Abstract methods and subclasses

I have a class, DfContainer that has a method whose return type is of an abstract class, DfObject.
There are several classes that extend the abstract class. Each of them has methods that are unique to them.
But in order to be able to execute the methods using the object returned through the method in DfContainer, I need to define each method as abstact in DfObject.
Now say, I need a method called getNames() in the class DfNames that extends DfObject, and I need a method called getRanks() in DfOrganization that also extends DfObject.
I would define those two methods as abstract in DfObject. But, I would need to add getRanks() to DfNames, and add getNames() to DfOrganization, even if I don't need them there.
That's what I'm doing right now. If I don't cast the object returned from the method in DfContainer to a subtype of DfObject, is this the correct approach?
Thanks,
Moshe

Why don't you define an empty method body in the abstract class, that does not have to be overridden in the subclass.
An abstract class can contain fully implemented methods. You can then override them or not as you choose. You don't have to define your individual methods as abstract.
Look at the way the various listeners and adapters are done in AWT. You have a listener that's an interface, then a dummy class that provides do-nothing implementations for all methods in the interface, so when you want to override one method you only need to override that specific method, not all the methods in the interface. You should be able to do something similar.

Similar Messages

Abstract method and class

I'm a beginner in Java and just learn about abstract method and class.
However, i am wondering what is the point of using abstract method/class?
Because when I delete the abstract method and change the class name to public class XXXX( changed from "abstract class XXXX), my program still runs well, nothing goes different.
Is it because I haven't encountered any situation that abstract method is necessary or ?
Thanks!

Yes - you probably haven't encountered a situation where you need an abstract.
Abstract classes are not designed to do anything on their own. They are designed to provide a template for other classes to extend by inheritance. What you have build sounds like a concrete class - one which you are creating instances of. Abstract classes are not designed to be ever instantiated in their pure form - they act like a partial building block, which you will complete in a class which extends the abstract.
An example might be a button class, which provides some core functionality (like rollover, rollout etc) but has an empty action method which has to be overwritten by a relevant subclass like 'StartButton'. In general, abstract classes may not be the right answer, and many people would argue that it is better to use an interface, which can be implemented instead of extended, meaning that you can ADD instead of REPLACING.
Not sure if that helps.. there are whole chapters in books on this kind of thing, so it's hard to explain in a couple of paragraphs. Do some google searches to find out more about how they work.

Abstract Methods and Override Don't work For SubPackages

I have an abstract class called "A" in a package called test.
package test;
public abstract class A
    int x;
    public A (int x)
        this.x = x;
    abstract void testMethod();
}And I have a sub class of "A" called "SubA" in a sub package of "test" called "test.subPkg":
package test.subPkg;
import test.A;
public class SubA extends A {
    public SubA(int x)
        super(x);
    @Override
    void testMethod()
        throw new UnsupportedOperationException("Not supported yet.");
}Now when I try to compile, it gives me the error:
/home/eric/NetBeansProjects/drum-tabber4/trunk/src/test/subPkg/SubA.java:6: test.subPkg.SubA is not abstract and does not override abstract method testMethod() in test.A
public class SubA extends A {
/home/eric/NetBeansProjects/drum-tabber4/trunk/src/test/subPkg/SubA.java:14: method does not override or implement a method from a supertype
If I put SubA in the above package "test" instead of "test.subPkg", it compiles fine. But I don't want to do that because I have a lot of sub classes that are organized differently in separate sub packages.
If I make A an interface, it compiles fine. But I don't want to do that either because I want A to implement the constructor.
Anyone have any idea of how to fix this issue?

Now when I try to compile, it gives me the error:
/home/eric/NetBeansProjects/drum-tabber4/trunk/src/test/subPkg/SubA.java:6: test.subPkg.SubA is not abstract and does not override abstract method testMethod() in test.A
public class SubA extends A {
/home/eric/NetBeansProjects/drum-tabber4/trunk/src/test/subPkg/SubA.java:14: method does not override or implement a method from a supertype
If I put SubA in the above package "test" instead of "test.subPkg", it compiles fine. But I don't want to do that because I have a lot of sub classes that are organized differently in separate sub packages.Thats because method testMethod() in class A has a default access modifier. Meaning, that testMethod() will only be visible within the same package and not outside of package test. Expand the access modifier of testMethod() in class A, either make it protected or public, like this...
protected abstract void testMethod();
OR
public abstract void testMethod();

About abstract interface and it's method

Hi !!
HttpSession session = request.getSession();
getSession() is the Method of HttpSession and it is the abstract interface..
right ??
so how can we call the abstract method and ..
here... request is the object of HttpServletRequest which is abstract interface so how can we make an Object of this abstract interface...
I m very confused Please solve my Querry ASAP

As far as I can tell, these three threads are all the same person asking the same question.
http://forum.java.sun.com/thread.jspa?threadID=5188609
http://forum.java.sun.com/thread.jspa?threadID=5188638
http://forum.java.sun.com/thread.jspa?threadID=5188603
How obnoxious.

Why does this abstract class and method work without implement it?

hi,
I have seen many times that in some examples that there are objects made from abstract classes directly. However, in all books, manual and tutorials that I've read explain that we MUST implement those methods in a subclass.
An example of what I'm saying is the example code here . In a few words that example makes Channels (java.nio.channel) and does operations with them. My problem is in the class to make this channels, because they used the ServerSockeChannel class and socket() method directly despite they are abstracts.
   // Create a new channel: if port == 0, FileChannel on /dev/tty, else
   // a SocketChannel from the first accept on the given port number
private static ByteChannel newChannel (int netPort)
      throws Exception
      if (netPort == 0) {
         FileInputStream fis = new FileInputStream ("/dev/tty");
         return (fis.getChannel());
      } else {
//CONFLICT LINES
         ServerSocketChannel ssc = ServerSocketChannel.open(); //<--I have never thought do that!! Anyway, how it is static method may work.
         ssc.socket().bind (new InetSocketAddress (netPort)); //<--but here, this method (socket) is abstract. WHY RETURN A SOCKET???????? this mehod should be empty by default.
         System.out.print ("Waiting for connection on port "
            + netPort + "...");
         System.out.flush();
         ByteChannel channel = ssc.accept();
         ssc.close();
         System.out.println ("Got it");
         return (channel);
   } I test this code and works fine. So why can it be??
Also, I read that the abstract classes can't have static methods. Is it true???
Please Help!!
PS: i have seen this kind of code many times. So i feel that I don't understand how its really the abstract methods are made.
PS2: I understand that obviously you don't do something like this: *"obj = new AbstractClass(); "*. I dont understand how it could be: ServerSocketChannel ssc = ServerSocketChannel.open(); and the compiler didn't warn.

molavec wrote:
ServerSocketChannel ssc = ServerSocketChannel.open(); //<--I have never thought do that!! Anyway, how it is static method may work.
The static method creates an instance of a class which extends ServerSocketChannel, but is actually another non-abstract class.I thought that, but reading the documentation I saw that about open() method:
Opens a server-socket channel.
The new channel is created by invoking the openServerSocketChannel method of the system-wide default SelectorProvider object.
The new channel's socket is initially unbound; it must be bound to a specific address via one of its socket's bind methods before connections can be accepted.
...and the problem is the same openServerSocketChannel is abstract, so i don't understand how it could return a ServerSocketChannel.There is a concrete implementation class that has implemented that method.
I guess that really the open() method use a SelectorProvider's subclase but it doesn't appear in the doc.It doesn't need to. First, you don't care about those implementation details, and second, you know that if the class is abstract, it must use some concrete subclass.
Ok, I speak Spanish by default (<-- this sounds like "I am a machine", ^_^' ). So, I didn't know how to say that the method would be {}. Is there a way to say that?? I recommendable for me to know, for the future questions o answers.Not sure what you're saying here. But the other respondent was trying to explain to you the difference between an abstract method and an empty method.
// abstract method
public abstract void foo();
// empty method
public void bar() {
Which class does extend ServerSocketChannel? I can not see it.It may be a package-private class or a private nested class. There's no need to document that specific implementation, since you never need to use it directly.

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."

Using an abstract method to assign an annotion property

Hi Guys
I have an annotation for specifiying that a method requires a transation. It's very simple, and looks like this:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface TransationRequired
    PersistenceUnitDescriptor persistenceUnitDescriptor();
}PersistenceUnitDescriptor is a simple enum, and I use Aspect J to weave code around methods annotated with this annotation.
When I annotate a method like this
@TransationRequired(persistenceUnitDescriptor = PersistenceUnitDescriptor.XXX)
     public Boolean func(String xx)
         //TODO: Something
     }It works fine, but when I annotate like this :
@TransationRequired(persistenceUnitDescriptor = getPersistenceUnit())
     public Boolean func(String xx)
         //TODO: Something
     }the apect code is never run.
getPersistenceUnit() is an abstract method and it is the only thing which is different between the two cases.
I'm not sure if this is beacuse of the annotation or because the weaving, but I thought I would ask in the annotation categroy just in case.
Anyone got an idea as to what may be causing this?
Thanks in advance,
Vackar

VackarAfzal wrote:
OK, Thanks for the tip. Pity that it has to be constant, in my opinion annotations would be a lot more powerful if you could dynamically change attributes. That really would make them more than just an alternative to annoying config files. But perhaps there are some technical details that I'm not aware of which makes this idea seem silly.Annotations were designed as a compile-time construct so the values within need to be compile time constants. At runtime, dynamic proxies are used to construct objects whose methods will return the values in question. You are free to construct your own objects implementing the interface of an annotation type at runtime that return values determined more dynamically.

Static abstract methods - how can I avoid them?

Hi everybody,
just found out that java doesn't allow static abstract methods nor static methods in interfaces. Although I understand the reasons, I can't think of how to change my design to gain the required behavior without the need of static abstract methods.
Here's what I want to do and how my thoughts lead to static abstract methods:
ClassA provides access to native code (c-dll, using jna). The path to the dll can be set programmatically. Here's a draft of the class:
public ClassA {
   private static String path;
   public static void setRealtivePath(String path) {
      //check if path exists and is not null -> get absolute path
      setPath(path);
   public static void setPath(String absolutePath) {
      this.path = path;
//code to provide access to native lib
}There is some more classes which provide access to different dlls (therefore the code for accessing the dlls differs from ClassA). Although this works, the setRelativePath method has to be implemented in each class. I thought it would be nice to put this method into a abstract super class that looks like this:
public abstract class superClass {
   public static void setRelativePath(String path) {
      //check if path exists and is not null -> get absolute path
      setPath(path);
   //force inherting class to implement a static method
   public static abstract void setPath(String absolutePath);
}thus simplifying ClassA and it's look-a-likes:
public ClassA {
   private static String path;
   @Override
   public static void setPath(String absolutePath) {
      this.path = path;
//code to provide access to native lib
}Since static abstract methods (and overriding static methods) is not allowed, this doesn't work.
I hope someone can catch my idea ;). Any suggestions how to do this in a nice clean way?
Thanks in advance,
Martin
Edited by: morty2 on Jul 22, 2009 2:57 AM

First of all, thanks a lot for your answer.
YoungWinston wrote:
Actually, you can "override" static methods (in that you can write the same method for both a subclass and a superclass, providing the superclass method isn't final); it's just that it doesn't work polymorphically. The "overriding" method masks the overridden one and the determination of what gets called is entirely down to the declared type.Yes, I know that. There's one problem: Your suggestion means that I simply have to drop the abstract modifier in the super classes setPath method. However, since the super class calls the setPath method (and not the inherting classes!) it will always be the super classes' method being called.
YoungWinston wrote:
Why are you so concerned with making everything static? Seems to me that the simplest solution would be to make all the contents instance-based.I want ClassA and it's look-a-likes to be set up properly at application start up, be accessible quite anytime and easily, they don't do anything themselves except for setting the path and calling into the native library which does the math. (Compareable to how you call e.g. Math.cos()). Therefore I don't think that an instance-based solution would be a better approach.
YoungWinston wrote:
Furthermore, you could make the class immutable; which might be a good thing - I'm not sure I'd want someone being able to change the pathname after I've >set up a class like this.Thanks for that!
PS: As mentioned in my first post, I do have a working solution. However, I'm really corious about finding a nicer and cleaner way to do it!
Martin

Override Abstract Methods in NetBeans 6.1

I just upgraded to NetBeans 6.1 and I can't figure out how to automatically override abstract methods... I I know in older versions you could select a menu option and select abstract methods and NetBeans would put them all in for you...

I'm sorry... I admit that I only looked quickly for a NetABeans forum and didn't find one... What is it under so I know for the future...

Where to put methods in a abstract base class - subclasses system

Hi,
I’d like to ask a question on some basic design practice.
When there are methods which are common in some subclasses so I would like to “move them up” in the base abstract class, I would also like to make sure that the ADT concept of the base class itself is not broken. So I don’t want to have methods in the base class that are not general enough to be there. How to resolve this?
For example I create a base abstract class Vehicle. Then I create subclasses Plane and Tanker and realize that the startEnginge() method in them is the same and in order remove the duplicated code, I can put it in Vehicle. But later there may be Bicycle or Sled subclasses which don’t need startEngine().
In a broader sense, I would like to keep the Vehicle class as similar to the real word concept of vehicles as possible. And not evey vehicle have engine of course.
What is the solution?
Extending the class hierarchy by injecting another abstract class between the base and the subclasses? (e.g: VehicleWithEngine)
I suppose I can’t use Interfaces because I need to have the common implemenations as well.
Thanks for any comments in advance,
lemonboston
ps: I am a beginner and don't know the terminology, so if there are programming expression for the followings for example, I would be thankful if someone could help with this too:
- moving common methods up in the class hierarchy
- injecting a class in the hierarchy
- abstract base class - subclasses system

lemonboston wrote:
Hi,
I’d like to ask a question on some basic design practice.
When there are methods which are common in some subclasses so I would like to “move them up” in the base abstract class, I would also like to make sure that the ADT concept of the base class itself is not broken. So I don’t want to have methods in the base class that are not general enough to be there. How to resolve this?
You are talking about code.
Instead you need to talk about the design.
The base class represents conceptually a 'type' of something. That 'type' defines behavior. That behavior is what goes in the base class nothing else (in terms of design.)
If you have common functionality which does not fit into the definition (design) of the 'type' then you put it in another class and use (composition) that class in the sub class.
For example I create a base abstract class Vehicle. Then I create subclasses Plane and Tanker and realize that the startEnginge() method in them is the same and in order remove the duplicated code, I can put it in Vehicle. But later there may be Bicycle or Sled subclasses which don’t need startEngine(). No that is not how it works.
You have specific examples of some vehicles and then you need to manage those types generically. That is the first step.
Second step is then to determine what the exact 'type' is that you want to manage. And you use the requirements of the need to create the 'type'.
Then you look at the specific examples to determine how they will meet the needs of the type.
Thus if I have an application that must start the engines of all the vehicles in the city then I must have a vehicle class which has startEngine.
But if I have an application that manages vehicles as inventory (like a retail store) and then decide that because my examples both have engines that I might as well move it into the base class. In that case there is no 'need' for the application to manage starting vehicles. The fact that both have engines is irrelevant.
So looking back at your example you have stated functionality about your specific types but you have not stated anything about why your application needs to deal with that functionality generically.
Another way to think about it is that you do not put the shared functionality in the base because you can but rather because you must.

Abstract classes and constructors - cannot call abs. methods in CONSTRUCTOR

Let me explain the scenario:
I'm building a program in which I need to read a file (among other things) and I intend to use object orientation to it's fullest in doing so. I thought of creating an abstract FILE class which has the commonalities, and two subclasses SERVER_FILE and PC_FILE, which implement the abstract method GET_CONTENTS in different ways (OPEN DATASET / GUI_UPLOAD), same for the CHOOSE method which allows to select the file from it's corresponding source.
Initially I've used an interface but since another tasks like setting the file path are common for both, switched to an ABSTRACT class.
Now, the problem is, from the main code I intend to use a FILE reference to handle either type of file. At the instantiation moment I'd like the path attribute to be set; if it was not set by parameter, i'd like to call the CHOOSE method which is abstract for the superclass. Since this is common for either subclass, I need a way to code it once in the superclass. But I get an error because the CHOOSE method is abstract.
This is the problem code (extracts):
*       CLASS lcl_file DEFINITION
CLASS lcl_file DEFINITION ABSTRACT.
PUBLIC SECTION.
    METHODS:
      constructor
        IMPORTING
          i_path TYPE string OPTIONAL
        EXCEPTIONS
          no_path_chosen,
      get_contents ABSTRACT
        RETURNING
          value(rt_contents) TYPE string_table
        EXCEPTIONS
          read_error.
PROTECTED SECTION.
    DATA:
      _v_path        TYPE string.
    METHODS:
      choose ABSTRACT
        EXCEPTIONS
          no_path_chosen,
      set_path
        IMPORTING
          i_path TYPE string.
ENDCLASS.                    "lcl_file DEFINITION
*       CLASS lcl_file IMPLEMENTATION
CLASS lcl_file IMPLEMENTATION.
METHOD constructor.
    IF i_path IS SUPPLIED.
      CALL METHOD set_path
        EXPORTING
          i_path = i_path.
    ELSE.
*---->>>> PROBLEM CALL - CAN'T BE DONE!!
      CALL METHOD choose
        EXCEPTIONS
          no_path_chosen = 1.
      IF sy-subrc = 1.
        RAISE no_path_chosen.
      ENDIF.
    ENDIF.
ENDMETHOD.                    "constructor
METHOD set_path.
    _v_path = i_path.
ENDMETHOD.                    "set_path
ENDCLASS.                    "lcl_file IMPLEMENTATION
*       CLASS lcl_server_file DEFINITION
CLASS lcl_server_file DEFINITION
                      INHERITING FROM lcl_file.
PUBLIC SECTION.
    METHODS:
      get_contents REDEFINITION.
PROTECTED SECTION.
    METHODS:
      choose       REDEFINITION.
ENDCLASS.                    "lcl_server_file DEFINITIO
*       CLASS lcl_server_file IMPLEMENTATION
CLASS lcl_server_file IMPLEMENTATION.
METHOD choose.
    DATA:
      l_i_path     TYPE dxfields-longpath,
      l_o_path     TYPE dxfields-longpath.
    CALL FUNCTION 'F4_DXFILENAME_TOPRECURSION'
      EXPORTING
        i_location_flag = 'A' " Application server
        i_path          = l_i_path
        fileoperation   = 'R' " Lectura
      IMPORTING
        o_path          = l_o_path
      EXCEPTIONS
        rfc_error       = 1
        OTHERS          = 2.
    IF sy-subrc = 0 AND l_o_path <> l_i_path.
      MOVE l_o_path TO _v_path.
    ELSE.
      RAISE no_path_chosen.
    ENDIF.
ENDMETHOD.                    "choose
METHOD get_contents.
    DATA: l_line   LIKE LINE OF rt_contents,
          l_osmsg TYPE string.
    CHECK NOT _v_path IS INITIAL.
    OPEN DATASET _v_path FOR INPUT
                             IN TEXT MODE
                             MESSAGE l_osmsg.
    IF sy-subrc <> 0.
      MESSAGE e000(oo) WITH l_osmsg
                       RAISING read_error.
    ELSE.
      DO.
        READ DATASET _v_path INTO l_line.
        IF sy-subrc = 0.
          APPEND l_line TO rt_contents.
        ELSE.
          EXIT.
        ENDIF.
      ENDDO.
      CLOSE DATASET _v_path.
    ENDIF.
ENDMETHOD.                    "get_contents
ENDCLASS.                    "lcl_server_file IMPLEMENTATION
*       CLASS lcl_pc_file DEFINITION
CLASS lcl_pc_file DEFINITION
                   INHERITING FROM lcl_file.
PUBLIC SECTION.
    METHODS:
      get_contents REDEFINITION.
PROTECTED SECTION.
    METHODS:
      choose       REDEFINITION.
ENDCLASS.                    "lcl_pc_file DEFINITIO
*       CLASS lcl_pc_file IMPLEMENTATION
CLASS lcl_pc_file IMPLEMENTATION.
METHOD choose.
    DATA:
      l_i_path     TYPE dxfields-longpath VALUE 'C:\',
      l_o_path     TYPE dxfields-longpath.
    CALL FUNCTION 'F4_DXFILENAME_TOPRECURSION'
      EXPORTING
        i_location_flag = 'P' " PC
        i_path          = l_i_path
        fileoperation   = 'R' " Lectura
      IMPORTING
        o_path          = l_o_path
      EXCEPTIONS
        rfc_error       = 1
        OTHERS          = 2.
    IF sy-subrc = 0 AND l_o_path <> l_i_path.
      MOVE l_o_path TO _v_path.
    ELSE.
      RAISE no_path_chosen.
    ENDIF.
ENDMETHOD.                    "choose
METHOD get_contents.
    CHECK NOT _v_path IS INITIAL.
    CALL METHOD cl_gui_frontend_services=>gui_upload
      EXPORTING
        filename                = _v_path
      CHANGING
        data_tab                = rt_contents
      EXCEPTIONS
        file_open_error         = 1
        file_read_error         = 2
        no_batch                = 3
        gui_refuse_filetransfer = 4
        invalid_type            = 5
        no_authority            = 6
        unknown_error           = 7
        bad_data_format         = 8
        header_not_allowed      = 9
        separator_not_allowed   = 10
        header_too_long         = 11
        unknown_dp_error        = 12
        access_denied           = 13
        dp_out_of_memory        = 14
        disk_full               = 15
        dp_timeout              = 16
        OTHERS                  = 17.
    IF sy-subrc <> 0.
      RAISE read_error.
    ENDIF.
ENDMETHOD.                    "get_contents
ENDCLASS.                    "lcl_pc_file IMPLEMENTATION
* Data
DATA: gr_file          TYPE REF TO lcl_file.
* Main Program
START-OF-SELECTION.
*   Get text lines from file
    IF p_srv = abap_true.
      CREATE OBJECT gr_file
        TYPE
          lcl_server_file
        EXCEPTIONS
          no_path_chosen = 1.
    ELSE.
      CREATE OBJECT gr_file
        TYPE
          lcl_pc_file
        EXCEPTIONS
          no_path_chosen = 1.
    ENDIF.
On a 4.6c system this code gave me a dump, while on my NW7.0 SP it doesn't even activate with the following error:
You cannot call abstract methods in the "CONSTRUCTOR" method.
- Following some suggestions from Java forums i've tried to define the constructor in the base class as PROTECTED or PRIVATE instead, then calling super->constructor from the subclasses, but I get this error in german:
Sichtbarkeit des Konstruktors darf nicht spezieller als die Sichtbarkeit der Instanzerzeugung (CREATE-Zuzatz) sein.
which Altavista translates like:
Visibility of the constructor may not be more special than the
visibility of the instance production (CREATE Zuzatz).
- I've also thought of defining the CHOOSE method as a class (not instance) one, then calling it before creating the file object which maybe solves the problem, but I see that approach more "procedural oriented" which i'm trying to avoid.
- Of course I could define a constructor for each subclass, but both would have exactly the same code.
I'm really lost on how should I code this. My main focus is on avoiding code dupplication.
I hope someone with more OO experience can see what I'm trying to do and sheds some light.
Many thanks for reading all this!

Dear Alejandro,
    When i saw your code, you are trying to access an astract method CHOOSE(which is actually implemented in sub class) from the constructor of the base class which is not possible. By this time, we don't know which sub class it is refering to, so it gives an error.   I see two solutions for this..
1. To define constructor in sub class and call the choose method from the consturctor of the sub class(which in this case is reputation of the same again for each sub class)
2. Remove the calling of choose method from the constructor of the main class and call it separately(after creating the object).   By now we know which sub class we are refering to.   I would have designed the program in the following way.
*       CLASS lcl_file DEFINITION
CLASS lcl_file DEFINITION ABSTRACT.
PUBLIC SECTION.
    METHODS:
      constructor
        IMPORTING
          i_path TYPE string OPTIONAL
        EXCEPTIONS
          no_path_chosen,
      get_contents ABSTRACT
        RETURNING
          value(rt_contents) TYPE string_table
        EXCEPTIONS
          read_errorm,
      set_path ABSTRACT
        EXCEPTIONS
          no_path_chosen.
PROTECTED SECTION.
    DATA:
      _v_path        TYPE string.
*    METHODS:
*      choose ABSTRACT
*        EXCEPTIONS
*          no_path_chosen,
*      set_path ABSTRACT
*        IMPORTING
*          i_path TYPE string.
ENDCLASS.                    "lcl_file DEFINITION
*       CLASS lcl_file IMPLEMENTATION
CLASS lcl_file IMPLEMENTATION.
METHOD constructor.
    IF i_path IS SUPPLIED.
      _v_path = i_path.
*      CALL METHOD set_path
*        EXPORTING
*          i_path = i_path.
*    ELSE.
**---->>>> PROBLEM CALL - CAN'T BE DONE!!
*      CALL METHOD choose
*        EXCEPTIONS
*          no_path_chosen = 1.
*      IF sy-subrc = 1.
*        RAISE no_path_chosen.
*      ENDIF.
    ENDIF.
ENDMETHOD.                    "constructor
* METHOD set_path.
*    _v_path = i_path.
* ENDMETHOD.                    "set_path
ENDCLASS.                    "lcl_file IMPLEMENTATION
*       CLASS lcl_server_file DEFINITION
CLASS lcl_server_file DEFINITION
                      INHERITING FROM lcl_file.
PUBLIC SECTION.
    METHODS:
      get_contents REDEFINITION,
      set_path     REDEFINITION.
* PROTECTED SECTION.
*    METHODS:
*      choose       REDEFINITION.
ENDCLASS.                    "lcl_server_file DEFINITIO
*       CLASS lcl_server_file IMPLEMENTATION
CLASS lcl_server_file IMPLEMENTATION.
METHOD set_path.
    DATA:
      l_i_path     TYPE dxfields-longpath,
      l_o_path     TYPE dxfields-longpath.
    CHECK _v_path IS INITIAL.
    CALL FUNCTION 'F4_DXFILENAME_TOPRECURSION'
      EXPORTING
        i_location_flag = 'A' " Application server
        i_path          = l_i_path
        fileoperation   = 'R' " Lectura
      IMPORTING
        o_path          = l_o_path
      EXCEPTIONS
        rfc_error       = 1
        OTHERS          = 2.
    IF sy-subrc = 0 AND l_o_path = l_i_path.
      MOVE l_o_path TO _v_path.
    ELSE.
      RAISE no_path_chosen.
    ENDIF.
ENDMETHOD.                    "set_path
METHOD get_contents.
    DATA: l_line   LIKE LINE OF rt_contents,
          l_osmsg TYPE string.
    CHECK NOT _v_path IS INITIAL.
*    OPEN DATASET _v_path FOR INPUT
*                             IN TEXT MODE
*                             MESSAGE l_osmsg.
    IF sy-subrc = 0.
*      MESSAGE e000(oo) WITH l_osmsg
*                       RAISING read_error.
    ELSE.
      DO.
        READ DATASET _v_path INTO l_line.
        IF sy-subrc = 0.
          APPEND l_line TO rt_contents.
        ELSE.
          EXIT.
        ENDIF.
      ENDDO.
      CLOSE DATASET _v_path.
    ENDIF.
ENDMETHOD.                    "get_contents
ENDCLASS.                    "lcl_server_file IMPLEMENTATION
*       CLASS lcl_pc_file DEFINITION
CLASS lcl_pc_file DEFINITION
                   INHERITING FROM lcl_file.
PUBLIC SECTION.
    METHODS:
      get_contents REDEFINITION,
      set_path     REDEFINITION.
* PROTECTED SECTION.
*    METHODS:
*      choose       REDEFINITION.
ENDCLASS.                    "lcl_pc_file DEFINITIO
*       CLASS lcl_pc_file IMPLEMENTATION
CLASS lcl_pc_file IMPLEMENTATION.
METHOD set_path.
    DATA:
      l_i_path     TYPE dxfields-longpath VALUE 'C:\',
      l_o_path     TYPE dxfields-longpath.
    CHECK _v_path IS INITIAL.
    CALL FUNCTION 'F4_DXFILENAME_TOPRECURSION'
      EXPORTING
        i_location_flag = 'P' " PC
        i_path          = l_i_path
        fileoperation   = 'R' " Lectura
      IMPORTING
        o_path          = l_o_path
      EXCEPTIONS
        rfc_error       = 1
        OTHERS          = 2.
    IF sy-subrc = 0 AND l_o_path = l_i_path.
      MOVE l_o_path TO _v_path.
    ELSE.
      RAISE no_path_chosen.
    ENDIF.
ENDMETHOD.                    "set_path
METHOD get_contents.
    CHECK NOT _v_path IS INITIAL.
    CALL METHOD cl_gui_frontend_services=>gui_upload
      EXPORTING
        filename                = _v_path
      CHANGING
        data_tab                = rt_contents
      EXCEPTIONS
        file_open_error         = 1
        file_read_error         = 2
        no_batch                = 3
        gui_refuse_filetransfer = 4
        invalid_type            = 5
        no_authority            = 6
        unknown_error           = 7
        bad_data_format         = 8
        header_not_allowed      = 9
        separator_not_allowed   = 10
        header_too_long         = 11
        unknown_dp_error        = 12
        access_denied           = 13
        dp_out_of_memory        = 14
        disk_full               = 15
        dp_timeout              = 16
        OTHERS                  = 17.
    IF sy-subrc = 0.
*      RAISE read_error.
    ENDIF.
ENDMETHOD.                    "get_contents
ENDCLASS.                    "lcl_pc_file IMPLEMENTATION
* Data
DATA: gr_file          TYPE REF TO lcl_file.
* Main Program
START-OF-SELECTION.
*   Get text lines from file
IF abap_true = abap_true.
    CREATE OBJECT gr_file
      TYPE
        lcl_server_file
      EXCEPTIONS
        no_path_chosen = 1.
ELSE.
    CREATE OBJECT gr_file
      TYPE
        lcl_pc_file
      EXCEPTIONS
        no_path_chosen = 1.
ENDIF.
gr_file->set_path( ).
Regards
Kesava
Edited by: Kesava Chandra Rao on Mar 19, 2008 11:44 AM

Is not abstract and does not override abstract method actionPerformed

I dont how to corr. Please help!! and thank you very much!!
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class test extends JFrame implements ActionListener, ItemListener
          private CenterPanel centerPanel;
          private QuestionPanel questionPanel;
          private ButtonPanel buttonPanel;
          private ResponsePanel responsePanel;
          private JButton b1,b2,b3,b4,b5;               //Create five references to Jbutton instances
     private JTextField t1,t2,t3,t4,t5;          //Create five references to JTextField instances
          private JLabel label1;                    //Create one references to JLabel instances
          private JRadioButton q1,q2,q3;               //Create three references to JRadioButton instances
          private ButtonGroup radioGroup;               //Create one references to Button Group instances
          private int que1[] = new int[5];           //Create int[4] Array
          private int que2[] = new int[5];
          private int que3[] = new int[5];
          private String temp1, temp2, temp3, temp4, temp5, temp6, temp7, temp8, temp9, temp10,
                    temp11, temp12, temp13, temp14, temp15;
public test (String header)
          super(header);
          Container container = getContentPane();
          label1 = new JLabel ("PLease click on your response to ");
          q1 = new JRadioButton("I understand most of the content of this subject",true);
          add(q1);
          q2 = new JRadioButton("I see the relevance of the subject to my degree",false);
          add(q2);
          q3 = new JRadioButton("The workload in this subject is appropriate",false);
          add(q3);
          radioGroup = new ButtonGroup();               //JRadioButton belong to ButtonGroup
          radioGroup.add(q1);
          radioGroup.add(q2);
          radioGroup.add(q3);
          JPanel buttonPanel = new JPanel();
          JPanel responsePanel = new JPanel();
          JPanel questionPanel = new JPanel();
          JPanel centerPanel = new JPanel();
          b1 = new JButton ("Strongly DISAGREE");          //Instantiate JButton with text
          b1.addActionListener (this);               //Register JButtons to receive events
          b2 = new JButton ("DISAGREE");
          b2.addActionListener (this);
          b3 = new JButton ("Neither AGREE or DISAGREE");
          b3.addActionListener (this);
          b4 = new JButton ("AGREE");
          b4.addActionListener (this);
          b5 = new JButton ("Strongly AGREE");
          b5.addActionListener (this);
          buttonPanel.setLayout(new GridLayout(5,1));
          buttonPanel.add(b1);
          buttonPanel.add(b2);
          buttonPanel.add(b3);
          buttonPanel.add(b4);
          buttonPanel.add(b5);
          t1 = new JTextField ("0",3);               //JTextField contains empty string
          t2 = new JTextField ("0",3);
          t3 = new JTextField ("0",3);
          t4 = new JTextField ("0",3);
          t5 = new JTextField ("0",3);
          t1.setEditable( false );
          t2.setEditable( false );
          t3.setEditable( false );
          t4.setEditable( false );
          t5.setEditable( false );
          responsePanel.setLayout(new GridLayout(5,1));
          responsePanel.add(t1);
          responsePanel.add(t2);
          responsePanel.add(t3);
          responsePanel.add(t4);
          responsePanel.add(t5);
          questionPanel.setLayout(new GridLayout(4,1));
          questionPanel.add(label1);
          questionPanel.add(q1);
          questionPanel.add(q2);
          questionPanel.add(q3);
          centerPanel.add(buttonPanel,BorderLayout.CENTER);
          centerPanel.add(responsePanel,BorderLayout.EAST);
          container.add(centerPanel,BorderLayout.WEST);
          container.add(questionPanel,BorderLayout.NORTH);
          q1.addActionListener(
               new ActionListener(){
          public void actionPerformed( ActionEvent e )
{                                        //actionPerformed of all registered listeners
          if (e.getSource() == b1) {
               que1[0] = Integer.parseInt(t1.getText()) + 1;
               String temp1 = String.valueOf(que1[0]);
          t1.setText(temp1);
          else if (e.getSource() == b2)     {
               que1[1] = Integer.parseInt(t2.getText()) + 1;
               String temp2 = String.valueOf(que1[1]);
          t2.setText(temp2);
          else if (e.getSource() == b3)     {
               que1[2] = Integer.parseInt(t3.getText()) + 1;
               String temp3 = String.valueOf(que1[2]);
          t3.setText(temp3);
          else if (e.getSource() == b4)     {
               que1[3] = Integer.parseInt(t4.getText()) + 1;
               String temp4 = String.valueOf(que1[3]);
          t4.setText(temp4);
          else if (e.getSource() == b5)     {
               que1[4] = Integer.parseInt(t5.getText()) + 1;
               String temp5 = String.valueOf(que1[4]);
          t5.setText(temp5);
} //end action performed
          q2.addActionListener(
               new ActionListener(){
          public void actionPerformed( ActionEvent e )
{                                        //actionPerformed of all registered listeners
          if (e.getSource() == b1) {
               que2[0] = Integer.parseInt(t1.getText()) + 1;
               String temp6 = String.valueOf(que2[0]);
          t1.setText(temp1);
          else if (e.getSource() == b2)     {
               que2[1] = Integer.parseInt(t2.getText()) + 1;
               String temp7 = String.valueOf(que2[1]);
          t2.setText(temp7);
          else if (e.getSource() == b3)     {
               que2[2] = Integer.parseInt(t3.getText()) + 1;
               String temp8 = String.valueOf(que2[2]);
          t3.setText(temp8);
          else if (e.getSource() == b4)     {
               que2[3] = Integer.parseInt(t4.getText()) + 1;
               String temp9 = String.valueOf(que2[3]);
          t4.setText(temp9);
          else if (e.getSource() == b5)     {
               que2[4] = Integer.parseInt(t5.getText()) + 1;
               String temp10 = String.valueOf(que2[4]);
          t5.setText(temp10);
} //end action performed
          q3.addActionListener(
               new ActionListener(){
          public void actionPerformed( ActionEvent e )
{                                        //actionPerformed of all registered listeners
          if (e.getSource() == b1) {
               que3[0] = Integer.parseInt(t1.getText()) + 1;
               String temp11 = String.valueOf(que3[0]);
          t1.setText(temp11);
          else if (e.getSource() == b2)     {
               que3[1] = Integer.parseInt(t2.getText()) + 1;
               String temp12 = String.valueOf(que3[1]);
          t2.setText(temp12);
          else if (e.getSource() == b3)     {
               que3[2] = Integer.parseInt(t3.getText()) + 1;
               String temp13 = String.valueOf(que3[2]);
          t3.setText(temp13);
          else if (e.getSource() == b4)     {
               que3[3] = Integer.parseInt(t4.getText()) + 1;
               String temp14 = String.valueOf(que3[3]);
          t4.setText(temp14);
          else if (e.getSource() == b5)     {
               que3[4] = Integer.parseInt(t5.getText()) + 1;
               String temp15 = String.valueOf(que3[4]);
          t5.setText(temp15);
} //end action performed
}//end constructor test
public void itemStateChanged(ItemEvent item) {
//int state = item.getStateChange();
//if (q1 == item.SELECTED)
          public class ButtonPanel extends JPanel
               public ButtonPanel()
          public class CenterPanel extends JPanel
               public CenterPanel()
          public class QuestionPanel extends JPanel
               public QuestionPanel()
          public class ResponsePanel extends JPanel
               public ResponsePanel()
public static void main(String [] args)
     test surveyFrame = new test("Student Survey") ;
     surveyFrame.setSize( 500,300 );
     surveyFrame.setVisible(true);
     surveyFrame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
     }//end main
}//end class test

is not abstract and does not override abstract method actionPerformed
Oh, I see that the title of your post is an error message? Ok. Well, the test class is declared as implementing an ActionListener. That means the test class must have an actionPerformed() method. Your test class apparently does not.
It does not appear that the test class needs to implement ActionListener. You are using annonymous classes as listeners.

Is not abstract and does not override abstract method tablechanged

I will remove all the gui code to make it shorter, but my problem lies with my InteractiveTableModelListener.
public class Meet extends JPanel{
private static void createAndShowGUI() {
        JFrame frame = new JFrame("MEET_dataTable");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new Meet(), BorderLayout.CENTER);
        frame.pack();
        frame.setVisible(true);
public class InteractiveTableModelListener implements TableModelListener {
     public void TableChanged(TableModelEvent evt) {
if (evt.getType() == TableModelEvent.UPDATE) {
      int column = evt.getColumn();
      int row = evt.getFirstRow();
      dataTable.setColumnSelectionInterval(column + 1, column + 1);
      dataTable.setRowSelectionInterval(row, row);
class InteractiveRenderer extends DefaultTableCellRenderer {
protected int interactiveColumn;
public InteractiveRenderer(int interactiveColumn) {
      this.interactiveColumn = interactiveColumn;
public Component getTableCellRendererComponent(JTable dataTable,
     Object value, boolean isSelected, boolean hasFocus, int row,
     int column)
Component c = super.getTableCellRendererComponent(dataTable, value, isSelected, hasFocus, row, column);
   if (column == interactiveColumn && hasFocus) {
     if ((Meet.this.tableModel.getRowCount() - 1) == row &&
        !Meet.this.tableModel.hasEmptyRow())
         Meet.this.tableModel.addEmptyRow();
    highlightLastRow(row);
return c;
public void highlightLastRow(int row) {
     int lastrow = tableModel.getRowCount();
if (row == lastrow - 1) {
      dataTable.setRowSelectionInterval(lastrow - 1, lastrow - 1);
else {
      dataTable.setRowSelectionInterval(row + 1, row + 1);
     dataTable.setColumnSelectionInterval(0, 0);
public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
            UIManager.put("swing.boldMetal", Boolean.FALSE);
            createAndShowGUI();
}As i say, i have removed all the gui code to make it shorter, but in this code i create the table and add all the model to it. I am being returned with the error
Meet.InteractiveTableModelListener is not abstract and does not override abstract method tableChanged(javax.swing.event.TableModelEvent)in javax.swing.event.TableModelListener
what would be causing this error?
Cheers

Sorry, just figured out my silly error, the method is tableChanged not TableChanged.
cheers
TOPIC CLOSED
Edited by: nick2price on Sep 11, 2008 7:08 AM

Is not abstract and does not override abstract method ERROR

Hello. I'm new at all this, and am attempting to recreate a sample code out of my book (Teach Yourself XML in 24 Hours), and I keep getting an error. I appriciate any help.
This is the Error that I get:
DocumentPrinter is not abstract and does not override abstract method skippedEntity(java.lang.String) in org.xml.sax.ContentHandler
public class DocumentPrinter implements ContentHandler, ErrorHandler
        ^This is the sourcecode:
import org.xml.sax.Attributes;
import org.xml.sax.ContentHandler;
import org.xml.sax.ErrorHandler;
import org.xml.sax.Locator;
import org.xml.sax.SAXParseException;
import org.xml.sax.XMLReader;
public class DocumentPrinter implements ContentHandler, ErrorHandler
// A Constant containing the name of the SAX parser to use.
private static final String PARSER_NAME = "org.apache.xerces.parsers.SAXParser";
public static void main(String[] args)
   // Check to see whether the user supplied any command line arguments. If not, print an error and exit.
   if (args.length == 0)
     System.out.println("No XML document path specified.");
     System.exit(1);
   // Create a new instance of the DocumentPrinter class.
   DocumentPrinter dp = new DocumentPrinter();
   try
     // Create a new instance of the XML Parser.
     XMLReader parser = (XMLReader)Class.forName(PARSER_NAME).newInstance();
     // Set the parser's content handler
    // parser.setContentHandler(dp);
     // Set the parsers error handler
     parser.setErrorHandler(dp);
     // Parse the file named in the argument
     parser.parse(args[0]);
   catch (Exception ex)
     System.out.println(ex.getMessage());
     ex.printStackTrace();
public void characters(char[] ch, int start, int length)
   String chars ="";
   for (int i = start; i < start + length; i++)
     chars = chars + ch;
System.out.println("Recieved characters: " + chars);
public void startDocument()
System.out.println("Start Document.");
public void endDocument()
System.out.println("End of Document.");
public void startElement(String namespaceURI, String localName, String qName, Attributes atts)
System.out.println("Start element: " + localName);
for (int i = 0; i < atts.getLength(); i++)
System.out.println(" Attribute: " + atts.getLocalName(i));
System.out.println(" Value: " + atts.getValue(i));
public void endElement(String namespaceURI, String localName, String qName)
System.out.println("End of element: " + localName);
public void startPrefixMapping(String prefix, String uri)
System.out.println("Prefix mapping: " + prefix);
System.out.println("URI: " + uri);
public void endPrefixMapping(String prefix)
System.out.println("End of prefix mapping: " + prefix);
public void ignorableWhitespace(char[] ch, int start, int length)
System.out.println("Recieved whitespace.");
public void processingInstruction(String target, String data)
System.out.println("Recieved processing instruction:");
System.out.println("Target: " + target);
System.out.println("Data: " + data);
public void setDocumentLocation(Locator locator)
// Nada
public void error(SAXParseException exception)
System.out.println("Parsing error on line " + exception.getLineNumber());
public void fatalError(SAXParseException exception)
System.out.println("Fatal parsing error on line " + exception.getLineNumber());
public void warning(SAXParseException exception)
System.out.println("Warning on line " + exception.getLineNumber());

Check to make sure that the arguments are consistent with your ContentHandler class. Probably the wrong type.
I think you forgot to include the skippedEntity method, it seems to be missing. Even if an implemented class has a method that you are not using, you still have to include the method in your code even if it doesn't do anything.
Message was edited by:
ChargersTule1

...is not abstract and does not override abstract method compare

Why am I getting the above compile error when I am very clearly overriding abstract method compare (ditto abstract method compareTo)? Here is my code -- which was presented 1.5 code and I'm trying to retrofit to 1.4 -- followed by the complete compile time error. Thanks in advance for your help (even though I'm sure this is an easy question for you experts):
import java.util.*;
   This program sorts a set of item by comparing
   their descriptions.
public class TreeSetTest
   public static void main(String[] args)
      SortedSet parts = new TreeSet();
      parts.add(new Item("Toaster", 1234));
      parts.add(new Item("Widget", 4562));
      parts.add(new Item("Modem", 9912));
      System.out.println(parts);
      SortedSet sortByDescription = new TreeSet(new
         Comparator()
            public int compare(Item a, Item b)   // LINE CAUSING THE ERROR
               String descrA = a.getDescription();
               String descrB = b.getDescription();
               return descrA.compareTo(descrB);
      sortByDescription.addAll(parts);
      System.out.println(sortByDescription);
   An item with a description and a part number.
class Item implements Comparable
      Constructs an item.
      @param aDescription the item's description
      @param aPartNumber the item's part number
   public Item(String aDescription, int aPartNumber)
      description = aDescription;
      partNumber = aPartNumber;
      Gets the description of this item.
      @return the description
   public String getDescription()
      return description;
   public String toString()
      return "[descripion=" + description
         + ", partNumber=" + partNumber + "]";
   public boolean equals(Object otherObject)
      if (this == otherObject) return true;
      if (otherObject == null) return false;
      if (getClass() != otherObject.getClass()) return false;
      Item other = (Item) otherObject;
      return description.equals(other.description)
         && partNumber == other.partNumber;
   public int hashCode()
      return 13 * description.hashCode() + 17 * partNumber;
   public int compareTo(Item other)   // OTHER LINE CAUSING THE ERROR
      return partNumber - other.partNumber;
   private String description;
   private int partNumber;
}Compiler error:
TreeSetTest.java:25: <anonymous TreeSetTest$1> is not abstract and does not over
ride abstract method compare(java.lang.Object,java.lang.Object) in java.util.Com
parator
            public int compare(Item a, Item b)
                       ^
TreeSetTest.java:41: Item is not abstract and does not override abstract method
compareTo(java.lang.Object) in java.lang.Comparable
class Item implements Comparable
^
2 errors

According to the book I'm reading, if you merely take
out the generic from the code, it should compile and
run in v1.4 (assuming, of course, that the class
exists in 1.4). I don't know what book you are reading but that's certainly incorrect or incomplete at least. I've manually retrofitted code to 1.4, and you'll be inserting casts as well as replacing type references with Object (or the erased type, to be more precise).
These interfaces do exist in 1.4, and
without the generics.Exactly. Which means compareTo takes an Object, and you should change your overriding method accordingly.
But this raises a new question: how does my 1.4
compiler know anything about generics? It doesn't and it can't. As the compiler is telling you, those interfaces expect Object. Think about it, you want to implement one interface which declares a method argument type of Object, in several classes, each with a different type. Obviously all of those are not valid overrides.

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