.
Somebody else might give you a better answer. I am just trying to answer one part of your question concerning abstract classes.

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" method in a non-abstract class

Hi all.
I have a class "SuperClass" from which other class are extended...
I'd like to "force" some methods (method1(), method2, ...) to be implemented in the inherited classes.
I know I can accomplish this just implementing the superclass method body in order to throw an exception when it's directly called:
void method1(){
throw new UnsupportedOperationException();
}...but I was wondering if there's another (better) way...
It's like I would like to declare some abstract methods in a non-abstract class...
Any ideas?

The superclass just models the information held by
the subclasses.
The information is taken from the database, by
accessing the proper table (one for each subclass).??
What do you mean by "models the information"?
You should use inheritance (of implementation) only when the class satisfies the following criteria:
1) "Is a special kind of," not "is a role played by a";
2) Never needs to transmute to be an object in some other class;
3) Extends rather than overrides or nullifies superclass;
4) Does not subclass what is merely a utility class (useful functionality you'd like to reuse); and
5) Within PD: expresses special kinds of roles, transactions, or things.
Why are you trying to force these mystery methodsfrom the superclass?
It's not mandatory for me to do it... I 'd see it
just like a further way to check that the subclasses
implements these methods, as they have to do.That's not a good idea. If the superclass has no relation to the database, it shouldn't contain methods (abstract or otherwise) related to database transactions.
The subclasses are the classes that handle db
transaction.
They are designed as a binding to a db table.And how is the superclass designed to handle db transactions? My guess (based on your description) is that it isn't. That should tell you right away that the subclasses should not extend your superclass.

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."

Why cannot create instance to an abstract class?

Dear Developers....
abstract class can create be its own constructor but why they didn't create object

We cann't create an object for abstract class
because they have abstract methods which depends on
its subclass for its implementation. if it is
possible to create the object for abstract class
means then you can call a method which has no
implementation(abstract methods) which is a illegal
one thats what the ruleWhat about classes with no abstract members?
public abstract class A {}No danger of accessing anything illegal here.
The rule is far more simple. By declaring a class as abstract, the creator defines that no instances of this class may exist. Period. If you could create instances, the class wouldn't be abstract anymore, per definition. It's like asking why a green light doesn't shine red. If it would, it wouldn't be green anymore.

Abstract Class Compiling Problem

This is my first time using the code tags, I apologize if I didn't do it right.
I can't seem to get the subclass to compile properly. I have defined an abstract class TemplateLoginCommand that encapsulates all of the login functionality. I have defined two abstract classes success and failure that should hold the code to forward the app to the right page on success, or handle any errors on login. I keep getting the following error message when I try to compile KlLoginCommand:
I am removing the package info so my boss doesn't flip
../command/KlLoginCommand.java [17:1] ../command.KlLoginCommand should be declared abstract; it does not define failure() in ../.command.TemplateLoginCommand
public class KlLoginCommand extends TemplateLoginCommand
^
1 error
Can anyone see what I am doing wrong here. I don't have a lot of experience with abstract classess/methods. Thanks
public abstract class TemplateLoginCommand extends PageVerificationCommand implements Constants
    // Required Form Fields
    public static final String[] REQUIRED_FIELDS = {USERNAME, PASSWORD};
    // Abstract Methods Must Be Subclasses
    abstract void success(UserEntity entity);
    abstract void failure();
    /** Creates new TemplateLoginCommand */
    public TemplateLoginCommand()
        super();
    public TemplateLoginCommand(String childDirectory)
        super(childDirectory);
    public void destroy(Map map) throws Exception
        super.destroy(map);
        // Validate
        if (verifyFields(REQUIRED_FIELDS, map))
            // Attempt login
            try
                String username = (String) map.get(USERNAME);
                String password = (String) map.get(PASSWORD);
                UserEntity user = UserEntity.login(username, password);
                // Forward to rolemenu
                success(user);
                return;
            catch( SQLException sqle )
                failure();
                return;
        else
            failure();
            return;
public final class KlLoginCommand extends TemplateLoginCommand
    /** Creates a new instance of KlLoginCommand */
    public KlLoginCommand()
    protected void failure()
    protected void success(UserEntity entity)
}

Hi,
Try making the methods
// Abstract Methods Must Be Subclasses
abstract void success(UserEntity entity);
abstract void failure();
protected.
Roger

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.

Final methods in abstract classes?

Hi, why is it possible to define a final method in an abstract class? The theory behind a final method doesn't say that a final method couldn't be overridden?
Marco

So it's formally correct but it doesn't have any
sense, does it?You sound very confused. A final method in an
abstract class has just the same semantics and
makes just as much sense as in a non-abstract
class.
The semantics of a final method is simply that
it cannot be overridden in subclassed. Both
abstract and non-abstract classes can be
subclasses. So why do you think there should be any
difference?Actually i was confused now it's clear. I was too binded to the concept that the extending class SHOULD(not for a formal reason, but for a 'design' one) write the implementation of the methods defined in the abstract class. Now i see that, actually, by defining a final method in an abstract class we are defining our design as implemented and clients(i.e. subclasses) can only use it.
Thank you,
Marco

Access overriden method of an abstract class

class Abstract
abstract void abstractMethod(); //Abstract Method
void get()
System.out.print("Hello");
class Subclass extends Abstract
void abstractMethod()
System.out.print("Abstract Method implementation");
void get()
System.out.print("Hiiii");
In the above code, i have an abstract class called "Abstract", which has an abstract method named "abstractMethod()" and another method called "get()".
Now, this class is extended by "Subclass", it provides implementation for "abstractMethod()", and also overrides the "get()" method.
Now my problem is that i want to access the "get()" method of "Abstract" class. Since it is an abstract class, i cant create an object of it directly, and if i create an object like this:
Abstract obj = new Subclass();
then, obj.get() will call the get() method of Subclass, but how do i call the get() method of Abstract class.
Thanks in advance

hey thanks a lot,, i have another doubt regarding Abstract classes.
i was just trying something, in the process, i noticed that i created an abstract class which does not have any abstract method, it gave no compilation errors. was wondering how come this is possible, and what purpose does it solve?

Accessing Enclosing Class Members From Inner Class Subclass

I have the following scenario that I cannot get to work. Notice the comments in B.doWork() for the problem code. In B.doWork(), how do I access m_strA?
* A.java
* Created on July 5, 2002, 2:20 PM
package Projects.InnerTrouble.Files;
public class A {
     public abstract class InnerA {
          public abstract void doWork ();
     public String m_strA;
     /** Creates new A */
     public A ()
               new InnerA() {
                         public void doWork ()
                                   System.out.println("A$InnerA$1's doWork() called!");
                                   m_strA = "Annonymous subclass of InnerA's doWork did this";
                    }.doWork();
     * @param args the command line arguments
     public static void main (String args[])
               A oTemp = new A();
               System.out.println(oTemp.m_strA);
               B.getB(oTemp).doWork();
               System.out.println(oTemp.m_strA);
class B extends A.InnerA {
     public B (A a)
               a.super();
     public void doWork ()
               System.out.println("B's doWork() called!");
               // How do I access m_strA within B's doWork() method? The following is what I would expect to be the answer, but it does not compile
               // A.this.m_strA = "B's doWork did this";
     private static A.InnerA sm_oInnerA;
     public static A.InnerA getB (A a)
               if (sm_oInnerA == null)
                    sm_oInnerA = new B(a);
               return (sm_oInnerA);

The whole point is that B is not an inner class of A
so it does not have access to A's member variables.
Eventhough B extends an inner class of A, that does
not make B an inner class of A. That is in the JLS,
but not so elegantly as I have put it, hehe.
If B were an innerclass of InnerA, then it would
qualify to access A's member variables.OK, I think that you are finally getting through to my thick skull on this one. Let me restate and get your check-off on my understanding of the situation.
The only classes with access to A's this reference are A and inner classes of A that are found within the definition of A. So, despite the fact that A and B are in the same package (and B should have access to A's non-private members because B and A are in the same package), and despite the fact that we would normally state that B "is a" InnerA (which is an inner class of A and would have access to a reference to the A.this reference), B is not allowed access to A.this (because B "is not really a" InnerA in the same way that the anonymous implementation of InnerA "is a" InnerA). However, nothing would prevent me from giving B access to a reference of the enclosing A as long as it was done via a method of InnerA, and as long as the implementation of that method is contained in A's implementation.
Does this "access" rule realy make sense? Are you aware of the justification for this rule? Or is the justification stated in the JLS? I would think that the compiler ought to be able to figure this kind of thing out and allow it. It seems to me the fact that I defined B in the way that I did, and the fact that B "is a" InnerA, implies that I desired a fairly tight relationship to A. In fact, I desired the exact relationship that exists for the anonymous implementation of InnerA.
The following is a modified version of my original example that runs as I originally wanted it to, but works around the access rules discussed on this forum thread:
* A.java
* Created on July 5, 2002, 2:20 PM
package Projects.InnerTrouble.Files;
public class A {
     public abstract class InnerA {
          public abstract void doWork ();
          /** added to allow implementors of InnerA that are not enclosed in A's class definition to have access to the enclosing class */
          public A myEnclosingInstance ()
                    return (A.this);
     public String m_strA;
     /** Creates new A */
     public A ()
               new InnerA() {
                         public void doWork ()
                                   System.out.println("A$InnerA$1's doWork() called!");
                                   m_strA = "Annonymous subclass of InnerA's doWork did this";
                    }.doWork();
     * @param args the command line arguments
     public static void main (String args[])
               A oTemp = new A();
               System.out.println(oTemp.m_strA);
               B.getB(oTemp).doWork();
               System.out.println(oTemp.m_strA);
class B extends A.InnerA {
     public B (A a)
               a.super();
     public void doWork ()
               System.out.println("B's doWork() called!");
               // The following is what I would expect to be the answer, but it does not compile
               // A.this.m_strA = "B's doWork did this";
               // added myEnclosingInstance() to get functionality desired above
               myEnclosingInstance().m_strA = "B's doWork did this";
     private static A.InnerA sm_oInnerA;
     public static A.InnerA getB (A a)
               if (sm_oInnerA == null)
                    sm_oInnerA = new B(a);
               return (sm_oInnerA);
}

Problems implementing abstract classes

hello.
this is james mcfadden. I am developing a multiplayer BlackJack card game in Java. the game consists of three programs: BlackJack.java, BlackJackServer.java and BlackJackClient.java (three 3 programs are shown below). i don't know how to implement abstract classes. i am trying to get the BlackJack.java program working with the BlackJackServer.java program. there should be "extends BlackJackServer" somewhere in the BlackJack.java program, but i don't know where.
import javax.swing.*;
public class BlackJack extends JPanel{
   public BlackJack(){
      //FlowLayout is default layout manager for a JPanel
      add(new JButton("Hit"));
      add(new JButton("Stay"));
      add(new JButton("New Game"));
   public static void main(String[] args){
      JFrame frame=new JFrame("BlackJack");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(500,500);
      frame.setLocation(200,200);
      BlackJack bj=new BlackJack();
      frame.setContentPane(bj);
      frame.setVisible(true);
import java.io.*;//Provides for system input and output through data streams, serialization and the file system
import java.net.*;//Provides the classes for implementing networking applications
import java.util.*;//Contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes
import java.awt.*;//Contains all of the classes for creating user interfaces and for painting graphics and images
import javax.swing.*;//Provides a set of lightweight components that, to the maximum degree possible, work the same on all platforms
public class BlackJackServer extends JFrame{
   private JTextArea jta=new JTextArea();//a text area for displaying text
   public static void main(String[] args){
          new BlackJackServer();//invokes the constructor BlackJackServer()
   }//end main
   public BlackJackServer(){
      setLayout(new BorderLayout());//places the text area on the frame
      add(new JScrollPane(jta),BorderLayout.CENTER);//lays out a text area, arranging and resizing its components to fit in the centre region;and provides a scrollable view of a lightweight component
      setTitle("BlackJack Server");//Sets the title for this frame to the specified string
      setSize(500,300);//Resizes this component so that it has a width and a height
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//Sets the operation that will happen by default when the user closes this frame
      setVisible(true);//shows the frame
      try{
         ServerSocket serverSocket=new ServerSocket(8000);//creates a server socket
         jta.append("Server started at "+new Date()+'\n');//displays the current date in the text area
         Socket socket=serverSocket.accept();//listens for a connection request
         DataInputStream inputFromClient=new DataInputStream(socket.getInputStream());//creates a data input stream
         DataOutputStream outputToClient=new DataOutputStream(socket.getOutputStream());//creates a data output stream
         while(true){
            float bet=inputFromClient.readFloat();//receives bet from the client
            float doublebet=bet+bet;//computes double the bet
            outputToClient.writeFloat(doublebet);//sends double the bet back to the client
            jta.append("Bet received from client: "+bet+'\n');//displays the bet in the text area
            jta.append("Double the bet found: "+doublebet+'\n');//displays double the bet in the text area
         }//end while
      }//end try
      catch(IOException ex){
         System.err.println(ex);//displays an error message
      }//end catch
   }//end constructor
}//end class BlackJackServer
import java.io.*;//Provides for system input and output through data streams, serialization and the file system
import java.net.*;//Provides the classes for implementing networking applications
import java.awt.*;//Contains all of the classes for creating user interfaces and for painting graphics and images
import java.awt.event.*;//Provides interfaces and classes for dealing with different types of events fired by AWT components
import javax.swing.*;//Provides a set of lightweight components that, to the maximum degree possible, work the same on all platforms
public class BlackJackClient extends JFrame{
   private JTextField jtf=new JTextField();//a text field for receiving text
   private JTextArea jta=new JTextArea();//a text area for displaying text
   private DataOutputStream toServer;//output stream
   private DataInputStream fromServer;//input stream
   public static void main(String[] args){
      new BlackJackClient();//invokes the constructor BlackJackClient()
   public BlackJackClient(){
      JPanel p=new JPanel();//holds the label and text field
      p.setLayout(new BorderLayout());//sets the layout of the content pane of this component by default
      p.add(new JLabel("Enter bet"),BorderLayout.WEST);//displays the bet and lays out a JLabel, arranging and resizing its components to fit in the western region
      p.add(jtf,BorderLayout.CENTER);//lays out the text field, arranging and resizing its components to fit in the centre region
      jtf.setHorizontalAlignment(JTextField.RIGHT);//Sets the horizontal alignment of the text to the right
      setLayout(new BorderLayout());//places the text area on the frame
      add(p,BorderLayout.NORTH);//lays out the text field, arranging and resizing its components to fit in the northern region
      add(new JScrollPane(jta),BorderLayout.CENTER);//lays out a text area, arranging and resizing its components to fit in the centre region;and provides a scrollable view of a lightweight component
      jtf.addActionListener(new ButtonListener());//invokes the ButtonListener class
      setTitle("BlackJack Client");//Sets the title for this frame to the specified string
      setSize(500,300);//Resizes this component so that it has a width and a height
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//Sets the operation that will happen by default when the user closes this frame
      setVisible(true);//shows the frame
      try{
         Socket socket=new Socket("localhost",8000);//creates a socket to connect to the server
         fromServer=new DataInputStream(socket.getInputStream());//creates an input stream to receive data from the server
         toServer=new DataOutputStream(socket.getOutputStream());//creates an output stream to send data to the server
      }//end try
      catch(IOException ex){
         jta.append(ex.toString()+'\n');//displays an error message
      }//end catch
   private class ButtonListener implements ActionListener{
      public void actionPerformed(ActionEvent e){
         try{
            float bet=Float.parseFloat(jtf.getText().trim());//gets the bet from the text field
            toServer.writeFloat(bet);//Converts the float argument to an int using the floatToIntBits method in class Float, and then writes that int value to the underlying output stream
            toServer.flush();//Flushes this output stream and forces any buffered output bytes to be written out
            float doublebet=fromServer.readFloat();//gets double the bet from the server
            jta.append("Bet is "+bet+"\n");//displays the bet in the text area
            jta.append("Double the bet received from the server is "+doublebet+'\n');//displays double the bet in the text area
         }//end try
         catch(IOException ex){
            System.err.println(ex);//displays an error message
         }//end catch
      }//end method
   }//end class
}//end class BlackJackClient

there should be "extends BlackJackServer" somewhere in the BlackJack.java programI very much doubt that.
It's possible you might need to create a BlackJackServer object or something like that. But I don't see the point in subclassing it.

Abstract class - subclass

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