Extending a class, its constructor inherited ?

class a extends JFrame
xxxxxxxx
public void a()
xxxxxxxxxx
class someclass
a myA = new a();
================
there is a constructor -> JFrame(String title) in JFrame.
when i create an instance of this class a this way which extends JFrame, do i overwrite the constructor JFrame(String title) ? what if i want to have a constructor that i can use to create the instance of a this way ->
a myA = new a("my title")
how should i do it? and as for a good java programmer, what is the best and most efficient way ?
thank you

hi,
class a extends JFrame
xxxxxxxx
public void a()
super();
public void a(String title)
super(title);
class someclass
a myA = new a("My Title");
Phil

Similar Messages

When extending class do constructors get implemented as well

Say i need to call a list as an object and it was an extension of another list. Would the constructor be the same but with the extended list name??
For example:
First class:
public class ListR
        //variables
     public ListModelR()
          trackCount = 0;
          firstTrack = null;
          lastTrack = null;
     }///rest of code...
Would the class that extended this hav a constructor of its class name or would i have to reference ListModelR?
Thanks James

You cant have a constructor that is different from your classes name.
class A
     public A()
          int a = 5;
class B
     public B()
          super(); //executes the super class's default constructor
}

Extending classes and constructors

Hi,
I am extending a class like this:
public class MyTreeCellRenderer extends CheckableTreeCellRenderer
public MyTreeCellRenderer()
The super class has a constructor:
public class CheckableTreeCellRenderer extends DefaultTreeCellRenderer
public CheckableTreeCellRenderer()
checkStyle = CHECKMARK;
But I cannot instantiate my newly created class. What is the reason for this? Something wrong with the constructors?
ERROR MESSAGE:
symbol : constructor MyTreeCellRenderer ()
location: class MyTreeCellRenderer
MyTreeCellRenderer renderer = new MyTreeCellRenderer();

Didn't you define the constructor of MyTreeCellRendered with some arguments?

Does extending a class require a hand-entered constructor in superclass?

Hello,
Trying to extend a class (which doesn't have a main() method, if this matters), I got the following compiler message
NewInput.java:1: cannot resolve symbol
symbol : constructor InputFileDeclared ()
location: class InputFileDeclared
public class NewInput extends InputFileDeclared
After I hand-entered a blank constructor to the superclass the subclass have compiled.
Doesn't a class have a constructor by default?

If your class has a contructor which takes arguments, then the compiler does not provide you with a no-arg constructor. In this case if you need to have a constructor that takes no arguments then you need to write one.
If your base class had a constructor like
public class InputFileRead {
//Constructor
public InputFileRead( String fileName ) {
//Methods
and you wrote your Derived class with no constructor as
public class NewInput extends InputFileRead {
//No constructors
//Methods
Now when you write
NewInput nInp = new NewInput();
since you have not written any constructor for your new class the compiler will provide you with a default no-args constructor which will call the default constructor of your base class. Since there is no default constructor for your base class, you won't be able to instantiate the derived class.
You need to write a no-arg constructor for the new class which will then call the appropriate constructor of the base class.
public class NewInput extends InputFileRead {
//Constructor
public NewInput() {
super( "InputFile.txt" );
//Methods
}

Constructor Inheritance Question

Here's a quote from the Java Tutorials at http://java.sun.com/docs/books/tutorial/java/javaOO/objectcreation.html :
"All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent's no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor (Object does have one), the compiler will reject the program."
In order to fully understand this concept, I created two classes: a ClassParent and a ClassChild.
public class ClassParent
    public static void main(String[] args) {
       ClassParent tester = new ClassParent();
public class ClassChild extends ClassParent
    public static void main(String[] args) {
        ClassChild child = new ClassChild();
}Both classes compiled successfully, which raised the following question:
I understand that the ClassParent default constructor calls the Object's no-argument constructor.
Does the ClassChild also call the Object's constructor once it realizes that the ClassParent does not have a no-argument constructor?
And a somewhat non-related question:
Seeing how ClassParent calls Object's no-argument constructor if it does not have one of its own, does it also extend the Object class?
Edit: After running the following code, I realized that the answer to my last question is yes:
public class ClassParent
    public static void main(String[] args) {
       ClassParent tester = new ClassParent();
       boolean test = tester instanceof Object;
       System.out.println(test);
}Edited by: youmefriend722 on May 26, 2008 1:54 PM

youmefriend722 wrote:
I think I'm getting a basic grasp now but want to make sure that I'm not misunderstanding anything.
Constructor inheritance:
If a no-argument constructor is invoked but one isn't declared in that class, the superclass's no-argument constructor will be invoked. Well, sort of. If you invoke a constructor that doesn't exist, you get an error. Keep in mind that the invocation and the constructor may both be automatically supplied by the compiler, and that the compiler won't automatically create a no-arg constructor for a class if you define any constructors for that class.
So if you don't define any constructors in a class, then a no-arg one is created automatically (at compile time) and (at runtime) when you instantiate that class, that no-arg constructor will try to invoke the superclass's no-arg constructor.
But suppose you do define a constructor, one that takes an argument. Then if you try to invoke a no-arg constructor on that class, you'll get an error, and the superclass's no-arg constructor won't be invoked (because the error happens first).
If the superclass does not have a constructor, then the superclass's superclass's constructor will be invoked.No. That never happens. Every class has a constructor (although it might have permissions which make it inaccessible, which is a whole other issue we'll worry about later).
If the superclass does have a constructor but doesn't have a no-argument constructor, then there will be a compile-time error.Only if you try to invoke the no-arg constructor, which might happen implicitly. For example, if you write a superclass with a 2-arg constructor, and you write a subclass with a 1-arg constructor, and the 1-arg subclass's constructor invokes the superclass's 2-arg constructor, then there's no problem, even though in this example, the superclass doesn't have a no-arg constructor.
Constructors in general:
In every constructor, the superclass's no-argument constructor will be invoked if the none of the superclass's constructors are explicitly invoked.Yeah, I think that's right.

Extending a class (newbie)

I'm very new to Java. I'm making use of custom web classes from a proprietary application.
I want to write my own class by extending another class. My class need to do exactly what the super class does. The super class extends another class.
Here is how it looks,
MY CLASS:
public RmCopyCommand extends DwCopyCommand
THE SUPER CLASS:
public DwCopyCommand extends DwDocbaseCommand
The super class, DwCopyCommand, has four methods defined (plus many inherited). To make my class, RmCopyCommand, mimic the super class, I did the following:
1. I made a constructor
RmCopyCommand()
super();
2. I duplicated the methods
public boolean hasTransaction()
return super.hasTransaction();
I did the above to all four methods defined in the super class. I also used TRY...CATCH blocks where necessary. The source compiled.
Off course, this doesn't work. I got a nullPointerException error. What I want to do is use the exact functionality of DwCommand, but write another method that makes a call to an inherited method.
Could anyone tell me what I might be doing wrong?
I hope I included enough details. If you need further details, let me know.
Thanks,
Moshe

Lets go through this point by point:
I'm very new to Java. I'm making use of custom web
classes from a proprietary application.
I want to write my own class by extending another
class. My class need to do exactly what the super
class does. The super class extends another class.Does you class really need to do exactly what the super class does? If the super class has everything you need why are you subclassing it.
Here is how it looks,
MY CLASS:
public RmCopyCommand extends DwCopyCommandThe above statement is all you need followed by {} according to your description.
THE SUPER CLASS:
public DwCopyCommand extends DwDocbaseCommand
The super class, DwCopyCommand, has four methods
defined (plus many inherited). To make my class,
RmCopyCommand, mimic the super class, I did the
following:
1. I made a constructor
RmCopyCommand()
super();
}You only need to do this for constructors with arguments. If you don't create a default constructor, one is created for you. Also if you don't call super() in your constructor it will automatically before anything else is executed.
2. I duplicated the methods
public boolean hasTransaction()
return super.hasTransaction();
}This is unneccesary. By subclassing a class all its public and protected members are part of the sub class.
I did the above to all four methods defined in the
super class. I also used TRY...CATCH blocks where
necessary. The source compiled.
Off course, this doesn't work. I got a
nullPointerException error. What I want to do is use
the exact functionality of DwCommand, but write
another method that makes a call to an inherited
method.Nothing that you have stated here would cause that.
Could anyone tell me what I might be doing wrong?
I hope I included enough details. If you need further
details, let me know.
Thanks,
Moshe

Why do we intialize other classes in constructor ????

Please see the question below:
class MThread extends Thread {
private Abc mfact = null;
private Xyz mo = null;
private Pro sts = null;
* Constructor.
public MThread(Abc mfact, Xyz mo ) { *// do i have to pass Pro here from calling method so that i can initialize in constructor in order to use its method like record ???*
this.mfact = mfact;
this.mo = mo;
public somemethod() {
sts.record();
public void somemethod1() {
mo.create();
***************************************************Another Class******************************************************
public class Xyz{
// Some code here
// this class calls Mthread and creates 10 such Threads sth like
for (int i = 0; i < 10; i++) {
mThreads[i] = new MThread(mfact, mo;
***********Another Class ****************
public class Pro {
// some code
public synchronized record() {
rat ++;
Explanation :
Xyz calls Mthread 10 times to create such threads.
Mthread has method that class Pro for stats recording
Mthread also uses other class files to do some functioning of its own.
Question:
Please see the question in the constructor of Mthread ??
for using method like record in Pro. do i have to initialize that in the constructor of Mthread ??
because other class like Abc initializes itself in the construtor??
Why do we have to initialize some class in constructor ?? I don't get it ?? Also when i press F3 (Open declaration ) on record call in somemethod ()
.. it properly opens the file of class Pro and points to my sychronized method in that class ??
CONFUSED... Please help ..
Thank you

javanewbie83 wrote:
To morgair:
thats right..so actually my question was do i need to change the requiremensts of the constructor so i can pass in the value of class Pro from the calling method.
Do i need to do that. is that compulsory to use one of its methods
ThanksYes, somehow you have to get visibility to your object that you want to use. If it's not passed in, then you have to have it in some global context with reference to where you want to use it.
class myClass{
A a;
B b;
C c;
myClass(A a, B b, C c){
    this.a = a;
    this.b = b;
    this.c = c;
public void myMethod(){
    c.doSomething(a, b);
// This can be done since a and b are supplied as arguments to the constructor
// but also consider this:
class myClass{
A a;
B b;
C c;
myClass(A a, B b){
    this.a = a;
    this.b = b;
    c = new C();
public void myMethod(){
    c.doSomething(a, b);
//this will also work but c is a local variable
//also consider this:
class myOuterClass{
A a;
B b;
C c;
myOuterClass(){
    a = new A();
    b = new B();
    c = new C();
    //do stuff
public void someStuff(){
    myInnerClass p = new myInnerClass(a, b);
    p.myMethod(); //this will also work since c is defined in myOuterClass
class myInnerClass{
    A a;
    B b;
    myInnerClass(A a, B b){
      this.a = a;
      this.b = b;
    public void myMethod(){
      c.doSomething(a, b);
}Note: this example is supplied off the top of my head without being tried in my IDE, but should show the concepts even if my fat fingers have hit the wrong keys.

Loading external swfs extending from classes in the same shared codebase

Hey there!
I'm currently developing a game in flash and want to be able to divide up my .fla assets in a way that means artists can work on a game menu .fla in isolation from the game.fla and rest of the game code.
If I could briefly explain how I've approached this so far, I would be extremely if people could shout in my general direction and tell me I am stupid, or even better still, give me helpful advice as to where I am going wrong and how I can correct it!
My project is setup like this:
HighScoreMenu.fla -> document class HighScoreMenu extending GameMenu class.
game.fla ->document class game.as
game.as class loads the published HighScoreMenu.swf and manipulates the menu i.e. animates on and off screen via inherited functions in the GameMenu class.
Now this seemed to work to begin with, until my code evolved and upon going to publish my HighScoreMenu.fla flash started complaining about symbols being used in Game.as that were in Game.fla... If I'm only publishing the HighScoreMenu which extends from GameMenu then why is it even looking to compile Game.as?
Can you spot the problem in the way I am doing this, or is there a better approach I should try?
Any advice greatly appreciated!
Stevie.

No it doesn't seem to, however though they both have a reference to the HighScoreManager so there is some overlap there between menu and game code, but the menu classes don't reference any symbols on the game.fla...
Just to clarify, I have an fla HighScoreMenu.fla, which has a child symbol HighScoreMenu which uses the following class.
public dynamic class HighScoreMenu extends GameMenu
//convenient storage for competition Manager object
private var _highScoreManager:HighScoreManager;
The game then loads this class as follows:
I load the swf "HighScoreMenu.swf" and once its loaded I obtain the menu class like so:
var cls:Class = Class( applicationDomain.getDefinition("Menus.HighScoreMenu") );
var object:* = new cls();
...and create a new instance of the menu.
Is there anything fundamentally wrong with approaching it in this way?

[svn:osmf:] 10996: Changing MediaElementLayoutTarget to be constructed via a static ' getInstance' method, instead of by its constructor.

Revision: 10996
Author:   [email protected]
Date:     2009-10-19 07:45:26 -0700 (Mon, 19 Oct 2009)
Log Message:
Changing MediaElementLayoutTarget to be constructed via a static 'getInstance' method, instead of by its constructor. This prevents the construction of multiple instances that reference one single media-element. Updating client code accordingly.
Extending layout unit tests. Adding a custom renderer, checking calculation and layout passes invokation counts.
Modified Paths:
    osmf/trunk/framework/MediaFramework/org/osmf/composition/CompositeViewableTrait.as
    osmf/trunk/framework/MediaFramework/org/osmf/composition/ParallelViewableTrait.as
    osmf/trunk/framework/MediaFramework/org/osmf/composition/SerialViewableTrait.as
    osmf/trunk/framework/MediaFramework/org/osmf/gateways/RegionSprite.as
    osmf/trunk/framework/MediaFramework/org/osmf/layout/DefaultLayoutRenderer.as
    osmf/trunk/framework/MediaFramework/org/osmf/layout/LayoutContextSprite.as
    osmf/trunk/framework/MediaFramework/org/osmf/layout/LayoutRendererBase.as
    osmf/trunk/framework/MediaFramework/org/osmf/layout/MediaElementLayoutTarget.as
    osmf/trunk/framework/MediaFramework/org/osmf/utils/MediaFrameworkStrings.as
    osmf/trunk/framework/MediaFrameworkFlexTest/org/osmf/composition/TestParallelViewableTrai t.as
    osmf/trunk/framework/MediaFrameworkFlexTest/org/osmf/gateways/TestRegionSprite.as
    osmf/trunk/framework/MediaFrameworkFlexTest/org/osmf/layout/TestDefaultLayoutRenderer.as
    osmf/trunk/framework/MediaFrameworkFlexTest/org/osmf/layout/TestLayoutUtils.as
    osmf/trunk/framework/MediaFrameworkFlexTest/org/osmf/layout/TestMediaElementLayoutTarget. as
Added Paths:
    osmf/trunk/framework/MediaFrameworkFlexTest/org/osmf/composition/CustomRenderer.as

AlexCox, can you clarify that once you get the default model out and save it in the temp var, that your ONLY access to the data is then through temp and never through the JList methods?
Quick question for everyone: Are any of you using JList.setListData to set you initial dataset? I think that is where the bug lies. When I do not use it and start with an empty list, I am able to add to that list, and then remove from that list using the following line:
((DefaultListModel)myList.getModel).remove(###);
And this cast works everytime.
But when I start that list out with some data such as:
myList.setListData(myStringArray);
and then try to execute the above line, boom! I get the ClassCast exception.
I think setListData creates a whole new model which is an inner class to JList (which seems stupid when DefaultListModel seems made for this).
Just my thoughts on where the problem lies. I'll add my data directly to the list and see if that works better.
PK

AS3.0: How to extend a class that extends MovieClip

When I try to set the base class of a library symbol to a
class that doesn't DIRECTLY extend MovieClip, but instead extends
another class that DOES extend MovieClip, it's disallowed, saying,
"The class 'Whatever' must subclass 'flash.display.MovieClip' since
it is linked..."
Is this just a validation bug in the property windows, only
checking one class deep into the inheritance hierarchy? Because the
specified class does extend MovieClip, just two levels in instead
of one. Is there a fix for this? Or must library symbols always
directly extend MovieClip? If so, why?

Which classes from flash.display have you imported. Just
because a class extends another doesn't mean that it inherits all
of its properties. Saying a class extends sprite doesn't give you
access to all of it's properties, members and display unless you
first import flash.display.* or flash.display.Sprite. If you aren't
already importing flash.display.*, try importing
flash.display.MovieClip and see what happens...

Abstract classes having constructors?

InputStream is an abstract class. But then according to API it has a public constructor. Well, since it cannot be instantiated, why does it need to have a constructor? What's the point? Thanks!

I don't know why it would have a public constructor, but it definitely needs at least one c'tor. Usually abstract classes' c'tors are protected.
A constructor does not create an object, it only initializes an already created object to a valid initial state. When creating an instance of a subclass, the ancestors' constructors, starting with Object, are run first, before the descendants'. This way a child knows that the "parent portion" of the object is in a valid state before its constructor starts.
Constructor rules:
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.

Error while extending controller: class name is wrong or not included

Hi All,
I am getting this error while I port my extended controller class to the custom top and assign this controller to the page. I have made sure its the class file that is copied. The directory is correct, the permissions were given using chmod 775. There exists a soft link betwen the custom top and the oracle top as well. What else am I missing here?
Error: oracle.apps.fnd.framework.OAException: Could not create Java class: (oracle.apps.ap.oie.entry.webui.XXEntryFlowPageCO) associated with region: (GeneralInformationPG). This is probably because the class name is wrong or not included in project.

:( Started out with that Gyan. If i do give the path with xx. appended to it, it lets me save and when i log back in and there are no changes to the page. I go to the personalise option to find the modification has been overwritten. I was told that this is so because Oracle doesnt recognise the xx.path and since there exists a soft link already the standard path with the new controller name should work.
i have really tried both of these options and am not sure what could be wrong. thanks for all your attempts to help. anything else i can try?

Not able to pass values to variables in extended Tree class

Hi,
I have a as class that extends from Tree, additionally this
custom class defines
new class level variables as follows:
public class MyTree extends Tree {
public var arrayColl:ArrayCollection;
and i call this tree from mxml as follows:
<customTree:MyTree
arrayColl={list}
xmlns:customTree="../../.*"/>
Though my 'list' collections is not null, when i place an
alert in the constructor of extended tree class
it shows as null.
Please advice.
Thanks,
Lucky

below is the canvas:
<?xml version="1.0" encoding="utf-8"?>
<mx:Canvas xmlns:mx="
http://www.adobe.com/2006/mxml">
<mx:Script>
<![CDATA[
import mx.collections.IViewCursor;
import mx.collections.ArrayCollection;
import com.citi.ascript.TreeData;
import mx.controls.Alert;
import com.citi.ascript.CitiTree;
public var availableTreeDataColl:ArrayCollection;
public var selectedTreeDataColl:ArrayCollection;
public var treeData:TreeData;
public var srcTree:CitiTree;
public var availableTreeDataArray:Array = [{a:1},
{b:2},
{c:3}
public function formTreeData():void {
var tempColl:ArrayCollection = new
ArrayCollection(availableTreeDataArray);
availableTreeDataColl = new ArrayCollection();
for (var i:int=0; i<tempColl.length; i++) {
var tempTreeData:Object = tempColl
treeData = new TreeData();
treeData.a = tempTreeData.a;
treeData.b = tempTreeData.b;
treeData.c = tempTreeData.c;
availableTreeDataColl.addItem(treeData);
]]>
</mx:Script>
<mx:VBox>
<mx:Label text="Drag & Drop the analysis sections
that you would like to include in the report" width="450"
height="20"/>
<mx:HBox id="treeContainer"
creationComplete="formTreeData();">
<customTree:MyTree treeList={availableTreeDataColl} //
here collection becomes null.
</customTree:MyTree>
</mx:HBox>
</mx:VBox>
</mx:Canvas>
The collection of TreeData is iterated in the customTree to
form xml which will act as dataprovider for tree.
and this canvas is added as a child for a panel in my main
application.
Hope this gives an idea.
Thanks,
Lucky

Constructor inheriting

Can we inherit a constructor of the base class in the derived class.
Please help.
Shravan

You can always write a constructor of your own, and call the super class constructor:
public Dummy(int dummy) { // Constructor for class Dummy which
// extends a class with a constructor which takes one int
super(dummy);
}/Kaj

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

Extending a class, its constructor inherited ?

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